~ CHAPTER 3 (Continued) ~
~ SOIL- AND SOIL-EROSION BASICS ~
Edition 9 of March 2010

NOTE: SECTIONS (3-A) through (3-F-c) below are found in another file. (se3.html)

(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]~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 ~

The top of this Chapter 3 (See topics above) is in another file. (se3.html)

(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 Section (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 ~ [Jb1]~Erosion, [Jb2]~Herbicides and Pesticides,
~ (3-J-c) ~
Reduced Tillage Technology and Economics ~

(3-K) ~ Aluminum Toxicity and Other Fertility Constraints ~

(3-L) ~ Cropping Intensity / Multiple Cropping ~

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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.)

Part [Fd] ~ 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,

Sub-Part [Fd1] ~ Selected Civilizations ~ Mesopotamia (Iraq) ~
Deforestation and over-grazing in the Armenian highlands (eastern Turkey) is believed to be the source of the silt load that is believed to be the root cause of the decline of the Mesopotamian civilization and the irrigation system on which it depended ((74C1), p. 53). The coastline change at the mouth of the Tigris and Euphrates Rivers due to siltation between 700 BC and the 20th century AD is described in Ref. (56D1) (See map, Fig.101). The Persian Gulf has been shortened by 150 miles by the silt load of these rivers (56D1).

The Ahkadian (spelling?) empire (Persian Gulf to headwaters of Euphrates River in modern-day Turkey) collapsed around 2200 BC as a (probable) result of a 300-year drought which drove people out of northern cities in Mesopotamia (93W1). By 2300 BC, wheat was no longer grown in the Sumarian city-state of Agode (Mesopotamia). By 2100 BC Ur also had abandoned wheat production, and wheat represented just 2% of the crop in the Sumerian region. By 2000 BC Isin (spelling?) and Larsa (spelling?) no longer grew wheat, and by 1700 BC, salt levels in soil throughout southern Mesopotamia were so high that no wheat was grown (90P3). In Iraq, the shift from wheat to barley was accompanied by a serious decline in fertility (attributed to salinization). In Girsu, yields were 253.7 m3/ km2 in 2400 BC; 146. m3/ km2 by 2100 BC, and 89.7 m3/ km2 in 1700 BC (58J1).

In southern Iraq, about 3500 BC, wheat and barley were nearly equal in occurrence. In 2500 BC the less-salt-tolerant wheat accounted for 1/6 of the crop. By 2100 BC wheat accounted for 2% of the crop in the Girsu area. By 1700 BC, wheat had been abandoned completely in the southern part of the alluvium (58J1).

Three major occurrences of salinity have been established from ancient records. The earliest, and most serious, affected southern Iraq from 2400 BC to 1700 BC. A milder salinity problem occurred in central Iraq from 1300-900 BC. Finally, the Nahruan area east of Baghdad became salty only after 1200 AD (58J1).

With the converging effects of mounting maintenance requirements and the declining capacity for more than rudimentary maintenance tasks, the virtual desertion of the lower Diyala area of Mesopotamia was inevitable. By the middle of the 12th century AD, most of the Nahrwan region was already abandoned. Mongol horsemen under Hulaga Khan arrived a century later, (but are blamed for the devastation they found ever since) (58J1).

Sub-Part [Fd2] ~ Selected Civilizations ~ Jordan ~
Current spectacular erosion in Jordan is said to be an old problem (Ref.3 of (92D1)). Erosion-induced productivity losses were extensive 1000+ years ago, and certainly reduced yield significantly over large areas in the hills prior to 1900. Many references note rocky soils in places that were once cultivated (92D1).

In Asia Minor, the interiors of the old Roman provinces of Caria (sp?) and Phrygia were completely deforested by the first century AD (90P3).

Sub-Part [Fd3] ~ Selected Civilizations ~ Israel ~
Extensive cultivation began in Israel between 5000 and 3000 BC (Ref. 36 of (92D1)). Shortly before 0 BC, crop rotation and terracing of hill slopes were practiced widely. After the Muslim conquest in 640 AD, terraces were destroyed through neglect, causing catastrophic soil erosion. Much erosion occurred in this region during Roman times also (Ref.4 of (92D1)). Others believe catastrophic erosion of productive lands occurred after the decline of Roman power in Palestine and Syria in the fifth century (Ref.15 of (92D1)).

Sub-Part [Fd4] ~ Selected Civilizations ~ Europe ~
The Greenland Norse cut their grasslands for turf to build homes and watched the strong winds blow away the underlying soil, reducing the carrying capacity for cattle below what the settlers needed to survive (
04D1). The Norse disappeared from Greenland after about 400 -450 years of settlement. They refused to switch from beef to blubber like their Inuit neighbors (04D1).

Tilling soil in Central Europe dates back to at least Neolithic times. Yet in the course of more than 50 centuries and considerable historic turbulence, continuous productivity of the land has been maintained (04D1).

[Fd4a] ~ Selected Civilizations ~ Europe ~ Italy ~
Aerial photographs prove that whatever damage was done by Ostrogoths, Lombards and Byzantine generals to Italian agriculture in the 5th and 6th centuries AD during temporary war-like operations was repaired quickly (56H1). Spanish methods of sheep-breeding after 1300 AD, and not Roman or early medieval agriculture mismanagement, are the real causes of the present emptiness of many regions of southern Italy ((56H1), p. 171). The Italian coast from south of Ravenna, north and eastward almost to Trieste, has been extending into the Adriatic Sea for at least 20 centuries. Old coastal cities are now 6 miles inland ((56D1), p. 511).

