~ CHAPTER 3 ~ SOIL- AND SOIL-EROSION BASICS ~
Edition 9 of March 2010
(Updated Sept. 2011)

NOTE: CHAPTER 3 PARTS (3-F-d) AND BEYOND ARE IN ANOTHER FILE. (se3f.html) 

TABLE OF CONTENTS:

(3-A) ~ Soil Formation ~

(3-B) ~ Soil Conservation Technologies ~

(3-C) ~ Land Classifications ~ [C1]~Net Primary Production (NPP), [C2]~Soil Carbon Pool, [C3]~Croplands by Erosion Potential, Class, ~

(3-D) ~ Soil Data ~
~ (3-D-a) ~
Soil Basics ~ [Da1]~Soil Organic Matter, [Da2]~Soil Compaction, [Da3]~Water-Holding Capacity, [Da4]~Mycorrhizae, [Da5]~Justus von Liebig's Law of the Minimum ~
~ (3-D-b) ~
Temperate Soils ~ [Db1]~Australia, [Db2]~North America, [Db3]~China, ~
~ (3-D-c) ~ 
Tropical Soils ~ [Dc1]~General, [Dc2]~Lateritic Soils, [Dc3]~African Soils, [Dc4]~Vertisol Soils, ~

(3-E) ~ Desertification and Land Degradation Generally ~

(3-F) ~ Antiquities ~
~ (3-F-a) ~
Climatic Changes ~
~ (3-F-b) ~
In the Beginning. ~
~ (3-F-c) ~
Salinization as a soil destroyer ~
~ (3-F-d) ~
Selected Civilizations ~ [Fd1]~Mesopotamia (Iraq), [Fd2]~Jordan, [Fd3]~Israel, [Fd4]~Europe, [Fd5]~China, [Fd6]~India, [Fd7]~Sri Lanka, [Fd8]~Greece, [Fd9]~Turkey, [Fd10]~North Africa, [Fd11]~US, [Fd12]~Mayans, [Fd13]~Oceania,
~ (3-F-e) ~
Some Lifetimes of Civilizations ~

(3-G) ~ Effects of Soil Erosion and Monoculture on Cropland Yield
~ (3-G-a) ~
Erosion-Effects on Crop Yield ~ [Ga1]~Evaluation of experimental procedures, [Ga2]~Studies of Productivity Loss vs. erosion in the US, [Ga3]~Studies of Productivity Loss vs. erosion outside the US,
~ (3-G-b) ~
Monoculture ~ Effects on Crop Yield ~
~ (3-G-c) ~
Organic Matter ~ Effects on Crop Yield ~

(3-H) ~ Causes and Dynamics of Soil Erosion ~
~ (3-H-a) ~
Anthropogenic Effects on Soil Erosion in General ~
~ (3-H-b) ~
Soil Erosion on non-crop lands (Also see (4 -H)) ~
~ (3-H-c) ~
Slope Dependence of Erosion ~
~ (3-H-d) ~
Crop Dependence of Erosion ~
~ (3-H-e) ~
Soil-Cover Dependence of Erosion ~
~ (3-H-f) ~
Positive Feedbacks (Instabilities) ~
~ (3-H-g) ~
Wind Erosion ~
~ (3-H-h) ~
Gully Erosion ~
~ (3-H-i) ~
Mulching- and Organic Matter Influence on Erosion ~

(3-I) ~ Off-Farm Impacts of Soil Erosion ~

(3-J) ~ Reduced Tillage Cropland Management ~
~ (3-J-a) ~
Extent of Reduced Tillage ~
~ (3-J-b) ~
Effects of Reduced Tillage ~
~ (3-J-c) ~
Reduced Tillage Technology and Economics ~

(3-K) ~ Aluminum Toxicity ~
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3
NOTE: The notation (su1) means that the data is used in the document analyzing the sustainability of the productivity of the world's food, fiber and water supply systems. (See elsewhere in this website after late 2007.)

Comments: Defining soil and soil-loss has difficulties. While "soil" is considered to have only several major components, a continuum extends from subsoil (about 0.5% organic carbon) through topsoil (typically 1-3% organic carbon) through soil-surface litter (about 50% organic carbon). The loss of each component has much different implications. "Topsoil" (the portion of the continuum rich in very-slowly-degrading organic matter) is seen here as the critical resource, so it is the focus of this document. Thus this document attempts to distinguish topsoil-loss from losses from other parts of the continuum. This is essential for estimating the remaining lifetime of agricultural soil productivity. Soil surface litter (the portion of the continuum rich in rapidly degrading organic matter) is not critical in temperate soils because litter is largely replaced annually by net primary production. In many tropical soils, loss (removal) of vegetation via cropping or grazing or logging also removes the bulk of the nutrients needed for fertility, and this does has serious implications.

There is growing evidence that farmers are able to adapt to environmental stress in ways that limit degradation (01M3) (01M2) (03N1). Comments: This statement is difficult to interpret without examples. The greater the population pressures on agricultural land the greater the rate of land degradation - Erosion in the Yellow Crescent from Korea to the Middle East is far larger than elsewhere in the world.

SECTION (3-A) ~ Soil Formation ~

Soil covers most of the land surface of the earth in a thin layer, ranging in thickness from a few centimeters to several meters. It is composed of inorganic matter (rock and mineral particles), organic matter (decaying plants and animals), living plants and animals (many of them microscopic), water and air (99F1).

The speed of the soil building process varies. In prairie regions with ample rain and organic inputs, it may take 50 years to build up a few centimeters of soil; in mountainous areas it can take thousands of years (99F1).

The belief that soils are renewable first appeared in the English literature in the 1970s (Refs. 5, 24 of (92F2)).

It takes 200-1000 years, averaging about 500 years, to form 2.5 cm. (1 inch) of topsoil (94K3) under normal agricultural conditions (82O3), (81H4), (84L4), (84L5), (85E2). Globally, current soil losses range from 2000-30,000 tonnes/ km2/ year (80E1). Comments: These loss-rate data should be compared to a cropland soil-formation rate of 337-1123 tonnes/ km2/ year (See below.)

About 200-1000 years are required to form 2.5 cm. of topsoil under cropland conditions, and even longer under pasture- and forest conditions (Refs. 24, 33, 61, 81 of (95P1)).

Under normal agricultural conditions, soil forms at a rate of 1"/ 100 years (1.5 ton/ acre/ year) (337 tonnes/ km2/ year) (Ref. 35 of (76P2)).

Under ideal soil management conditions soil forms at a rate of 1"/30 years (1123 tonnes/ km2/ year) (Ref. 32 of (76P2)).

The average rate of soil formation (the rate of conversion of parent material into soil in the A, E and B horizons) is about 100 tonnes/ km2/ year (Ref. 17 of Ref. (95P1)).

Rates of (sub-) soil development from bedrock are on the order of 0.2 ton/ acre/ year (45 tonnes/ km2/ year) in humid climates (87A1) and (Physical Geography, 6, pp. 25-42). Rates range from under 0.04 ton/ acre/ year (9 tonnes/ km2/ year) for siliceous plutonic rocks to over 0.4 ton/ acre/ year (90 tonnes/ km2/ year) for some calcareous clastic sedimentary rocks (87A1).

Chamberlin's 1908 estimate of rates of (sub-) soil development from bedrock: 0.14 ton/ acre/ year (31 tonnes/ km2/ year) (87A1).

Typical soil formation rates: 1 cm/ 100-400 years (Ref. 32 of Ref. (95D3)). Comments: 1 cm/ 100 years = 133 tonnes/ km2/ year at a density of 33,700 tonnes/ km2-inch. This rate seems low for cropland - reasonable for grassland, forestland etc.

Total chemical denudation, by water, of the landmasses of the earth is on the order of 21 tonnes/ km2/ year ((56T1), p. 546). Whole-earth weathering rate of bedrock: 27 tonnes/ km2/ year = 0.01 mm/ year ((75P1), Ref. 4) Comment: These numbers should be interpreted as the rate of formation of sub-soil from bedrock. This is to be distinguished from the rate of formation of topsoil (the "A horizon") from sub-soil that occurs much faster - about 5 tons/ acre/ year (1123 tonnes/ km2/ year) (1/30 inch/ year) on cropland.

Current global rate of erosion of bedrock: 22.5 Gt./ year, as compared to the erosion rate over the past 350 million years of 7.5 Gt./ year (=57.7 tonnes/ km2/ year on the world's 130 million km2 of ice-free land). This factor-of-three increase over the past century or so is attributed to Man (See the Garrels and Mackenzie 1971 reference in (76G1).) 20% of the increase may be due to increases in continental land mass ((76G1), p. 307). Comments: 22.5 Gt./ year = 173 tonnes/ km2/ year for the world's 130 million km2 of ice-free land. This increase in rate of rock erosion could reflect the weathering of bedrock bared to the elements by gross anthropogenic soil erosion above it. If so, one should not assume that it reflects the rate of subsoil formation under productive soils.

Rates of chemical denudation (rock-weathering) in the US: 35 tonnes/ km2/ year. Sediment yields in the US: 316 tonnes/ km2/ year ((56L1), p. 640). Comments: This sediment-yield figure is obviously the rate of sediment discharge into oceans. It neglects the fact that only 5-10% of anthropogenic erosion sediments make it to the oceans. The remainder is deposited on river bottoms, flood plains, in pools behind dams, etc.

Topsoil forms at a rate of about 1.5 tons/ acre/ year (337 tonnes/ km2/ year) under favorable agricultural conditions. Such conditions do not exist in many places on the Great Plains of the US (Ref. 292, p. 77 of (81S1)).

Formation of the "A" horizon exceeds 1/30 inch/ year (about 5 tons/ acre/ year) in medium- to moderately coarse-textured soil, but is slower in fine-textured soils (Ref. 5 of (83S1)).

The rate of subsoil formation from unconsolidated material (bed rock) is less than 90 tonnes/ km2/ year (less than 0.4 tons/ acre/ year) ((83L1), Ref. 34, p. 463). Sub-soil formation from consolidated material (bed rock) is even slower (83L1).

Subsoil renews at about 0.5 ton/ acre/ year (112 tonnes/ km2/ year) for unconsolidated parent material (bed rock), and is much less for consolidated material (bed rock) (Ref. 11 of (83S1)).