The city of Adria (gave its name to the Adriatic Sea) is now 15 miles from the sea, and 15 ft. higher in elevation. It was an Etruscan town built in 550 BC on an island a few miles north of the mouth of the Po River. It was an important Roman seaport. By the end of the Western Roman Empire, Adria was on the mainland. It was an important port on the Adriatic Sea in the 12th century AD, but it was then several miles from the sea and joined to it by a canal ((74C1), p.153).

About 300 BC, Italy and Sicily were still well forested (90P3).

[Fd4b] ~ Selected Civilizations ~ Europe ~ Spain ~
Much of the land of eastern Spain was seriously exhausted during Roman times (500 BC-400 AD). Moors cultivated it intensively from the 8th to 13th centuries AD, so by the 15th century, much of it was severely eroded and exhausted. Thus the possibility of new land across the ocean appealed to Spain before it appealed to Europe that still had adequate good land (due to crop rotation and earlier elimination of the feudal system) ((74C1), p. 174).

Sub-Part [Fd5] ~ Selected Civilizations ~ China ~
Chinese civilization began around 2000 BC in North China, probably in the Loess-hills region of the Yellow River basin. It spread to the coastal plain of North China and, for 15 centuries, flourished there. Then it spread to the Yangtse Valley in central China and Manchuria around 500 BC. By then, the original Chinese civilization was disintegrating, and a dark age ensued. The new civilization shifted to the south, and reached the extreme southern part of China in medieval and modern times. China's best farms are now in the region south of the Yangtse River where civilized people have lived for the shortest time. This region is the most progressive and prosperous ((74C1), p.198). Comments: Note that this history is consistent with the premise that a progressive civilization can survive for only about 50 generations (12.5 centuries) in one place unless there is some mechanism for replenishing the soil.

China's Great Wall (2500 miles long, built to keep out marauding hoards from the north where agriculture had collapsed) (74H1) was built in 221-206 BC (Ch'in Dynasty).

Sub-Part [Fd6] ~ Selected Civilizations ~ India ~
Glover (Ref. 12 of (92D1)) claims that there was no erosion of the uplands in the Punjab region of India and Pakistan before the British pacified the region in the 1800s. (This action stopped wars and encouraged population increase (92D1).)

Sub-Part [Fd7] ~ Selected Civilizations ~ Sri Lanka ~
The central highlands suffered severe erosion with the introduction of tea and coffee plantations in the early 19th century (Ref. 27 of (92D1)). By 1873 erosion had become so severe that the government ruled that no land above 8000 ft. could be sold to private buyers (92D1).

Sub-Part [Fd8] ~ Selected Civilizations ~ Greece ~
Over-grazing began around 650 BC on the 80% of the land that was unsuitable for cultivation (90P3).

Sub-Part [Fd9] ~ Selected Civilizations ~ Turkey ~
Around 500-300 BC, a Greek writer referred to the erosion and ruin of once-fertile lands along the Turkish coast ((82S1), p. 7).

Sub-Part [Fd10] ~ Selected Civilizations ~ North Africa ~
In Roman times as many as 15,000,000 bushels of grain were transported annually from North Africa (where there is now only desert) and Egypt to Rome (56G2). North Africa had a large, prosperous agriculture during 200 BC-600 AD, as much evidence shows. Erosion is argued to be the reason for the decline (70L1). The land was probably already more-or-less ruined by erosion when Arabs conquered the area near the end of the 7th century AD ((70L1), p. 246). Presumptive evidence suggests erosion in the granary of Rome in the waning days of the empire, but the first real threat seems to have appeared in the 7th century, after the Arab conquest of present-day Tunisia, Algeria, and Morocco (Ref. 26 of (90D1)). By the 10th century the situation was said to have become bad. Deforestation was the principal contributor to erosion (90D1).

No more than 10% of the original forests that once stretched from Morocco to Afghanistan as late as 2000 BC still exist (90P3).

Sub-Part [Fd11] ~ Selected Civilizations ~ United States ~
Cotton, like tobacco, a clean-cultivated row crop, and a cash crop, caused southern US fields to surface-wash, especially in winter. The southern US uplands gradually lost their organic horizons, color and protection; gullies were noted even before the Civil War ((56S1), p. 64).

Jappatown MD was a bustling tobacco port in the 18th century. But as the land was cleared and farmed, topsoil eroded into the harbor. Today, two miles of dry land separate early 18th century mooring posts from navigable waters (69T1).

Sub-Part [Fd12] ~ Selected Civilizations ~ Mayans ~
The migrations and decay of the Mayan civilization is often attributed to the soil destruction instability that is inherent in the Ladang (shifting cultivation) system that requires a 15-25 year fallow period ((56G1) p. 338) (Copied to definition list).

Mayans in the Guatemala lowlands expanded continuously over 17 centuries (68 generations) beginning in 800 BC, and reaching a population of 5 million (doubling time: 408 years) by AD 900 when the population density became comparable to that of the most agriculturally intense societies of today. Within decades the population dropped to a tenth of its peak. Core samples from two lakes in the region implicated soil erosion. The area was almost totally deforested by AD 250 (82B4). The earliest Mayan settlements date from about 2500 BC. Within a few decades after 800 AD the whole Mayan society began to disintegrate. The peak population of the Mayan lowland jungle might have been near 5 million in an area that today supports only a few tens of thousands (90P3).

Sub-Part [Fd13] ~ Selected Civilizations ~ Oceania ~
Easter Island, off the coast of Chile, although drier than most of Polynesia, is not as dry as most of Australia, and has more fertile volcanic soils. Archaeological finds and ancient pollen show that when Polynesians arrived on Easter Island, it was covered with a lush subtropical forest of palm trees and giant sunflowers, inhabited by land birds and breeding sea birds (Wilson da Silva, "Long dry spells, outlook gloomy", New Scientist 10/12/96).

Easter Island sustained a population of 58 people/ km2. But the population stripped the forests bare and killed the native animals. By 1500, all that was left was grassland. "People turned to the largest protein source around-cannibalism. Easter Island society collapsed in an epidemic of warfare. When the first Europeans reached the island in 1722, two-thirds of its population had died (Ref. Citation above).

In the past few thousand years, 12 other dry land masses in the Pacific subject to the whim of El NiŅo have seen human societies collapse, in some cases leading to the complete disappearance of a people. Polynesian societies on wetter islands with more predictable climates, such as Tonga and Samoa, survived the changes wrought by humans." The main difference seems to be that societies in low-rainfall environments were the ones especially prone to collapse by destroying vegetation. Low rainfall means that vegetation regrows slowly, so it can easily happen that regrowth doesn't keep pace with cutting (Wilson da Silva, "Long dry spells, outlook gloomy", New Scientist 10/12/96).

Population of Easter Island in 1550: 7000. Population in 1850 after deforestation and collapse of natural and agricultural systems: 100 (Clive Pointing, A Green History of the Environment and the Collapse of Civilization, New York, Penguin Books, 1991).

Part [Fe] ~ Some Lifetimes of Civilizations ~
Historical records of the past 60 centuries show that civilized man, with few exceptions, was never able to continue as a progressive civilization in one locality for more than 30-80 generations (7.5-20 centuries). The 3 notable exceptions: the Nile valley (200 generations), Mesopotamia (160 generations), and the Indus Valley (175 generations) ((74C1), p. 7). Comments: Note that the three civilizations (Mesopotamian, Egypt, Indus) that had large river valleys that replenished the soil annually lasted over 150 generations. Civilizations that lacked this feature lasted only 50 or so generations. In these three long-lived civilizations, foreign conquerors, high taxes, etc. came and went. Only when the soil replenishment system failed did progressive civilization end.

Mesopotamia (Tigris-Euphrates River Valley) maintained a progressive civilization from about 4000 BC to the mid-13th century AD (74C1).

A progressive civilization existed in the Indus River Valley (Pakistan) from around 1500 BC until about 1200 AD (74C1). Most of the lower Indus Valley (Pakistan) is now relatively barren country (74C1).

The Indus civilization had its rise in 2600 BC and declined around 1900 BC (John Noble Wilford, NY Times - See Pittsburgh Post Gazette, 2/16/98).

A progressive civilization existed in the lower Nile River Valley for about 6000 years (240 generations) (74C1).

Minoans on Crete lasted 11 centuries as a progressive civilization (2500 BC to 1400 BC) (44 generations) and then declined and vanished in 2 centuries (74C1).

Phoenicians (Lebanon) lasted as a progressive civilization from around 2000 BC until 480 BC (61 generations) ((74C1), p. 74). Deforestation and scavenger goats (over-grazing) bought on most of the erosion that turned Phoenicia (Lebanon) into a well-rained-upon desert ((74C1), p. 72).

Greek civilization lasted as a progressive civilization from 1600 BC until 400 BC (12 centuries: 48 generations) (74C1).

The Roman Empire started around 500 BC and was powerful in 400 AD, but was gone by 476 AD (74C1).

The Syrians lasted as a somewhat progressive civilization from the 16th century BC until the 13th century AD (30 centuries) (It was rarely powerful; usually it was caught in cross-fire between major civilizations.) (74C1).

Ceylon's civilization disappeared around 1200 AD due to deforestation of upland forest that filled irrigation systems with silt ((74C1), p. 202).

The Anasazi Indians (S.W. USA) lasted as a progressive civilization for about 30 centuries (120 generations) until around 1200 AD ((87B2), 74C1?).

The Hohokam Indians (S.W. USA) lasted as a progressive civilization from several centuries BC to 1400-1500 AD (E. W. Haury, National Geographic (year?)).

Teotihuacan (NE of Mexico City) grew rapidly in population during 200-900 AD and then collapsed mysteriously (Ref. 10 of (87N2)). The second population cycle began around 1168 AD - the Tenochas (Aztecs). Hispanics arrived in 1519 and bought smallpox etc. that decimated the population (87N2).

Easter Island's civilization was established by Polynesians around 400 AD in a well forested environment. It was gone (and the island treeless) by the 18th century (50 generations) (87B2).

Marajo Island's advanced civilization collapsed due to soil destruction in a shifting-cultivation system ((56G1) (p. 341)).

Terraced mountain land in Yemen has been continuously cultivated for about 30 centuries. The rich soils were derived from volcanic lava (Ref. 7 of (87V1)).

The Sumerian civilization (in modern-day Iraq) lasted from 2400 BC to 1800 BC (Ref.1 of (96G2)).

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SECTION (3-G) ~ Effects of Soil Erosion and Monoculture on Cropland Yield ~ [Ga]~Erosion~Effects on Crop Yields, [Gb]~Monoculture~Effect on Crop Yields, [Gc]~Organic Matter~Effect on Crop Yields. ~

Soil degradation is clearly cutting into crop yields in parts of Africa, South Asia, and Latin America, according to Sara Scherr, an analyst for Forest Trends (04K1).