Under ideal soil management conditions, topsoil may form at 1"/30 years (5 tons/ acre/ year), and under natural conditions at 1" per 300-1000 years (0.5-0.15 tons/ acre/ year). Under normal agricultural conditions, topsoil forms at 1"/ 100 years (1.5 tons/ acre/ year). Compare this to the average loss of topsoil from agricultural land of 12 tons/ acre/ year (2700 tonnes/ km2/ year) ((76P2), p. 150). Comments: Grasslands (grazing lands) would probably be considered as "natural conditions".

Bartelli (1980) cited in Ref. (83C1), p.11, claims that on permeable, unconsolidated material, the "A" horizon (roughly the topsoil - usually the most productive soil layer) may develop at a rate of about 1 cm/ year (60 tons/ acre/ year) - far above the 5 tons/ acre/ year (1123 tonnes/ km2/ year) rate of topsoil regeneration assumed by the SCS in setting tolerable erosion standards. Bartelli believes that, on the Tama, Decatur and Davidson soils found in much of Illinois, the rate of formation of the "A" horizon is not less than 12 tons/ acre/ year (83C1). A good discussion of the Soil Loss Tolerance ("T") (usually 5 tons/ acre/ year) is given in Ref. (87J1).

World-wide Soil Formation Rates by Direct Determination (92F2) (Formation rates (Col. 5) are in units of years per inch)
Soil Type |Location~ | Notes~ ~ ~ ~ ~ ~ ~ ~ | Age of | Form.
Generic - | ~ ~ ~ ~ ~|~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ |Material| Rate
Name- - - | ~ ~ ~ ~ ~|~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ |(years) | - -
Alluvium~ |Colorado~ |- - - ~ ~ ~ ~ ~ ~ ~ ~ | ~ 2000 | 271
Alluvium~ |Pakistan~ |- - - ~ ~ ~ ~ ~ ~ ~ ~ | ~12000 | 190
Alluvium~ |St.Vincent|Volcanic ash~ ~ ~ ~ ~ | ~ 4000 | ~44
Alluvium~ |Wisconsin |Decalcified Loess ~ ~ | ~ 8000 | 203
Argiaquoll|Oregon~ ~ |A-1*~ ~ ~ ~ ~ ~ ~ ~ ~ | ~ ~133 | ~21
Entisol ~ |Hawaii~ ~ |Azonal, volcanic ash~ | ~ ~ 45 | ~ 3
Alluvium~ |Iowa~ ~ ~ |Grassed loess ~ ~ ~ ~ | ~ ~100 | ~ 8
Ferosol ~ |Senegal ~ |Laterite~ ~ ~ ~ ~ ~ ~ | ~ ~ 35 | ~ 6
Hapludalf |Iowa~ ~ ~ |Podsol on loess ~ ~ ~ | ~ 4000 | 102
Hapludalf |Iowa~ ~ ~ |A-1, A-2@ ~ ~ ~ ~ ~ ~ | ~ 2500 | 211
Hapludalf |Wisconsin |A-1@~ ~ ~ ~ ~ ~ ~ ~ ~ | ~ ~265 | ~97
Hapludalf |Iowa~ ~ ~ |A-1@, only~ ~ ~ ~ ~ ~ | ~ ~400 | ~30
Mollisol~ |Iowa~ ~ ~ |Alluvium~ ~ ~ ~ ~ ~ ~ | ~ ~110 | ~76
Oxisol/s~ |Afric ~ ~ |3-ft. solum ~ ~ ~ ~ ~ | ~75000 |1905
Spodosol~ |Europe~ ~ |Podsol in glacial sand| ~ 1200 | ~53

* The age of soil material is as determined by paleological and/or chemical techniques, assuming "maturity" of soil has not been reached, except in the case of the African oxisols, which has been deleted from the primary calculation of soil formation rate.
@ Not full profiles/ solum

SECTION (3-B) ~ Soil Conservation Technologies ~

Reliable and proven soil conservation technologies include ridge-planting, no-till cultivation, crop rotation, strip-cropping, grass strips, mulches, living mulches, agro-forestry, terracing, contour planting, cover crops and wind-breaks (Ref. 98 of Ref. (95P1)).

Terracing at Debre Berhan, 100 km. north of Addis Ababa Ethiopia, has increased barley yields by 30-100 tonnes/ km2/ year by retaining water while controlling erosion (87M3).

Since early 1980's, government has concentrated exclusively on introduction of bench terraces. Indonesia government subsidizes bench terrace construction on slopes up to 50% (90S7).

Bench terraces reduced erosion about 70% relative to traditional practice. Bench terraces did not increase yields but did reduce cropping area up to 32%. Local farmers do not like bench terraces so they do not maintain the terraces. Indonesia (Sumatra) (Kerenci) (90S7).

Grass bunds and grass-legume bunds reduced erosion 79% and 86%, but grass bunds reduce crop area 17%. (Indonesia (Sumatra) (Kerenci)) (90S7).

Go to Top of this Review's Appendices (units, conversions, definitions)
Go to
Top of this Review's Reference List
Go to
Topsoil Loss-Causes, Effects, Implications (Table of Contents)
Go to
Home Page of this entire web site
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3

SECTION (3-C) ~ Land Classifications ~ [C1]~Net Primary Production (NPP), [C2]~Soil Carbon Pool, [C3]~Croplands by Erosion Potential, Class,

Part [C1] ~ Land Classification ~ Net Primary Production ~

It is estimated that the world's forests store 283 gigatonnes (Gt.) of carbon in their biomass alone, and 638 Gt. of carbon in the ecosystem as a whole (to a soil depth of 30 cm). Thus the world's forests contain more carbon than the entire atmosphere. Roughly half of total carbon is found in forest biomass and dead wood combined and half in soils and litter combined. Carbon in forest biomass decreased in Africa, Asia and South America in the period 1990-2005, but increased in all other regions. For the world as a whole, carbon stocks in forest biomass decreased by 1.1 Gt. of carbon annually, owing to continued deforestation and forest degradation, partly offset by forest expansion (including planting) and an increase in growing stock per hectare in some regions (05F2). (See p.14 of D:\Forest\A0400E03.doc) (also in df99.doc and terra-preta-data.doc)

Presently, humans appropriate over 40% of the earth's Net Primary Productivity (Rojstaczer S., Sterling S. M., Moore N. J., "Human appropriation of photosynthesis products", Science 294 (2001) pp. 2549-52.). Comments: A paper by Vitousek et al (86V1) say the same basic thing, but that paper contains some basic errors that cause the appropriation number to be significantly under-estimated. (See http://home.windstream.net/bsundquist1/gcia.html)

Adding up all the farming, fishing, mining, building and fuel consumption gives a total human demand on the earth's resources of 120% of the Earth's capacity to sustain these activities (Mathis Wackernagel, Norman Myers, Jorgen Randers, Richard Norgaard, Proceedings of the National Academy of Sciences, 6/25/02).

Net Primary Production and Biomass of Land Biota (la)
((75W2), (73W1), Ref. 23 of Ref. (78W2), p.143 and (78W3), p. 42)
(Col.5 = Area from Atlay, Ketner and Duvigneaud (1979) (79A1) (See p. 123 of (90W1))
(Col.6 = Area from Olson, Watts and Allison (1983) (83O1) (See p. 123 of (90W1))
Land Biota Type - - - |Area @ |Net.Prod|Biomass

- - - - - - - - - - - |(million|(Gt. C/|(Gt.C) |Area |Area
- - - - - - - - - - - | km2) ~ | Year) | ~ ~ ~ |Col.5|Col.6
Tropical Rainforest ~ |17.0~ ~ | ~16.8 | ~344. | 10.0| 12.0
Trop.Seasonal Forest~ | 7.5~ ~ | ~ 5.4 | ~117. | ~4.5| ~6.0
Temp.Forest, evergreen| 5.0~ ~ | ~ 2.9 | ~ 79. | ~6.0| ~8.2
Temp.Forest, deciduous| 7.0~ ~ | ~ 3.8 | ~ 95. | (in above)
Boreal* Forests ~ ~ ~ |12.0~ ~ | ~ 4.3 | ~108. | ~9.0| 11.7
Woodland/Shrub land ~ | 8.5~ ~ | ~ 2.7 | ~ 22. | ~4.5| 12.8
- -SUBTOTALS (forests)|57.0~ ~ | ~35.9 | ~765. | 34.0| 50.7
Savanna ~ ~ ~ ~ ~ ~ ~ |15.0~ ~ | ~ 6.1 | ~ 27. | 22.5| 24.6
Temperate Grass land~ | 9.0~ ~ | ~ 2.4 | ~ ~6.3| 12.5| ~6.7
Tundra/Alpine Meadow~ | 8.0~ ~ | ~ 0.5 | ~ ~2.3| ~9.5| 13.6
Desert Shrub~ ~ ~ ~ ~ |18.0~ ~ | ~ 0.7 | ~ ~5.9| 21.0| 13.0
Rock, ice and sand~ ~ |24.0~ ~ | ~ 0.03| ~ ~0.2| 24.5| 20.4
- -SUBTOTALS(dry land)|74.0~ ~ | ~ 9.7 | ~ 14.7| 90.0| 78.3
Swamp and Marsh ~ ~ ~ | 2.0~ ~ | ~ 2.7 | ~ 13.5| ~2.0| ~2.5
Bogs and peatland ~ ~ | ~. ~ ~ | ~.~ ~ | . ~ ~ | ~1.5| ~0.4
Lakes and streams ~ ~ | 2.0~ ~ | ~ 0.4 |0.02-2.| ~3.2|- - -
- -SUBTOTALS(wetlands)| 4.0~ ~ | ~ 3.1 | ~ 13.5| ~5.5| ~6.1
Cultivated Land ~ ~ ~ |14.0~ ~ | ~ 4.1 | ~ ~6.3| 16.0| 15.9
Built-up Areas~ ~ ~ ~ | ~. ~ ~ | ~ ~.~ | ~.~ ~ | ~2.0|- - -
- -TOTAL CONTINENT~ ~ |149.~ ~ | ~52.8 | ~827. |147.5|151.0
- -TOTAL MARINE ~ ~ ~ |361.~ ~ | ~24.8 | ~ 1.74|- - -|- - -
- -FULL TOTAL ~ ~ ~ ~ |510.~ ~ | ~77.6 | ~829. |- - -|- - -

* "Boreal" = of, related to, or located in northern (and mountainous regions of the northern hemisphere. @ Ref. (73W1)

Literature values for the global phytomass carbon pool size (in Gt. Carbon) (87E1)

Comments: Clearly the Whittaker-Liken estimate of the global phytomass carbon pool is larger than all other estimates and should be considered as an upper limit rather than the best estimate. More recent data later in this document suggest that Whittaker/ Likens significantly over-estimate the size of global forest biomass inventory.

Total phytomass produced by cropland and hay land of Russia in 1990 is estimated at 2186.9 Tg of dry matter (Table 1). Cropland comprises 1441Gt. (or 65.9%), hay land 667.5 Gt. (or 30.5%). Phytomass of perennials comprises only 3.6%. Above ground phytomass is about 58%, and below ground - 42%. The phytomass density differs by agricultural land and is 1.106 kg/m2, 0.845 and 3.062 kg/ m2 for cropland, hay land, and for perennials, respectively. NPP of cropland is the highest compared with other land uses (Table 2). (1 Tg. = 1 billion tonnes = 1 Gt.) (03S2) The research is derived from available crop statistic (Agriculture of Russia, 1995). To convert amount of yield and by-products expressed in metric phytomass units into carbon units we apply coefficient 0.86 for grain and 0.5 for rest phytomass fractions. Following assumptions have been done in the calculation: 1) living biomass (LB) is equal to NPP; 2) phytomass of agricultural land is considered as having a yearly life cycle (03S2).

Table 3. Distribution of Agricultural Phytomass (Tg. C) (Gt. C) by natural zones in Russia (1990) (03S2)
Bio-climate |Polar |Pre-Tundra +~ |FT,|Middle|
zone - - - -|Desert|Northern Taiga|ST | Taiga|
Crop~ ~ ~ ~ | 0~ ~ | 0.1~ ~ ~ ~ ~ |1.1| 27.0 |
Hay Land~ ~ | 0~ ~ | 6.1~ ~ ~ ~ ~ |3.3| 28.5 |
Total ~ ~ ~ | 0~ ~ | 6.2~ ~ ~ ~ ~ |4.4| 55.2 |

1 Tg = 1 billion tonnes = 1 Gt.

Bio-climate|Southern|Temperate|Steppe|Semi-Arid|
zone - - - | Taiga~ | Forest~ | ~ ~ ~| Desert~ |Total
Crop ~ ~ ~ | ~141.8 | 107.6 ~ | 351.5| 19.4~ ~ |648.5
Hay land ~ | ~ 34.2 | ~24.8 ~ | 137.0| 68.5~ ~ |302.1
Totals ~ ~ | ~176.0 | 132.4 ~ | 488.5| 87.9~ ~ |950.6

Part [C2] ~ Land Classification ~ Soil Carbon Pool

The stock of carbon stored in organic matter in the upper 1 meter of the world's soils is estimated to be about 1220 Gt. (about 1.5 times the amount of carbon stored in the world's biomass). Additional carbon is stored in deep soils as charcoal (50 Gt.) and carbonate carbon (720 Gt.) (W.G. Sombroek, F.O. Nachtergaele, and A. Hebel, "Amounts, Dynamics and Sequestering of Carbon in Tropical and Subtropical Soils." Ambio 22(7) (1993) pp. 417-426.)

Estimated average aboveground biomass and carbon content for different agroforestry types in sub-Saharan Africa (93U1)
Note: To compute carbon contents in kg/ ha, divide the aboveground biomass numbers by 2. This assumes that the biomass is 50% (by weight) carbon.

~

Above-ground biomass (kg/ ha)

Agroforestry type

Low density

Medium density

High density

Silvo-pastoral

.

.

.

0-400 mm rainfall

180

360

720

400-800 mm rainfall

175

350

700

>800 mm rainfall

4,850

9,700

19,400

Fruit tree

3,000

7,500

15,000

Fuelwood

15,400

38,500

77,000

Shelterbelts

.

6,490

.

Timber trees

130,000

240,000

270,000

Additional carbon is stored below-ground, in the roots and in the soil. Little information exists on the magnitude of this storage, although some estimates suggest that it could be as large as the above-ground storage (93U1).

Comparison of carbon stored in soil and in above-ground biomass (J. Goudriaan, "Biosphere Structure, Carbon Sequestering Potential and the Atmospheric 14C Carbon Record," Journal of Experimental Botany 43 (1992) pp.1111-1119). (Data are in units of kg C/ m2) (From a bar graph in Ref. (99P1))

Above-ground

Category

Litter

Litter+Wood

Litter+Wood+Leaf

Tropical Forest

0.2

6.0

6.3

Temperate Forest

0.7

8.7

9.1

Grassland

0.6

0.6

0.9

Desert Tundra

0.1

0.8

0.9

Agricultural Land

0.4

0.4

0.7

Below-ground

Category

Roots

Roots+
Humus

Roots+Humus+
charcoal, carbonates, etc.

Tropical Forest

0.8

2.4

6.9

Temperate Forest

1.0

15.4

23.0

Grassland

0.2

11.4

21.5

Desert Tundra

0.05

2.0

2.8

Agricultural Land

0.06

1.8

8.4

 

The Earth's soils contain 1500-2000 Gt. of organic carbon and 800-1000 Gt. of inorganic carbon. The ISRC calculated total global soil carbon as 2150-2300 Gt. in the upper meter, with soil organic matter being about 1500 Gt. of the total (96B3).

The pre-cultivation stocks of carbon on the present area of cultivated land were 222 Gt. and corresponding present levels of carbon are about 168 Gt. (93C1).

The total mass of organic carbon stored in soil globally is 1576 Gt., of which 32% is found in the tropics (93E2).

The world's grasslands (semi-arid) have an area of 73 million km2 and hold 500 Gt. of carbon (97S1).

The world's croplands have an area of 14 million km2 and hold 140 Gt. of carbon (97S1).

Soil organic carbon (SOC) levels decline when land is converted from grassland or forest to cropland (99B1). After 50-100 years, the levels stabilize at 50-60% of original values (99B1). The historic losses of soil organic carbon (SOC) of 41-55 Gt. C have been due to conversion of native grasslands and forests to croplands. Some estimates of SOC loss are as high as 50-100 Gt. because there is evidence that losses from tropical soils have been very high and, in general grossly under-estimated (99B1).

Soil carbon contents in semi-arid lands are inherently low, around 15 grams of soil organic carbon (SOC)/ kg or 2500 tonnes/ km2 (99B1).

Soils under native vegetation in sub-humid climates contain 20-30 grams of soil organic carbon/ kg. Continuous cropping with plowing rapidly decreases the SOC to as low as 5-10 grams of SOC/ kg within 5-10 years of cultivation (99B1).

In West Africa about 3 million km2 are used for agriculture and animal husbandry. Soil carbon values range from 2.5-6.0 grams/ kg for grazed savannahs and 1.5-4.5 grams/ kg on croplands without manure and 40% greater carbon contents in some locations with manure added (99B1).

The critical limit of soil organic carbon (SOC) concentration for most soils of the tropics is 1.1% (04L1). Comments: The limit is significantly larger in temperate soils.

Estimates of historic SOC loss range from 44 to 537 Gt., with a common range of 55-78 Gt. (R. Lal, Adv. Agron. 71 (2001) p. 145.), (04L1).

Between 1850 and 1998, emissions from fossil fuel combustion (270+30 Gt.) was about twice that from terrestrial ecosystems (136+55 Gt.) (Ref. 2 of (04L1)). The latter includes 78+12 Gt. from soils, of which about one third is attributed to soil degradation and accelerated erosion and two-thirds to mineralization (conversion to CO2) (04L1). Comments: Presumably "emissions" here refers to total C emissions.

Terrestrial ecosystems contributed to atmospheric CO2 enrichment during both the pre-industrial and industrial eras. During the pre-industrial era the total Carbon emission from terrestrial ecosystems was 320 Gt. or 0.04 Gt. Carbon/ year for 7800 years. During the industrial era the total Carbon emission from terrestrial ecosystems was 160 Gt. or 0.8 Gt. Carbon/ year for 200 years (W. Ruddiman, Climatic Change 61 (2003) p. 261.) (04L1).

Severe depletion of the soil organic carbon (SOC) pool degrades soil quality, reduces biomass productivity and adversely impacts water quality (as a result of reduced erosion resistance) (04L1).

Conversion of natural to agricultural ecosystems causes depletion of the SOC pool by as much as 60% in soils of temperate regions, and by 75% or more in cultivated soils of the tropics. The depletion is exacerbated when the output of Carbon exceeds the input, and when soil degradation is severe (04L1).

The global soil carbon (C) pool of 2500 Gt. includes about 1550 Gt. of soil organic carbon (SOC) and 950 Gt. of soil inorganic carbon (04L1). The soil Carbon pool is 3.3 times the size of the atmospheric pool (760 Gt.) and 4.5 times the size of the biotic pool (560 Gt.). The SOC pool to 1-meter depth ranges from 30 tons/ ha in arid climates to 800 tons/ ha in organic soils in cold regions and a predominant range of 50-150 tons/ ha (04L1). Comments: The word "tons" above probably refers to metric tons (tonnes) since it is used in conjunction with the metric quantity "ha".