A study headed by Christoffel den Biggelaar at Appalachian State University in Boone North Carolina combined erosion rates, estimated by soil type and climate, with data from hundreds of field studies for various crops and concluded that global average potential yield losses were 0.3%/ year (04K1). (See Keith Wiebe, editor, "Land Quality, Agricultural Productivity, and Food Security: Biophysical Processes and Economic Choices at Local, Regional, and Global Levels" (Edward Elgar Publishing, 2003)) Comments: The figure 0.3%/ year would be typical for soil depths that exceed the depth of the root zone of plants. But this is deceptive. When soil depths fall below root-zone depths, yield loss rates become much larger.

Oldeman (98O1) estimates the global cumulative loss of cropland productivity at about 13%, but there are large regional differences. Africa and Central America may have suffered declines of 25 and 38% respectively since 1945. Asia and South America, on the other hand, may have lost only about 13%, while Europe and North America have lost only 8%. Ref. (99U4) does not accept Crosson's assessment and argues that land degradation is so bad that it has negated many of the gains in land productivity of recent decades. Support for this view comes from detailed analysis of resource degradation under intensive crop production systems in the Pakistan and Indian Punjab (01M1) (03N1).

The cumulative productivity loss from soil degradation over the past 50 years has been roughly estimated using GLASOD data, to be about 13% for croplands and 4% for pasture lands (98O1) Comments: Rangeland degradation is probably a lot more, since rangeland is more arid, generally, than pastureland, and therefore more susceptible to degradation from overgrazing etc.

Grain-producing regions in industrialized countries typically have deep, geologically "new" soils that can withstand considerable degradation without having yield affected. (00W1) Comments: Also note that productivity decreases very little with erosion until soil depths become less that the root zone. Thereafter productivity drops off rapidly as erosion proceeds. Data on this is given elsewhere in this document.

In developing countries agricultural productivity is estimated to have declined significantly on 16% of agricultural lands. The GLASOD study estimates that almost 74% of Central America's agricultural land (defined by GLASOD as cropland and planted pastures) is degraded, as is 65% of Africa's and 38% of Asia's ((99S1), p. 18). (la)

Detailed studies based on predictive models for Argentina, Uruguay and Kenya calculate agricultural yield reductions of 25-50% over the next 20 years. ((97M2), p. 39-40) Comments: This seems high, relative to rates in recent decades.

Part [Ga] ~ Erosion Effects on Cropland 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,

Erosion reduces water-availability (85F1), as well as nutrients to growing plants, and diminishes soil organic matter and soil biota (91L3).

Minimum soil depths for productive yields (European data) (87M4)
Sandy loam soil| 20 cm.
Clay loams ~ ~ | 15 cm.

Globally, about 10% of presently cultivated land, and 8% of rangeland has probably experienced a severe loss of productivity. Perhaps 65% of rain-fed croplands have sustained a moderate (10-50%) loss. The situation is considerably worse in developing countries (85D2).

Sub-Part [Ga1] ~ Analysis/ Validity of experimental procedures ~
Crosson (97C1) suggests that recent rates of land degradation and particularly soil erosion have had only a small impact on productivity, and argues that the average loss for cropland productivity since the mid-1950s was lower than 0.3%/ year (
03N1). Comments: This is because the soil is still deeper than the rooting zone of plants. Once this is no longer true, far more drastic rates of degradation are expected to take over. So Crosson's statement is extremely and dangerously misleading.

A paper ("Soil Erosion Effects on Soil Productivity: A Research Perspective", Journal of Soil and Water Conservation 36(2) pp. 82-90 prepared by the Nat. Soil Erosion-Soil Productivity Research Planning Committee, Science and Education Administration- Agricultural Research) says that damage to soil water-holding capacity is probably the most important mechanism by which soil erosion reduces productivity ((83C1), p. 26). Other mechanisms include nutrient-loss damage to soil structure and farm-management difficulties (83C1). Soil erosion depletes soil productivity, but the relationship between erosion and productivity is not well defined (Refs. 63, 67, 80, 83, 99, 100 of (81N1)). Effects of soil erosion on cropland productivity are discussed in depth in Ref. (89S1) and (89B3).

Some tests on simulated erosion effects are not comparable to actual erosion because "In most instances, instantaneous removal of topsoil would likely affect productivity more drastically than would removal by erosion". Note that removal of surface soil has sometimes increased productivity (expose better soil). Note that fertility deficiency can be corrected, but poor physical condition is very difficult to correct (85B6).

Some studies on the effects of erosion focus only on changes in soil depth, and neglect the importance of biodiversity, organic matter and the other complex of interdependent variables. These studies indicate crop-yield reductions of only 0.13-0.39%/ cm. of soil lost (Refs. 88 and 89 of (95P1)). A reduction in soil depth of 2.8 cm. reduces productivity by about 7% (95P1).

Sub-Part [Ga2] ~ Studies of Productivity Loss vs. erosion in the US ~
Of all known measurements of erosion's effects on soil productivity, about 60% have been made in the US, and 2/3 since the 1980s (90D1) (Ref. 54 of (92D1)).

Cumulative topsoil erosion has reduced the production potential of US croplands 10-15%. An earlier estimate (35%) by Bennett (Ref. 27 of (76P2)) is generally considered to be an over-estimate (76P2).

Erosion-productivity relations were evaluated on 3 Indiana soils from 1981-86 (6 years). (SCS criteria used for slight, moderate, and severe erosion phases). Corn yields reduced 15%, and soybeans 24%, on severely eroded sites compared to slightly eroded sites (89S5).

Compared erosion-productivity relations on four soils: Otero (eastern Colorado), Athena (eastern Washington), Cecil (Georgia Piedmont), and Marshall (eastern Iowa) (Used EPIC (no gully erosion) to do computations.). If these four soils eroded at 20 tons/ acre/ year (4500 tonnes/ km2/ year) for 100 years (losing 450,000 tonnes/ km2 of soil), Otero productivity would decline 35%, Athena 35%, Cecil 20%, and Marshall 20-25% (89B6). Comments: Typical topsoil inventory: 400,000 tonnes/ km2. Stripping away all topsoil would reduce topsoil productivity far more than the numbers given above.

Tables 5-1, -2, -3 of Ref. (80C1) give results of numerous studies on the effects of erosion on yield reductions for wheat, corn, and grain sorghum agriculture.

Crop yields drop significantly as topsoil depth drops below 6" (15 cm.) ((76P2), Table 2, p. 153). Comments: Decreasing yields mean: (1) reduced soil cover, and (2) increased pressure on the land to produce at previous levels. These two effects provide positive feedback, i.e. instability that leads ultimately to gullies, which themselves represent an additional positive feedback that increases soil erosion rates.

Some 26 papers on the effects of soil erosion on soil properties and crop yields are collected in Ref. (84U1).

Loess (wind-deposited soil, typically with low organic matter content) may be more than 100 meters thick, so losing several meters would have little effect on productivity (92D1). Comments: Loess soils are low in organic matter and are therefore very erosion-prone and low in productivity.

In Idaho (Kimberly) irrigation removed up to 28 cm. of topsoil at top of fields and deposited up to 28 cm. at the bottoms of fields. Yields decreased sharply at the tops of the fields where sub-soil mixed with remaining topsoil. Yields increased - but less sharply - at the bottoms of the fields as topsoil depth increased due to deposition of as much as 28 cm. to a total topsoil depth of 66 cm. (85C7).

In the Palouse River Basin, average wheat yields basin-wide are 22% less than what they would be if erosion had been controlled over the past 50 years (83F1) (83F2).

In the Pacific Northwest (Palouse?) a 1960 soil survey showed 15% of land had lost all original topsoil since first cultivated. Others had estimated 25% in that condition (85S1).

In the Pacific Northwest soil erosion had robbed wheat farmers of 1/3 of the increased productivity provided by technological improvements in past 50 years (said in 1981). In Whitman County, yields would be 80 bushels/ acre instead of the 55-60 now (Reference apparently lost - could be (85C8) -see below.).

Crop production over 1 million acres in Idaho is probably reduced by 10% over what it would be if there were no erosion (85C8).

Soil erosion has robbed Pacific Northwest wheat farmers of 1/3 of the increased productivity provided by technological improvements in the past 50 years (said in 1981). In Whitman County, yields would be 80 bu./ acre instead of the 55-60 now obtained (this is 33% less based on present yield of 60 bu./ acre, or 25% less based on 80 bushels/ acre yield). Soil survey in 1960 showed 15% of land lost all original topsoil since first being cultivated. Kaiser figured 25% in that condition in 1981 (85S2).

The loss of 650 tons of soil is equivalent to the loss of one acre of cropland productivity (650 tons = 4 acre-inches) ((79S1), p. 91). Comments: This data is highly dependent on how much soil is left after erosion occurs. If this depth is greater than the root-zone, productivity loss per ton of erosion sediments will be very small; otherwise it will be very large. Ignore the data as virtually meaningless.

At 1977 erosion rates, corn- and soybean yields on highly erodible land in the Corn Belt (43% of US cropland) will be 15-30% less in 50 years than they would be if erosion were controlled ((84U2), p. 54).

US crop yields would drop 0.5-1%/ decade from erosion at current rates (84U2). Comments: This statement almost has to refer to erosion on flat bottomland with deep soil.

In the humid eastern US, grain yields decline 30-40% and forage yields drop 20-30% when the "A" horizon erodes away (7 Refs., (81N1)). Because sub-soil contains fewer plant nutrients than topsoil, added fertilizer is needed. Although fertilizer can partially compensate for low crop yields on exposed sub-soil, production costs and susceptibility to drought increase (many refs. of (81N1)). Comments: Fertilizer cannot compensate for the moisture-holding capacity of topsoil, nor for the tilth and erosion resistance provided by organic matter in topsoil - three of the most vital properties of soil.

Wind erosion on sandy soil near Sparta Wisconsin prior to 1938 caused productivity loss in red pine. Original A horizon of 30+ cm. Eroded "A" horizon was about 5cm thick (87F1).

Corn yields were checked annually (1968-1984) for 4 erosion classes (slight, moderate, severe, depositional) in SW Iowa. Erosion classes are those of the Soil Survey manual. Differences in yields between slight and severe erosion were generally small and non-significant. Corn yields from well-managed deep loess soils were not affected by topsoil loss from natural erosion when studied on a field scale (85S3).

Soybean yields were checked at 40 farm sites near Watkinsville (Georgia Piedmont) with varying amounts of erosion (Soil Survey manual). 1982- and 1983 yields on severely eroded sites were 50% of those on slightly eroded sites (85W1).