Estimate of the World Soil Carbon Pool (82P2) (la)
Life-Zone Group - - - - -| ~ Area | Carbon| Soil
- - - - - - - - - - - - -|-million|Density|(Gt.C)
- - - - - - - - - - - - -| ~km2 ~ | kg/m2~ | - - -
Tropical Forest, wet~ ~ | ~4.1 ~ | 19.1 | ~78.3
Tropical Forest, moist~ | ~5.3 ~ | 11.4 | ~60.4
Tropical Forest, dry~ ~ | ~2.4 ~ | ~9.9 | ~23.8
Tropical Forest, very dry| ~3.6 ~ | ~6.1 | ~22.0
Temperate Forest, warm ~ | ~8.6 ~ | ~7.1 | ~61.1
Temperate Forest, cool ~ | ~3.4 ~ | 12.7 | ~43.2
Boreal Forest, wet ~ ~ ~ | ~6.9 ~ | 19.3 | 133.2
Boreal Forest, moist ~ ~ | ~4.2 ~ | 11.6 | ~48.7
Trop. Woodlands, Savanna | 24.0 ~ | ~5.4 | 129.6
Temperate Thorn Steppe ~ | ~3.9 ~ | ~7.6 | ~29.6
Cool Temperate Steppe~ ~ | ~9.0 ~ | 13.3 | 119.7
Tropical Desert Bush ~ ~ | ~1.2 ~ | ~2.0 | ~ 2.4
Warm Desert~ ~ ~ ~ ~ ~ ~ | 14.0 ~ | ~1.4 | ~19.6
Cool Desert~ ~ ~ ~ ~ ~ ~ | ~4.2 ~ | ~9.9 | ~41.6
Boreal Desert~ ~ ~ ~ ~ ~ | ~2.0 ~ | 10.2 | ~20.4
Tundra ~ ~ ~ ~ ~ ~ ~ ~ ~ | ~8.8 ~ | 21.8 | 191.8
Cultivated Lands ~ ~ ~ ~ | 21.2 ~ | ~7.9 | 167.5
Wetlands ~ ~ ~ ~ ~ ~ ~ ~ | ~2.8 ~ | 72.3 | 202.4
Lakes and streams~ ~ ~ ~ | ~3.2 ~ | ~0.0 | ~ 0.0(90W1)
Global Soil Carbon Pool~ |132.8 ~ | - - -|1395.3

Four major ecological zones of the tropics are savannas and associated grasslands (49%), evergreen forests (24%), desert (16%), and semi-desert (11%) ((75S1), Ref. 16) (la).

Part [C3] ~ Croplands by Erosion Potential, Class ~

Distribution of US Croplands by Inherent Erosion Potential ((84U2), p. 57)
(Col. 1 = Inherent erosion potential (tons/ acre/ year))
(Col. 2 = Total croplands in category (million acres) (excludes 7 million acres of wild hay and mountain meadow))
(Col. 3 = Croplands planted in major crops (million acres))
(Col. 4 = Croplands treated with contour plowing, crop residues, or minimum tillage (million acres))
(Col. 5 = Not treated = Col. 3 minus Col. 4)
Col.1 |Col.2 |Col.3=|Col.4+Col.5
0 -10 |201.6 |188.6=|96.4 + 92.2
10-25 |112.7 | 98.6=|48.6 + 50.0
25-50 | 44.7 | 37.0=|15.7 + 21.3
50-100| 26.6 | 22.9=| 8.4 + 14.5
100+~ | 20.7 | 18.3=| 6.2 + 12.1
Totals|406.3 |365.4=|175.3+190.1

100 million acres (405,000 km2) of US cropland (out of 460 million acres of land suitable for crops) are flat, alluvial lands of negligible erosion hazard (53L1). (la)

Distribution of US croplands by Class ((79S1), p. 92)
Class -|1977~ ~ |Annual ~ | Productivity
- - - -|cropland|Soil Loss|Loss (million
- - - -|(million|(million |acres
- - - -| acres) | tons) ~ |Equivalent**)
I~ ~ ~ | 31.529 | ~ 91.244|0.140
II ~ ~ |187.702 | ~709.722|1.092
III~ ~ |131.710 | ~709.388|1.091
IV, VII| 62.226 | ~506.603|0.779
Totals |413.167 | 2017. ~ |3.102

** 650 tons (4 inches) = 1 acre

Go to Top of this Review's Appendices (units, conversions, definitions)
Go to
Top of this Review's Reference List
Go to
Topsoil Loss-Causes, Effects, Implications (Table of Contents)
Go to
Title Page of this entire web site
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3

SECTION (3-D) ~ Soil Data ~ [Da]~Soil Basics, [Db]~Temperate Soils, [Dc]~Tropical Soils,

Part [Da] ~ Soil Basics ~ [Da1]~Soil Organic Matter, [Da2]~Soil Compaction, [Da3]~Water-Holding Capacity, [Da4]~Mycorrhizae,

Porous loamy soils are the richest of all, laced with organic matter which retains water and provides the nutrients needed by crops. Sand and clay soils tend to have less organic matter and have drainage problems: sand is very porous and clay is impermeable. Only 11% of the earth's soils have no inherent limitations for agriculture. (See table below.) (99F1)

Percent of total world land area that has:
Soils too dry for agriculture - 28%
Soils limited by chemical problems for use in agriculture - 23%
Soils too shallow for agriculture - 22%
Soils too wet for agriculture - 10%
Permafrost - 6%
Soils with no limits for agriculture - 11%
Total - 100%

Soil texture varies with particle size from clay (fine) through silt (medium) to sand (coarse). The larger the particles the larger the spaces between them so water drains fast through sand but clay gets waterlogged quickly. Texture depends largely on the bedrock - shales yield finer soils than sandstones -but most soils contain a mixture of particle sizes in different proportions. Loam is the best soil for plant growth (99F1).

The basics of soil characteristics (texture, structure, organic matter, depth of rooting zone) are outlined in Chapter 3 of Ref. (83C1). As a general rule, the most productive soil ecosystems occur in temperate climates and/or areas of young soil parent material such as the glaciated, alluvial, or loess-covered zones of North America, Europe, and Asia (88L1). Africa's soils are derived from old, highly weathered landscapes under severe climatic conditions (88L1).

Sub-Part [Da1] ~Soil Basics ~ Soil Organic Matter ~
In Africa, non-intensive agriculture (bush-fallowing) results in organic matter levels a little more than half of those found in uncultivated lands. More intensely cultivated lands may have even lower organic matter contents (99B1).

African savannahs are characterized by sandy soils of generally low organic matter contents (99B1).

In a 1973 study of Nigerian soils, soil carbon levels were estimated at 0.68% (1.36% organic matter).
In Kenya soil carbon levels had a mean soil carbon level of 3.33% (99B1).

In northeastern Brazil, where the predominant cropping regime is shifting agriculture with 5 years of crops followed by 20 or more years of bush fallow, soil carbon levels in the native land is around 8 grams/ kg or 1500 tonnes/ km2. Clearing results in rapid losses (30-50%) of carbon within the first 6 years, at which point the land is usually abandoned (99B1).

Global warming is expected to exacerbate regional food crises. As the ground heats up, organic matter decomposed more readily, reducing soil fertility (04K1). Comments: Tropical soils contain roughly 1% organic carbon while temperate soils contain about 3% organic carbon. Organic carbon makes soils more fertile and increases erosion resistance.

Organic matter is a major source of plant nutrients and is the glue that holds soil particles together and stabilizes the pore structure. Organic matter makes soils less vulnerable to wind erosion and functions as a sponge for holding water and slowing down its loss from the crop root zone by drainage or evaporation. Moreover, nutrients added to soil as organic residues are released more gradually than those from mineral fertilizers and are therefore less prone to leaching, volatilization or fixation (86A1). In addition, higher soil organic matter levels are commonly associated with greater levels of humic acid, which increases phosphate availability and thus can be beneficial in those areas with strongly phosphate-fixing soils found in sub- Saharan Africa and Latin America (03N1).

Agriculturally modified topsoils average 25% less carbon than unmodified (native) topsoils (Ref. 1 of Ref. (93H5)).

Cultivated soils contain about 180 Gt. of carbon (per km2?) (Ref. 2 of Ref. (93H5)). Based on radiocarbon data, soil carbon can be divided into fast- and slow-turnover time pools. The fast pool (75% of soil-carbon) has a turnover time of 25 years, while the slow turnover pool (25% of soil carbon) has a turnover time of 3700 years. Conversion to croplands produces a carbon loss - mainly from the fast turnover pool (93H5).

Humus comprises, typically, 6% of topsoil mass, but up to 25% of its volume (95W1).

About 95% of the Nitrogen, and 25-50% of Phosphorus are contained in the organic matter component of soil (Ref. 34 of (95P1)).

Fertile topsoil typically contains 10,000 tonnes/ km2 of organic matter (4% of total soil weight) (Refs. 68 and 69 of Ref. (95P1)). Comments: If 45% of organic matter is carbon, then 2% of soil weight is carbon. (0.45*4%)

Good-quality soil contains 100 tonnes/ km2 of earthworms, 100 of arthropods, 15 of protozoa, 15 of algae, 170 of bacteria, and 270 of fungi (Ref. 74 of (95P1)). Straw mulches may increase soil biota by 3 times (Ref. 79 of (95P1)), and organic matter or manure may increase earthworms and microorganisms by as much as 5 times (Ref. 80 of (95P1)). Comments: Total soil biota = 670 tonnes/ km2 out of 400,000 tonnes/ km2 of topsoil.

Soil organic matter serves many functions: (su1)

Polysaccharides, a form of carbohydrate produced by microbes as they decompose plant residue, increase a soil's ability to clump into small aggregates. The compounds also increase soil strength and mechanical resistance to rain-drop impact (88U4). This helps to explain why soils high in organic matter (which typically contain more microbes) are less erodible than soils low in organic matter (e.g. tropical soils and soils that have been cropped for many years). It also helps to show why crop-residue management is so effective in protecting soil from water erosion. Crop residues enhance microbial activity near the soil surface. Bodies of microbes themselves also have been found to help glue soil particles together (88U4).

Only a small portion of soil organic matter is in a free state since it is "mineralized" (e.g. to CO2) relatively rapidly. Most is chemically bound with the mineral part of the soil where its rate of mineralization is much slower. The nature of metallo-organic compounds (organo-mineral compounds) that form when soil organic matter bonds chemically with the minerals in the soil is different for different soils and is not yet clear ((66K1), p. 190). Comments: Soil carbon is also found in the form of fulvic acid and humic acid.

Naturally occurring organic acids (humic acids) are effective agents in the colloidal transport of Cu, Pb, Zn, Fe, and Al. Concentrations of organic acids in the range 4-40 ppm carbon can increase the amount of these metals stabilized in solution by several orders of magnitude. Stability of these metal-organic colloids increases with organic acid content and with pH (optimal pH is 6- 9). Experiments on the role of organic acids in the decomposition of rock and minerals are reviewed in Ref. (70O1).

Some Organic Carbon Contents of Soils of Saskatchewan (96J1) (typically semi-arid land for growing wheat)
Moderate fertilizer, 70-80 years| 0.99%
Heavily manured soils ~ ~ ~ ~ ~ | 3.83, 3.88, 4.77%
Fallow-cereal-crop soils~ ~ ~ ~ | 1.33-2.88%
Fallow-cereal-no-till soils ~ ~ | 2.54-2.83%
Research plot soils ~ ~ ~ ~ ~ ~ | 3.34, 3.79, 2.97, 5.56%

(Semi-Arid Lands) Soil Organic Carbon Contents in Tall Grass Prairie Sites in eastern Kansas (96S1):
Prairie Site - - - (depth = 0.2 meter) 2.2% organic carbon
Cultivated Site* (depth = 0.2 meter) 0.8% organic carbon
* Near the prairie site, but cropped continuously in winter wheat since sometime between 1939 and 1950.

Comments: The depletion of organic matter in arid croplands is also found in Canada's Great Plains - about half of the original organic matter there has been lost. See elsewhere in this document.

(Arid Lands) Some soil characteristics within Los Alamos National laboratory Environmental Research Park in northern New Mexico, elev.2140 meters, 36 cm. precipitation/ year (semi-arid) with vegetation mainly of pinon-juniper woodlands with an understory of mixed grasses, annual forbs and cryptogamic crust. (96D1)
A-horizon thickness: 8-13 cm.

Organic carbon content of the "A" horizon: about 0.4% on bare ground, about 0.8% under grass, roughly 1.1% under pinon-juniper canopy. Comments: Other researchers contend that, in arid lands, the organic carbon content of the "A" horizon (top layer) is usually not more than 1%, and in temperate, reasonably humid climates soil organic carbon contents are typically 2-4%. Since the site described here is considered semi-arid, the organic carbon contents seem reasonable. "A"-horizon thicknesses are also known to be thinner in arid climates than in more humid climates. The thickness listed above seems consistent with this.

Sub-Part [Da2] ~Soil Basics ~ Soil compaction ~
Natural alleviation of subsoil compaction (e.g. by heavy tractors) is usually very slow (
05S1).

Soil compaction has increased because average tractor-weight has more than doubled in the past 30 years (Ref. 10 of (79U1)).

Increased soil compaction reduces soil tilth and increases runoff (79U1).

Sub-Part [Da3] ~Soil Basics ~ Water-Holding Capacity ~

Available-water-holding Capacity of Various Soils
(Capacity (Col. 2 and 4) is in units of inches of water/ inch of soil-depth) (Refs. 2, 5, 15, 23 of (80G1))
Textural Class -|Capacity| Textural Class |Capacity
Sand~ ~ ~ ~ ~ ~ | 0.070~ | Silt loam~ ~ ~ | 0.20
Loamy sand~ ~ ~ | 0.065~ | Silt ~ ~ ~ ~ ~ | 0.23
Sandy loam~ ~ ~ | 0.095~ | Silty clay loam| 0.25
Sandy clay loam | 0.150~ | Silty clay ~ ~ | 0.22
Loam~ ~ ~ ~ ~ ~ | 0.165~ | Clay ~ ~ ~ ~ ~ | 0.20
Clay loam ~ ~ ~ | 0.185~ |

Comments: Kinetic factors are neglected here. If rain comes too fast, water won't have time to soak down to lower soil levels and start to run off the soil surface sooner than one would predict from the above table. Sand tends to be the coarsest-grained soil component; silt is finer-grained, and clay is the finest-grained. Thus clay has the most surface area, and would be expected to be able to hold the most water.

Sub-Part [Da4] ~Soil Basics ~ Mycorrhizae ~
The soils of boreal, temperate, alpine and tropical forests and the soils of grasslands and tundra all have mycorrhizae. The few exceptions are lava fields, strip mines, places robbed of topsoil and farmland that had been heavily fertilized. Many plants seem to benefit from plant-fungi partnerships. The more plentiful the fungal species, the more diverse the floral landscape above. Many horticulturists inject mycorrhizae in plant beds to reduce the damage done by (chemical) fertilizers and to stimulate productivity (
04P1). (See more of this issue in the document on grazing land degradation.)

Sub-Part [Da5] ~Soil Basics ~ Justin von Liebig's Law of the Minimum ~
In the mid-19th century, Justus von Liebig formulated his law of the minimum that states that plant growth is limited by the availability of whatever nutrient is scarcest (
76J1).

Part [Db] ~ Temperate Soils ~ [Db1]~Australia, [Db2]~North America, [Db3]~China,

Sub-Part [Db1] ~ Temperate Soils ~ Australia ~
Maps showing the extent of arid lands, saline soils and sodic soils are in Ref. (87H2).

Sub-Part [Db2] ~ Temperate Soils ~ North America ~

[Db2a] ~ Temperate Soils ~ North America ~ Canada ~
Some typical temperate soil characteristics: a sandy chernozemic prairie soil from the Canadian Great Plains originally contained about 8750 grams of Carbon/ m2, and loses 50% of its organic matter after 65 years of cultivation (94T1).

[Db2b] ~ Temperate Soils ~ North America ~ United States ~
A soil map of the conterminous US is found in Ref. (90P1). Southern US soils are mainly Ultisols (Ref.3, 5 of (85H1)) and are not very fertile because their clay mineral fraction is dominated by non-expanding amorphous clay silicates that do not retain nutrients. They are relatively low in organic matter, and are extremely susceptible to erosion (Ref. 28, 29 of (85H1)). Crude protein concentration in Kansas wheat in 1940, 1949 and 1951 is plotted (Fig. 139) on a map of Kansas. A marked decline in protein content is shown ((56A2), p. 671)

Organic Matter Contents and Clay Ratios of 13 Minnesota soils to provide a perspective on these two parameters and their variability (77Y1)
Soil- - - - - - - - - -Organic|Clay
- - - - - - - - - - - -|Matter|Ratio
Barnes loam~ ~ ~ ~ ~ ~ | 5.86%|0.30
Sverdrup loamy sand~ ~ | 1.73 |0.09
Hamerly loam ~ ~ ~ ~ ~ | 6.18 |0.35
Kranzburg clay loam~ ~ | 3.42 |0.58
Waukon sandy loam~ ~ ~ | 3.49 |0.19
Rothsay clay loam~ ~ ~ | 3.53 |0.41
Rockwood sandy loam~ ~ | 4.13 |0.18
Forman clay loam ~ ~ ~ | 3.76 |0.51
Nebish sandy loam~ ~ ~ | 2.20 |0.11
Clarion sandy clay loam| 4.28 |0.44
Sioux sandy loam ~ ~ ~ | 2.55 |0.17
Storden sandy clay loam| 3.60 |0.31
Flak sandy loam~ ~ ~ ~ | 1.36 |0.09

Comments: Carbon contents are typically 45-50% of organic matter contents.

Sub-Part [Db3] ~ Temperate Soils ~ China ~
Soil structure is generally poor; organic matter content is often less than 1% (3% is normal for soil in a temperate climate.) (82B2).

Soils in China are low in organic matter because crop residues are typically removed from fields and used for forage or fuel (95B2).

Part [Dc] ~ Tropical Soils ~ [Dc1]~General, [Dc2]~Lateritic Soils, [Dc3]~African Soils, [Dc4]~Vertisol Soils,

Sub-Part [Dc1] ~Tropical Soils ~ General ~
Except for irrigated systems or areas with relatively fertile young soils and/or lower rainfall, few rain forests have experienced large-scale intensive agriculture (
04V1).

Some argue that the soils of most tropical forests are suited only for long-fallow shifting cultivation (04V1) (B. Meggers, "Amazonia, Man and Nature in a Counterfeit Paradise", Smithsonian Institution, Washington DC, 1996). Others contend that many rain forests have been shaped by a long history of intensive cultivation (M. J. Heckenberger et al, Science 301 (2003) p.1710.) (04V1).

Poor soils dominate the tropical latitudes. The world's most fertile soils are limited to certain areas of the temperate zone. Most areas of tropical rain forest are underlain by soils classified as infertile (Ref. 17 of (94H4)).

Many tropical soils are low in inherent fertility because they formed from geological parent materials that were low in essential elements, or because they have lost most of their nutrients and became acidic as a result of warm temperatures that accelerate chemical or biological processes, and high rainfall that leaches nutrients out of the soil (Refs. 17 and 18 of (93H4)).

Geological stability and the lack of glaciations in much of the lowland tropics have reduced the input of fresh, mineral-rich substrates for soil formation. High-fertility soils in the tropics are generally limited to areas of active volcanism or alluvial sediments from young mountain ranges (Ref. 19, 20 of (93H4)).

Agricultural productivity based on inherent soil fertility is probably inadequate to finance the transition from an agrarian to an industrial society in many tropical countries. But such an agriculturally driven transition may not be necessary in an age of globalization of world trade (93H4).

Some typical tropical soil characteristics: a sandy ferralsol in semi-arid northeastern Brazil originally contains 3380 g.C/m2 and loses 40% of its organic carbon in 6 years of cultivation (94T1).

An extremely poor Amazonian soil has no potential for agriculture beyond the 3-year life-span of the forest litter mat once biological nutrient cycles are interrupted by slash-burning (94T1).

Factors that make tropical soils much less productive than soils in temperate regions are outlined in Ref. (76E2), pp. 136-139.

The soils underlying 95% of the remaining tropical forests are infertile and are degraded easily through erosion, laterization and other processes once the vegetative cover is removed ((90W1), p. 107, Ref. 30).

High base-status soils called aridisols occur in tropical deserts and occupy 14% of the tropics. When irrigated and with added nitrogen, they can be extremely productive (86B1), (75S1).

Tropical agriculture first developed in areas of high base-status soils (alfisols, vertisols, mollisols, certain entisols and inceptisols (Ref. 13). These cover 18% of tropical land area (Ref. 16 of (75S1), (86B1)). Tropical centers of population are in areas having these soils. Impacts of the green revolution are very much limited to areas with high base-status soils, particularly those which are irrigated. These soils have generally developed from alluvial sediments or volcanic ash rich in Ca, Mg, and K. High base-status soils are synonymous with high native soil fertility and relatively low cost of supplying additional nutrients (mainly nitrogen) ((86B1), referencing a study by Sanches and Buol of 1975).