In the Southern Piedmont most land is classed as severely eroded (Class IV) or moderately eroded (Class II). Difference: 15 cm. of soil eroded away. Average corn yield loss due to 15 cm. lost soil was 42% - same as yield difference found 28 years ago when yields compared then (79L3).

15 cm. of soil lost by erosion caused about 40% reduction in corn yield (Southern Piedmont). The same 40% loss was noted 28 years ago when total yield/ acre was about half of what it was in 1979 (79L2).

Similar test on Grenada silt loam in west Tennessee where soils with varying degrees of erosion compared for crop yields. In Tennessee on moderately- to severely eroded soil, yield reductions were (79L2): fescue 25%; wheat 28%; cotton 33%; corn 42%; soybeans 50%.

Sub-Part [Ga3] ~ Studies of Productivity Loss vs. erosion outside the US ~
(Africa) Crop yield reductions in Africa during 1970-1990 due to water erosion alone are estimated to be 8% ((95L3), p. 666).

(Australia) In Australia (New South Wales) soil erosion of 3 cm. reduced yields 39% in 1980 and 11% in 1981 (90S4). Total dryland cropping area: 121,000 km2 (90S4).

(Australia) In Australia's wheat-growing areas, (control yield of 150-220 tonnes/ km2/ year.) loss of 75 mm. of soil caused yield declined 6%-46%. Loss of 150 mm soil caused yield declined 19%-52% (87B3).

(Canada) In Canada's (3) Prairie Provinces 362,000 km2 of croplands are being used (88P4). Prairie Farm Rehabilitation Administration estimated in 1983 that a 65-year production period in the prairies would have experienced sufficient wind/ water erosion to reduce long-term crop productivity by 13.7%. Another source says Prairie Farm Rehabilitation Administration estimated 8% yield loss was due to wind erosion, and 5.7% to water erosion. This may be fertility (OM?) loss (88P4).

(Ethiopia) Debre Berhan, 100 km north of Addis Ababa: In old days, grain yields were up to 100-150 tonnes/ km2/ year. Current yields: 20 tonnes/ km2/ year. Reduction is attributed to erosion (87M3). Comments: The reduction might also be due to nutrient mining, due to lack of fertilizer.

(Hungary) On badly eroded fields in Hungary, crop yields are 20-50% less than on flat fields. On degraded (moderately?) fields, yields are 10-30% less. On less eroded soils yields are 0-20% less (92S2).

(Nepal) Crop yields in Nepal have declined 22-30% due largely to erosion (91N2).

(Thailand) Erosion in Thailand is blamed for an average yield decline of 50% for corn and upland rice (Ref. 2 of (92D1).

(Philippines) Some Zamboanga del Sur farmers claim corn yields have dropped 80% in the previous 15 years due to erosion (92D1).

(Ukraine) In Southeastern Ukraine Grain crop (barley, wheat) yield is 40-50% of that on non-eroded soils. At Voroshilovgrad, barley yields on severely eroded land is 57% of that on non-eroded (75D1).

Effects of Topsoil Depth on Corn Yields (76P2) (Yields in bushels/ acre)
Depth - - - - | 0-2" |2-4" |4-6" |6-8" |8-10" |10-12"
Yield Range ~ | 25-56|28-69|39-83|49-97|50-102|50-125
Yield Average | 36.2 |47.0 |56.3 |64.7 |69.0~ |74.3

Effects of Artificial Soil Removal on Winter Wheat Yields (bushels/ acre) in a New South Wales Australia study (57B1)
Removal|Yield|Removal|Yield|Removal|Yield
inches | ~ ~ |inches | ~ ~ |inches | ~ ~ ~
0~ ~ ~ |24.7 | ~3~ ~ |16.0 | 6 ~ ~ |10.0
0~ ~ ~ |20.1 | ~3~ ~ |14.7 | 6 ~ ~ | 9.3
0~ ~ ~ |19.6 | ~3~ ~ |13.4 | 6 ~ ~ | 8.4

Same Experiment repeated at 3 other Research Stations in same year (1955). Results (winter wheat yields in bushels/ acre) (57B1)
Station - -| Soil removed
- - - - - -| 0 in.|3in.|6in.
Gunnedah ~ | 21.5 |14.7| 9.2
Wellington | 18.5 |15.1|13.6
Cowra~ ~ ~ | 10.8 |10.8| 5.0
Wagga~ ~ ~ | 24.2*|22.5|16.2 (* = crop lodged)
Comments: Not everyone agrees that artificial soil removal is a good proxy for actual soil erosion.

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Part [Gb]~ Monoculture's Effects on Cropland Yield ~
Nearly 50% of U.S. corn land is grown continuously as a monoculture (
Mario Giampietro, David Pimentel, "The Tightening Conflict: Population, Energy Use, and the Ecology of Agriculture", http://www.dieoff.com/page55.htm (1994)).

When monoculture is practiced, yields tend to decrease with time regardless of how much fertilizer is applied. A steady annual presence of a particular root system favors a few organisms - bacteria, fungi, nematodes -that are potagenic (spelling?) to plant roots. Changing to a different crop alters the circumstances, and all but the most unspecialized pathogens are unable to thrive in the absence of their usual host (90A1).

Many US farmers have returned to crop rotations, but monoculture is still the norm for 40% of US corn (Ref. 49 of (96G1)).

Background pros and cons related to monoculture are in Ref. (87P1).

Fig. 133 of Ref. (56H2) shows a plot of soil nitrogen vs. time during 50 years of continuous cropping, with and without manure, for timothy, oats, wheat, and corn. Bad effects of monoculture (no crop rotation) are clear. Also see Fig.135 p. 665.