A large group of tropic soils are of low base-status (oxisols, utisols, some inceptisols, and sandy entisols). They cover 51% of the tropics (86B1, Ref. 16 of (75S1)) in vast areas of the interior of South America, central Africa, and smaller areas of the hill country of Southeast Asia (86B1). They are highly leached, commonly deficient in bases, and often present aluminum-toxicity problems. Phosphorous deficiency is also hard to correct. No great centers of population are found in areas with this type of soil (75S1), (86B1). A sustainable crop-production system developed for low-base-status tropical soils requires lime, P, S, K, Mg, B, Mo, and Cu, and costs $87,500/ km2/ year (86B1).

A fourth group of tropical soils consists of shallow soil and dry sands. They occupy the remaining 17% of the tropical land mass and have virtually no agricultural potential (75S1), (86B1).

(Tropical Soils) In a study of soil organic carbon (SOC) pools in tropical soils in Belize, Central America, mean residence times of 5-8 years were found for labile pools, vs. 250-388 years for stable pools. In temperate soils, mean residence times has been found to be 15-72 years for labile SOC, and 850-3000 years for stable SOC (3 supporting references cited for each range). It is generally assumed that stable SOC is physically or chemically protected in a clay-organic matter complex, and probably affects only soil properties. Labile SOC is inferred to be the main contributor to nutrient cycling and to intrinsic sustainable soil productivity, and to be more dependent on soil management. The size of the labile SOC pool varied between 0.15 to 6.0% carbon over the 2 soil types and 3 management practices and varied significantly with management practice (sugarcane, orchard, forest). The size of the stable SOC pool varied between 2.6-6.5% carbon over the same 2 types of soils and 3 types of management, and did not vary with management practice - only soil type (96H1).

Sub-Part [Dc2] ~Tropical Soils ~ Lateritic Soils ~
Latosols, the reddish or yellowish-brown lateritic soils of the savannas and forests of tropical and subtropical regions, are the most severely weathered and leached soils in the world. They are low in P, potash, MgO, sulfates and N. The extreme form of latosol is Laterite, a soil high in iron oxide that hardens irreversibly when dried and exposed to air. Non-arable latosols cover 14 million km2. Yet latosols also include 10 million km2 of arable land. These must be extensively treated with various supplements, including lime, to reduce acidity and trace metals (76R1). The total area of the tropics where laterite may be found at, or close to, the soil surface is on the order of 7% (2% for tropical America, 5% for tropical Brazil, 7% for the tropical part of the Indian sub-continent, 15% of sub-Saharan West Africa). Laterite is iron-rich, humus-poor material that hardens on repeated wetting and drying (75S1).

Main Soil Constraints in the Amazon Basin ((82B1), Ref. 11) (la) (Areas are in millions of km2)
Constraint- - - - - - - - - |Area~ |% of
- - - - - - - - - - - - - - | ~ ~ ~|Basin)
P deficiency~ ~ ~ ~ ~ ~ ~ ~ | 4.36 |90%
Aluminum toxicity ~ ~ ~ ~ ~ | 3.15 |73
Low K reserves~ ~ ~ ~ ~ ~ ~ | 2.42 |56
Poor drainage/bad flooding~ | 1.16 |24
High P fixation ~ ~ ~ ~ ~ ~ | 0.77 |16
Low cation exchange capacity| 0.64 |15
High erodability~ ~ ~ ~ ~ ~ | 0.39 | 8
No major limitations~ ~ ~ ~ | 0.32 | 6
Steep slopes (over 30%) ~ ~ | 0.30 | 6
Laterite hazard ~ ~ ~ ~ ~ ~ | 0.21 | 4

Comments: Don't add up these areas: some overlap.

Sub-Part [Dc3] ~Tropical Soils ~ African Soils ~
African soils are, by nature extremely poor. They are very low in both organic matter and nutrients. Farmers in Sub-Saharan Africa once used fallow periods to replenish soil nutrients, but population pressures made this option unaffordable, so they have been essentially mining soil for some decades (
02F1).

An excellent discussion of African agriculture in general is contained in Ref. (87H1).

Soils and soil science of Africa are also discussed in Ref. (87L1).

Soil types, aridity, and precipitation in Africa are plotted on a map in Ref. (76R1). Humid areas suffer from high acidity, toxic levels of aluminum, and impervious layers of iron oxide just below the surface. Semi-arid areas have sandy soil which hold water poorly. African soils are low in clay and often susceptible to erosion and the formation of crusts that keep rain out (87H1).

Most of Africa's arable soils are coarse; clay content is so low that they cannot absorb and hold moisture, and therefore are highly susceptible to erosion ((90W1), p. 90, Ref. 21).

Sub-Part [Dc4] ~Tropical Soils ~ Vertisol Soils ~
New ways are being found to farm the world's 3 million km2 of black, sticky vertisol soils that occur mainly in India, Australia and the Sudan ((85A2), Ref. 11, p. 409).

Go to Top of this Review's Appendices (units, conversions, definitions)
Go to
Top of this Review's Reference List
Go to
Topsoil Loss-Causes, Effects, Implications (Table of Contents)
Go to
Home Page of this entire web site
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3

SECTION (3-E) ~ Desertification and Land Degradation Generally ~

Over-plowing and overgrazing are creating a huge dust bowl in northern and western China. The numerous dust storms originating in the region each year in late winter and early spring are now regularly recorded on satellite images. For instance, on March 20, 2010, a suffocating dust storm enveloped Beijing, prompting the city's weather bureau to warn that air quality was hazardous, urging people to stay inside or to cover their faces when outdoors. Beijing was not the only area affected. This particular dust storm engulfed scores of cities in five provinces, directly affecting over 250 million people (11B2).

Wang Tao, one of the world's leading desert scholars, reports that from 1950 to 1975 an average of 600 square miles of land in China's north and west turned to desert each year. By the turn of the century, nearly 1,400 square miles of land was going to desert annually (11B2).

By 2025, 67% of the arable land in Africa will disappear, along with 33% of Asia's and 20% of South America's (04H1). Comments: Much of this land being desertified is probably in semi-arid regions where productivities tend to be low. (su1)

Some 17.7% of the 937,000 km2 of Brazil's semi-arid region are desertified (research by the Federal University of Ceara.) (Brazil's semi-arid region occupies 9.3% of Brazil and is concentrated in northeastern Brazil (06B1).) (la)

The spread of deserts in China in recent years has been so rapid that as many as 20,000 villages have been abandoned (04D1).

Deforestation and soil erosion were factors in almost every civilization collapse studied by Jared Diamond in his book (04D1). (This data is also in the forest land degradation review.)

See a 2-page global map pointing out major soil degradation problems (physical and chemical) in numerous regions of the world in Science 304 (2004) pp. 1614-1615.

(NOTE: Global data is listed first here: Regional- and national data is at the end of this section.)

Land at Risk of Human-Induced Desertification (global) (in millions of km2) (la)

Low risk~ ~ ~ | 7.1
Moderate risk | 8.6
High risk ~ ~ |15.6
Very High risk|11.9

Total ~ ~ ~ ~ |49.2

Hari Eswaran, Paul Reich, Fred Beinroth, "Global Desertification Tension Zones", in D. E. Stott, R. H. Mohtar, G.C. Steinhardt, eds., Sustaining the Global farm (2001) pp. 24-28).

Table 12.4 ~ Global Assessment of Human-induced Soil Degradation (GLASOD) (2) (03N1) (91O1) (la)
Col.2: Total land affected (millions of km2)
Col.3: % of the region that is degraded to moderate
Col.4: % of the region that is degraded to strong and extreme degrees
Region - - - - | (2) |(3)|(4)
Africa ~ ~ ~ ~ | 4.94| 39| 26
Asia ~ ~ ~ ~ ~ | 7.47| 46| 15
Australasia~ ~ | 1.03| ~4| ~2
South America~ | 2.43| 47| 10
Central America| 0.63| 56| 41
Europe ~ ~ ~ ~ | 2.19| 66| ~6
North America~ | 0.96| 81| ~1
Totals ~ ~ ~ ~ |19.64| 46| 16

(2) GLASOD (91O1) defines four levels of degradation:

The most comprehensive global assessment is still the Global Assessment of Human-induced Soil Degradation (GLASOD) mapping exercise ((91O1), Table 12.4) (03N1).

The assessment of land degradation is greatly hindered by serious weaknesses in our knowledge of the current situation (99P1) (01B2). According to some analysts, land degradation is a major threat to food security, it has negated many of the productivity improvements of the past, and it is getting worse (95P1) (99U4) (01B3). Others believe that the seriousness of the situation has been overestimated at the global and local level (97C1) (99S1) (00L2) (01M2) (03N1).

The 1990 Global Assessment of Soil Degradation (GLASOD), based on a structured survey of regional experts, provides the only continental and global-scale estimates of soil degradation (91O1). The GLASOD study suggested that 19.7 million km2 had been degraded during 1945-90 ((99S1), p. 17) (00W1) (15% of ice-free land). To assess the extent and severity of soil degradation on agricultural lands in particular, PAGE researchers overlaid the GLASOD data on a map of agricultural land (land with over 30% agricultural use). This revealed that 65% of agricultural lands have some amount of soil degradation. 24% were classified as "moderately degraded" that, according to GLASOD, signifies that their agricultural productivity has been greatly reduced. An additional 40% of agricultural land fell into the GLASOD categories of "strongly degraded" (requiring major financial investments and engineering work to rehabilitate) or "very strongly degraded" (land that cannot be rehabilitated at all) (00W2). Among the most severely affected areas are South and Southeast Asia where populations are among the densest and agriculture the most extensive ((00W1), p. 62). Comments: "Agricultural" land here is probably defined to be cropland plus pasture land not including rangeland.

Degradation Scale vs. Productivity-Loss (PL) Scale as used by Dregne and Chou (Ref. 17 of (97C1))
Degradation |Cropland~ |Grazing Lands
Slight~ ~ ~ | 0-10% PL | 0-25% PL
Moderate~ ~ |10-25% PL |25-50% PL
Severe~ ~ ~ |25-50% PL |50-75% PL
Very Severe |over 50%PL|over 75%PL

Degradation Status of Global Land in Crops, Permanent Pasture and Woodlands based on work of Oldeman et al (Ref. 15 of Ref. (97C1)) and Dregne and Chou (Ref. 17 of (97C1)) (la)
- - - - - - - - - -|Area* |Lost
Total Land- - - - -|87.35 |Production
Not degraded ~ ~ ~ |67.70 | 0%
Lightly degraded ~ | 6.50 | 5%
Moderately degraded| 9.04 |18%
Strongly degraded~ | 4.11 |50%

* millions of km2
Comments: Obviously includes forest land.