Part [Gc] ~ Organic Matter's Effect on Yield ~
An increase of 1 ton (tonne?) of Soil Organic Carbon (SOC) increases wheat grain yield by 27 kg./ ha in North Dakota (
Ref. 29 of (04L1)) and by 40 kg./ ha in semi-arid pampas of Argentina (Ref. 31 of (04L1)), 6 kg./ ha of wheat and 3 kg./ha of maize in alluvial soils of northern India (Ref. 32 of (04L1)), 17 kg./ ha of maize in Thailand (Ref. 33 of (04L1)) and 10 kg./ ha of maize and 1 kg./ ha of cowpeas in western Nigeria (Ref. 34 of (04L1)).

Within 50 years of the introduction of moldboard plowing on the US Great Plains in the 19th century, 60% of the soil's organic matter was washed or blown away (96G1).

Soil degradation in developing countries is growing worse owing to increased burning of crop residues and dung for fuel. This reduces soil nutrients (94M1), (90D4) and intensifies soil erosion.

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

Burning of Agricultural Waste (Gt./ year (dry matter)) (91A2)
Tropical America |0.20 |USA and Canada ~ |0.25
Tropical Africa~ |0.16 |Western Europe ~ |0.17
Tropical Asia~ ~ |0.99 |USSR / E. Europe |0.23
Tropical Oceania |0.017|World Total~ ~ ~ |2.02
Assumptions: 80% of waste is burned in developing countries and 50% of waste is burned in developed countries, with a combustion efficiency of 90%. Comments: Presumably the remainder is left on the soil to enhance the organic content of soil.

Seiler and Crutzen (1980) estimated that agricultural waste is burned at 1.7-2.1 Gt./ year (91A2). Comments: Presumably the remainder is left on the soil to enhance the organic content of soil.

Carbon losses from soils due to erosion are much less than losses due to oxidation (Ref. 4 of (93H5)).

Soils high in organic matter (OM) content have significantly higher available water capacity (AWC). E.g., the AWC of a silt-loam with 4% OM (by weight) (= 15% by volume) was over twice that of a silt-loam containing 1% OM by weight (94H3).

Reduction in soil organic matter content from 4.3% to 1.7% lowers yield potential for corn by 25% in Michigan (Ref. 71 of (95P1)).

Organic matter plays four important roles in soils ((66K1), p. 228):

Organic Matter in the Upper 50 cm. of Topsoil Relates Closely to Corn Yield (Ref. 23 of (83S1)), i.e.:
Organic Matter (tonnes/ ha) = | 40| 50| 70| 90|110
Corn Yield (tonnes/ ha/year)= |1.6|2.6|3.0|3.4|4.0

During 1948-84 organic matter content of 10 sites in western Kansas declined by an average of 19% (86L1). Organic matter levels are now 50-60% of original levels in prairie soils of Canada, and 60-70% of original levels in croplands under hay rotations in central and eastern Canada (Refs. 5, 7 of (86D1)).

Fulvic acid (an organic constituent of soil) has a positive effect on root-initiation, which appears to be related to its metal-complexing ability (69S1).

Humic acids (one of several forms of organic material in soils) may decompose silicates, forming silic acid; Crenic and apocrenic (fulvic) acids (other forms of organic mater in soils) destroy silicates. Solutions of humic- and fulvic acids decompose various minerals. Utilization of P2O5 by plants was increased in the presence of humic acids (p.189 of (66K1)). Low molecular-weight organic acids in the soil (formic, acetic, propionic, aspartic, etc.) promote plant growth. Poor growth occurred in soils in which these compounds were absent (p. 212 of (66K1)).

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SECTION (3-H) ~ Causes and Dynamics of Soil Erosion ~ [Ha]~Anthropogenic Effects on Soil Erosion in General, [Hb]~Soil Erosion on Non-Croplands, [Hc]~Slope Dependence of Erosion, [Hd]~Crop Dependence of Erosion, [He]~Soil Cover Dependence of Erosion, [Hf]~Positive Feedbacks, [Hg]~Wind Erosion, [Hh]~Gully Erosion, [Hi]~Mulching and Organic Matter Influence on Erosion, ~

Table 12.5 ~ Shares of agricultural land in South Asia affected by different forms of degradation (03N1)
Type of Degradation - -| % of Agricultural Land Affected **
Water erosion~ ~ ~ ~ ~ | 25
Wind erosion ~ ~ ~ ~ ~ | 18
Soil fertility decline | 13
Salinization ~ ~ ~ ~ ~ | ~9
Falling water tables ~ | ~6
Waterlogging ~ ~ ~ ~ ~ | ~2
** (p.50-51 of (94F1))

Part [Ha] ~ Anthropogenic Causes of Soil Erosion in General ~

There are widespread efforts to restore saline and other degraded soils that have gone out of production. In the view of some analysts, restored lands could total 2-3 million km2 by 2025 (99G1) (03N1).

Cropland productivity losses arise from soil erosion, salinization, waterlogging, urbanization, nutrient depletion, over-cultivation, acidification, and soil compaction. These factors decrease soil productivity and substantially reduce crop yields (85M2), (85F1), (90P5), and will reduce crop productivity for the long term (91T2).

Soil erosion is the single most serious cause of degradation of arable land (91E1), (91B4), (93P1).