From the estimates in the table above (97C1), one can calculate the weighted average loss for each land-use category: 10.9% for irrigated land, 12.9% for rain-fed cropland, and 43% for rangeland (97C1). Crosson (97C1) notes that these degradations must be attributed to at least several decades. (He uses 4 decades in his calculations.) Comments: Lost production and topsoil loss are not linearly related. Until the depth of the root-zone reaches sub-soil (typically at a topsoil depth of 6 inches), productivity losses per inch of topsoil loss are very small, typically reflecting preferential erosion of nutrients and organic matter.

Soil degradation has affected 67% of the world's agricultural lands in the last 5 decades. (UN Development Program, "World Resources 2000-2001: People and Ecosystems, The Fraying Web of Life", UNEP (3/10/00)).

The UNEP Global assessment of soil degradation survey reveals that over 3 billion acres (12 million km2) of land (11% of the earth's vegetated surface) have been seriously degraded since 1945. 2/3 of these degraded lands are in Asia and Africa (92M3). Destructive agricultural practices account for 28% of global land degradation. Over-grazing accounts for another 35%. (World Resources Institute Press Release of 3/24/92).

Nearly 40% of the world's agricultural land is seriously degraded, say scientists at the International Food Policy Research Institute (IFPRI). Evidence compiled by IFPRI suggests that soil degradation has already had significant impacts on the productivity of 16% of the globe's agricultural land (00L1).

Most of the world's 31 million km2 of grazing lands are overgrazed (82W1) and (81S1). Comments: More recent data give significantly larger figures for grazing lands area, and also consider pastures (typically hilly land in more humid climates).

As much as a third of the Earth's ice-free land is undergoing desertification (84U3).

Climatic data suggests that over 33% of the Earth's ice-free land area is desert or semi-desert. But soil- and vegetation data indicate that 43% of the Earth's ice-free land is desert or semi-desert. The 10% difference (9.1 million km2) is accounted for by the estimated extent of man-made deserts (81S1). Comments: Does "desert" and "semi-desert" refer to hyper-arid, arid and semi-arid lands? If the Earth's ice-free land area is about 131 million km2, then a 10% difference would amount to 13.1 million km2.

Estimate of the rate of growth of the world's deserts and wastelands

The 1977 UN Conference On Desertification reported that just fewer than 20% of the world's croplands are degrading at a rate that is intolerable over the long run. Productivity on this land is estimated by the UN report to have been reduced by an average of 25% (Ref. 40, p. 25 of (78B2) (81B3). A 1977 UNEP report indicates that, by 2000, soil erosion, salinization, waterlogging, and urban sprawl could render 6 million km2 of land useless (though 3 million km2 of grazing land and forest land could be converted to cropland) (77U2).

A UNEP survey estimates that 45 million km2 (35% of the world's ice-free land) are in various stages of desertification (88J1), (89J1). (la)

During the past 40 years, 30% of the world's total arable land area has had to be abandoned (Ref. 6 of (97P3)). Comments: Pierre Crosson (Resources For the Future) states that he can find no such statement in Ref. 6. This statement seems unlikely to be true since other estimates give abandonment rates of only a fraction of 1%/ year.

Some 100,000 km2 of the world's cropland is being eroded and abandoned each year throughout the world. (David Pimentel, "How Many Americans can the Earth Support?", Population Press, 4/99) Comments: This number also includes irrigated cropland salinization and urbanization of cropland.

Globally, about 100,000 km2/ year of arable land are abandoned, and the rate is increasing (Ref. 7 of (97P3)). Comments: This figure typically includes salinization of irrigated lands, urbanization and abandonment caused by erosion etc.

Desertification claims 60,000 km2/ year irreversibly, and 200,000 km2/ year reversibly (Ref. 8 of (88J1)).

About 60,000 km2/ year of land becomes so severely degraded that it loses its productive capacity and becomes wasteland (Ref. 11 of (91B2)).

At least 20% of the 40,000 km2 of Amazon ranch land has been abandoned as a result of soil degradation ((90W1), p. 43, Ref. 40). Comments: But it may only take a decade or two of fallow to restore nutrients to tropical soils.

Areas of Moderate- to Excessive Soil Degradation (in millions of km2) (la)
Region- - - - |Water|Wind | Chem. | Phys. |Urban | Total
- - - - - - - |Eros.|Eros.|Degrad.|Degrad.|Devel.|
Africa~ ~ ~ ~ |1.70 |0.98 | 0.36~ |0.17 ~ |? ~ ~ | 3.21
Asia~ ~ ~ ~ ~ |3.15 |0.90 | 0.41~ |0.06 ~ |? ~ ~ | 4.52
South America |0.77 |0.16 | 0.44~ |0.01 ~ |? ~ ~ | 1.38
N./Cent. Amer.|0.90 |0.37 | 0.07~ |0.05 ~ |? ~ ~ | 1.39
Europe~ ~ ~ ~ |0.93 |0.39 | 0.18~ |0.08 ~ |? ~ ~ | 1.58
Australasia ~ |0.03 |?~ ~ | 0.01~ |0.02 ~ |? ~ ~ | 0.06
Totals~ ~ ~ ~ |7.48 |2.80 | 1.47~ |0.39 ~ |4.745*|16.89

* Urban Development data from Ref. (00W1). A partial allocation among various regions is given in Table FG.5.
Source: Ref. (90O1) (See (92N1), Table 4)

Dry-Land Regions at Least Moderately Desertified (1) (85D2) (la)
Region- - - - - - - |Range|Rain-Fed|Irrig.|Total
- - - - - - - - - - |Land |Cropland| Land |Dry Land
Sudano-Sahelian ~ ~ | 90 %| 80 % ~ | 30 % |88%
Africa S. of Sahel~ | 80~ | 80 ~ ~ | 30 ~ |80
Mediterranean Africa| 85~ | 75 ~ ~ | 40 ~ |83
Western Asia~ ~ ~ ~ | 85~ | 85 ~ ~ | 40 ~ |82
Southern Asia ~ ~ ~ | 85~ | 70 ~ ~ | 35 ~ |70
Asiatic USSR~ ~ ~ ~ | 60~ | 30 ~ ~ | 25 ~ |55
China and Mongolia~ | 70~ | 60 ~ ~ | 30 ~ |69
Australia ~ ~ ~ ~ ~ | 22~ | 30 ~ ~ | 19 ~ |23
Mediterranean Europe| 30~ | 32 ~ ~ | 25 ~ |39
Latin America ~ ~ ~ | 72~ | 77 ~ ~ | 33 ~ |71
North America ~ ~ ~ | 42~ | 39 ~ ~ | 20 ~ |40

(1) Includes sub-humid land (50-75 cm. rainfall/ year)

Human-Induced Soil Degradation between 1945-90. Source: Oldeman et al. (1990) and WRI (1992) in (96N1))
Degradation is in units of millions of km2/ % of vegetated land. (la)
Region- - - - - | Light ~ ~ | ~moderate,|
- - - - - - - - | ~ ~ ~ ~ ~ | ~severe,~ |
- - - - - - - - | ~ ~ ~ ~ ~ | ~Extreme~ | TOTAL
Europe~ ~ ~ ~ ~ | 0.606/ 6.4| 1.583/16.7| 2.189/23.1
Africa~ ~ ~ ~ ~ | 1.736/ 7.8| 3.206/14.4| 4.942/22.1
Asia~ ~ ~ ~ ~ ~ | 2.945/ 7.8| 4.525/12.0| 7.470/19.8
Oceania ~ ~ ~ ~ | 0.966/12.3| 0.062/ 0.8| 1.029/13.1
North America ~ | 0.168/ 0.9| 0.787/ 4.4| 0.955/ 5.3
Cent. Amer./Mex.| 0.019/ 0.7| 0.609/24.1| 0.628/24.8
South America ~ | 1.048/ 6.0| 1.385/ 8.0| 2.434/14.0
World ~ ~ ~ ~ ~ | 7.490/ 6.5|12.154/10.5|19.644/ *

* 17% of 115.5

A global map showing degree of desertification of arid lands from Dregne's Book Cropland Degradation (1991 UN study) (97G1) (la): (Areas are in units of 1000 km2)
Cropland degraded (1945-90) (total) ~ ~ ~ ~ | 5520
Cropland "Strongly" or "Extremely" degraded*| ~860

- -(* Beyond restoration, or requiring major engineering work)

Cropland lost via degradation (1945-90) (annually) | 20
Cropland lost via degradation (current) (annually) | 50-100

Globally, 1.6 million km2 of hillside land were identified as "severely eroded" in 1989 (96G2).

If the most severe degradation (that which leads to abandonment) continues at its 1945-90 rate, 470,000 km2 will be lost by 2020 (Ref. 53 of (96G2)). (1945-90 losses were 860,000 km2 (Ref. 44 of Ref. (96G2)).) (Current cropland loss rate from degradation severe enough to pull land from production = 50-100,000 km2/ year (Ref. 44 of (96G2)).)

Share of agricultural land with degraded soil (1945-1990) (Ref. 44 of Ref. (96G2))
Australia~ ~ | 16% | N. America| 26% | Europe| 25%
Asia ~ ~ ~ ~ | 38% | S. America| 45% | Africa| 65%
Cent. America| 74% |

(A 1994 study of South Asia estimated that soil damage extends to 10% more area than the above UN study had indicated (Ref. 43 of (96G2)).) Comments: The meaning of "1945-1990" above is not clear.

Desertification of Arid Lands (1984) is shown in Fig. 1 of Ref. (85D2).

The UNEP GLASOD study concluded that, globally, 2.95 million km2 are so degraded that restoration to full productivity is beyond the normal means of a farmer, but could be restored with major investments. 9.1 million km2 are moderately degraded to the point that original biotic functions are partly destroyed, and agricultural productivity is greatly reduced (92N1) (la). Comments: Urbanized lands (about 4.75 million km2) should be added to this list.

A study by Oldeman et al (Ref. 15 of (97C1)) calculated that of the world's 87.35 million km2 of croplands, grasslands and forestlands: (la)

Comments: *** The 77% are probably forestland and non-grazed grasslands.

Soil degradation induced by human activity, globally, since 1945: 20 million km2 - 17% of the Earth's vegetated land (UNEP study). 38% are lightly degraded - full potential for recovery; 46% are moderately degraded -restorable only through considerable financial and technical investment; 15% are severely degraded - no agricultural utility under local management systems and reclaimable only with major international assistance; 0.5% are extremely degraded - incapable of supporting agriculture and irreclaimable (95D3). These degraded lands occupy 20% of Asia's vegetated lands, 22% of Africa's and 23% of Europe's. Direct causes: overgrazing (35%), deforestation (30%), other agricultural activities (28%), over-exploitation for fuel wood (7%), and bio-industrial activities (1%) (95D3). (la)

Status of Global Dry-land* Degradation as of 1983-84 (89P2) (la)
(Col. 2= Area at least moderately degraded (millions of km2))
(Col. 3= Total global area of that land category (millions of km2))
(Col. 4= Area deteriorating to zero net economic return (millions of km2/ year))
Land Category - - |Col.2|Col.3 | Col.4
Rangeland ~ ~ ~ ~ |31.0 |36.90 | 0.177 (0.48%/ year)
Rain fed Cropland | 3.35| 5.68 | 0.020 (0.35%/ year)
Irrigated Cropland| 0.40| 1.29 | 0.006 (0.47%/ year)
Totals~ ~ ~ ~ ~ ~ |34.75|43.87 | 0.203 (0.46%/ year)

* arid, semi-arid, sub-humid climatic zones
Comments: Arid lands tend to be more fragile than moister lands. Over-grazing is more widespread than soil erosion. (Most rangeland is arid land and is overgrazed; flat cropland usually shows negligible erosion.) Rangeland economics may be inherently more marginal than cropland economics. So conclusions comparing rangeland erosion to cropland erosion are hard to draw.

Agronomist H. Dregne's Classification of Cropland Erosion (90B2) (la)
SLIGHT -| (yield potential reduced by less than 10%)
MODERATE| (yield potential reduced 10-50%)
SEVERE -| (yield potential reduced by more than 50%)

Distribution of Cropland Among Dregne's Categories (90B2) (Figures are in percent) (la)
Continent |Slight|Moderate|Severe
Africa~ ~ | ~ 60 | ~ 23 ~ |17
Asia~ ~ ~ | ~ 56 | ~ 28 ~ |16
Australia | ~ 38 | ~ 55 ~ | 7
Europe~ ~ | ~ 69 | ~ 25 ~ | 6
N. America| ~ 70 | ~ 23 ~ | 7
S. America| ~ 73 | ~ 17 ~ |10

According to the secretariat of the UN Convention to Combat Desertification, if desertification is not arrested and reversed, 1/3 of Asia's arable land, 1/5 of South America's and 2/3 of Africa's will be lost (03E1).

There is no clear consensus as to the area of degraded land, even at the national level. In India, estimates by different public authorities vary from 53-239 million ha (97K1) (03N1).

Regional hot spots of land degradation (03N1) (96S4)
Southern and Western Asia
Salinization in the Indus, Tigris and Euphrates River basins
Erosion in the foothills of the Himalayas
East and Southeast Asia
Salinization in northeast Thailand and the North China Plain
Erosion in the non-terraced slopes of China and Southeast Asia
Africa
Salinization in the Nile Delta
Erosion in southeastern Nigeria, the Sahel, and mechanized farming areas of northern and western Africa
Latin America / Caribbean
Salinization in Northern Mexico and the Andean highlands
Erosion in the slopes of Central America, semi-arid Andean highlands, Andean Valley, and the cerrados of Brazil

A follow-up study for South Asia addressed some of the weaknesses of GLASOD (91O1). It introduced more information from national studies and greater detail on the different forms of degradation (94F1). The broad picture for South Asia remains similar in the two studies: 30-40% of agricultural land is degraded to some degree, and water erosion is the most widespread problem (Table 12.5) (03N1).

Central America: Almost 75% of cropland in Central America is seriously degraded (001L).

Asia: About 11% of cropland in Asia is seriously degraded (00L1).

China loses 1200 km2/ year of farm- and pastureland to drifting sand dunes (86W4).

Africa
About 20% of cropland in Africa is seriously degraded (
00L1).

Desert + cumulative desertified land in Africa = 7.42 million km2 (25% of Africa) (Ref. 38 of (88L1)). Others estimate that as much as 65% of Africa is prone to some degree of desertification (Ref. 6, 7, 69 of (88L1)).

Estimates based on the GLASOD (Global Assessment of Soil Degradation) approach indicate that, by 1992, nearly 3.2 million km2 of dry lands in sub-Saharan Africa had degraded soils, ranging from light and moderate (77%) to strong and extreme (23%). These estimates, however, represent a considerable (almost two-thirds) reduction in the area previously thought to be suffering desertification as a result of human activities (95M1).

National Environment Secretariat (NES) says 483,830 km2, 85% of Kenya's land area of 569,137 km2, is experiencing some form of desertification. NES says 19.3% of Kenya is severely affected, and 9.4% is moderately affected (91D2).

Go to Top of this Review's Appendices (units, conversions, definitions)
Go to
Top of this Review's Reference List
Go to
Topsoil Loss-Causes, Effects, Implications (Table of Contents)
Go to
Home Page of this entire web site
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3

SECTION (3-F) ~ Antiquities ~ [Fa]~Climatic Changes , [Fb]~"In the Beginning . . ." [Fc]~Salinization as a Soil Destroyer, [Fd]~Selected Civilizations, [Fe]~Some Lifetimes of Civilizations,

Ronald Wright has shown that previous civilizations collapsed be because they allowed the greed of a selfish short-sighted minorities to prevail over the interests of the majority. On page 87 of "A Brief History of Progress", Ronald Wright wrote: "The Athenians became alarmed by deforestation early in the sixth century BC. Greek city populations were growing quickly at the time, most of the timber was already cut, and the poor were farming the goat-stripped hills with disastrous results. Unlike the Sumerians, who may have been unaware of the destruction caused by their irrigation methods until it was too late, the Greeks understood what was happening and tried to do something. In 590 BC, the statesman Solon realizing that rural poverty and land alienation by powerful Athenian nobles lay behind much of the trouble, outlawed debt-serfdom and food exports; he also tried to ban farming on steep slopes. (Continued below)

A generation later, Pisastratus, another ruler of Athens, offered grants for olive planting, which would have been an effective reclamation measure, especially if combined with terracing. But as with such efforts in our day, funding was unequal to the task. Some 200 years later in his unfinished dialogue Criatus, Plato wrote an account of the damage, showing a sophisticated knowledge of the connection between water and woods. "What now remains compared to what then existed is like the skeleton of a sick man, all the fat and soft earth having wasted way "Mountains which have nothing but food for bees" had trees not very long ago. [The land]~was enriched by the yearly rains, which were not lost to it as now, by flowing from the bare land into the sea; but the soil was deep, and therein received the water, and kept in the loamy earth feeding springs and streams running everywhere. Now only abandoned shrines remain to show where the springs once flowed." (James Sinnamon (12/01/05). frodo000@bigpond.net.au http://www.candobetter.org/james)

With the appearances of grasses during the Cenozoic Era, the relations between climate, vegetation, erosion and runoff became much as today, except for the influence of man (Schumm, 1968, in (90M1)).

In 1911, Franklin King's "Farmers of Forty Centuries: Permanent Agriculture in China, Korea and Japan" documented how farmers in parts of eastern Asia worked fields for 4000 years (160 generations) without depleting soil fertility (90R3).

Horseback riding started in the Ukraine 6000 years ago. It spread east, then west in 3500-3000 BC. Army cavalries appeared in the Middle East about 1000 BC (91A1). Comments: Collapse of agriculture in arid central Asia (in several centuries BC?) is understood to have been hastened by marauding bands of horsemen who had probably abandoned their own degraded land.

Part [Fa] ~ Antiquities ~ Climatic Changes ~

The history of the climate of Europe and Asia are reviewed in Ref. (56V1). The implication is that climatic changes have been very minor over recent millennia. A vegetation map of Europe and Asia during the last ice age is given on p. 280 of Ref. (56V1). Climatic changes since then are summarized. Temperatures during the last ice age were 5-7oC cooler than today (56V1). The arguments that climate has not changed much in the past 5000 years, and that the decline of civilizations had nothing to do with the small climate changes that did occur, are advanced on pages 57-62 of Ref. (74C1).

Part [Fb] ~ Antiquities ~ In the Beginning ~

The origin of planting and some domestic animals (dogs, pigs, fowl) was in the moist tropics, on the riverbanks and coasts of southern Asia around the Bay of Bengal in the period 13000 to 9000 BC ((56V1), p. 283).

Domestication of sheep and goats, and cultivation of grain, began in the wooded steppe and steppe areas of northwest India and adjoining western Adriatic(??) Mountain region during the 6th millennium BC ((56V1), p. 283).

The ancient civilizations of the Near East (western Iran, northern Mesopotamia, Syria, Palestine, and Egypt) derived from a farming and herding culture. About 3000 BC, with the invention of writing, they were completely developed ((56V1), p.283).

A history of earliest agricultural civilizations and the breakdown of agriculture is given in Ref. (56V1) (p.291-299).

The fertility potential of soil was rarely endangered in classical antiquity (1100 BC-565 AD). Plows and other agricultural tools of the period were not strong enough to cause what we now call soil destruction (____).

Part [Fc] ~ Antiquities ~ Salinization as a Soil Destroyer ~

Wheat is less salt-tolerant than barley. Around 3500 BC, wheat and barley grew in nearly equal proportions in Mesopotamia (Iraq). In 2500 BC, wheat accounted for only 1/6 of grain crops. In 1700 BC, wheat cultivation ceased entirely in southern Iraq ((76E1), p. 116). Comments: Another document (like the one you are reading now) is available on irrigation-related issues. Salinization is discussed in greater detail there.

Wheat can tolerate soil salt level up to 0.5%; Barley can tolerate soil salt levels up to 1% (90P3).

Go to Top of this Review's Appendices (units, conversions, definitions)
Go to
Top of this Review's Reference List
Go to
Topsoil Loss-Causes, Effects, Implications (Table of Contents)
Go to
Home Page of this entire web site
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - se3