In Third World, problems of making soil conservation effective are much worse than in the US. Steep land is used more frequently for cultivated crops. Pressure is growing to expand production on to even more marginal lands. Land use rights of villagers so deeply ingrained in some places that change is impossible. Peasants cannot afford to take risks (83H3).

A study by Douglas (67D1) of soil erosion in small catchments in eastern Australia (which had few effects of Man), compared to a 1960 study by Fournier (60F1) of a large, world-wide data-base of small-catchments erosion data, indicate that anthropogenic effects increase soil erosion by about two orders of magnitude.

Clay soils and soils low in organic matter are particularly prone to compaction. Compaction increases risk of erosion, and can reduce crop yields by up to 60% (Ref. 8 of (86D1)).

Sediment Yield (tonnes/ km2/ year) vs. Fraction of the Drainage Basin in Croplands as Measured on Some Drainage Basins of area 10-1300 km2 (Data scatter: factor of two) (Wark and Keller, 1963, in (90M1))
% Croplands - | 10| 20| 30| 40| ~50| ~60
Sediment Yield| 10| 30| 60| 90| 120| 140

Sediment yields from construction sites in Baltimore-Washington DC area are 2000-50,000 tonnes/ km2/ year (Wolman and Schick, 1967, in (90M1)). Extra sediment produced by urbanization near Washington was sufficient to double the discharge of suspended sediment by the Potomac River to its estuary (Guy, 1965, in (90M1)).

Sediment Yield (tonnes/ km2/ year) vs. land-use and Precipitation Rate (100 and 175 cm./ year) on small Drainage Basins (0.04 - 0.12 km2) (Ursic and Dendy, 1965, in (90M1))
Land Use- - - - |Sediment Yield
~ - - - - - - - |100 cm.|175 cm.
~ - - - - - - - |/ year |/ year
Woodlands ~ ~ ~ | ~ 2 ~ | ~ 5
Abandoned Fields| ~ 9 ~ | ~80
Pasture ~ ~ ~ ~ | 250 ~ | 500
Croplands ~ ~ ~ |2000 ~ |9000

Part [Hb] ~ Soil Erosion on non-Croplands ~
Soil erosion rates on off-road-vehicle (ORV)-used desert hill slopes are commonly 10-100 times natural erosion rates (Ref. 13 of (81I1)). About 100 years are needed for desert soils to recover from off-road vehicle use (Ref. 17 of Ref. (81I1)).

The 2,484,000 km2 of undisturbed forestland in the contiguous US contribute over 100 million tons/ year of sediment to US waterways (36.6 tonnes/ km2/ year). Non-point pollution from undisturbed forestland is 0.001-0.009 tons/ acre/ year (0.2-2.3 tonnes/ km2/ year) in scattered producing areas up to 740 tonnes/ km2/ year on isolated forests in southern and Pacific-Coast areas (81F1). Comments: High forested-land erosion rates along the Pacific Coast are often attributed to steep, unstable slopes resulting from volcanic activity in recent geologic times.

As a result of catastrophic runoff events, lands at the margin of arid- and semi-arid regions typically show the highest rates of mechanical weathering (Ref. 86 of (90S3)), and high concentrations of suspended solids in rivers (90S3).

Part [Hc] ~ Slope Dependence of Erosion ~
In Southeast Asia, land pressure caused by increasing population has extended the use of steep hill slopes particularly for maize production. This has led to a significant increase in erosion on lands with slopes of over 20% (91H5) (
03N1).

Globally, 1.6 million km2 of hillside land were identified in 1989 as "severely eroded" (96G1).

In Nigeria, cassava on 12% slopes lose 22,100 tonnes/ km2/ year, compared to 300 tonnes/ km2/ year on slopes of under 1% (Ref. 38 of (95P1)).

Southern Ontario-continuous corn: erosion = 1200 tonnes/ km2/ year (0 slope); 4900 tonnes/ km2/ year (10% slope) (Corn-hay rotations cut this to 700 tonnes/ km2/ year (84S2).

Soil loss/ unit-area is plotted against land slope in Fig.2 of (57S1), based on 4 sets of data and studies. Data on four Midwest US soils can be summarized by a table of erosion rate factors normalized to 1.00 at 3% land slope: (57S1)

Slope(%)| 0~ | 1~ | 2~ | 3~ | 5~ | 6~ | 8~ | 9
Factor~ |0.25|0.45|0.70|1.00|1.75|2.19|3.04|3.85
Slope(%)|10~ | 12 | 14 | 16 | 18 | 20
Factor~ |4.49|5.94|7.59|9.44|11.5|13.7

About 70% of US croplands in 1977 had slopes of 6% or less; 90% had slopes of 12% or less. (See Ken Cook, Journal of Soil and Water Conservation, 37 (1982) p. 92) (RCA appraisal).

Part [Hd] ~ Crop Dependence of Erosion ~
The average soil loss from 4 years of corn following 2 years of clover and grasses was 47% of the loss from 6 years of continuous corn (p. 89 of (71R1)). Comments: Erosion from clover- and grasslands is negligible relative to that from cornfields. But the effects of crop rotation are much greater than that computed by merely averaging soil erosion rates as can be seen from the figures above (and much other data). This suggests that crop rotation affects soil chemistry and soil structure in such a way as to reduce soil erosion.

Part [He] ~ Soil-Cover Dependence of Erosion ~
Energetics of erosion by rain drops (87B1) (Erosion is in units of tonnes/ km2)
Erosion| Environmental Condition
120~ ~ |(no soil-cover and no tree-cover (5 cm. rainfall)
80 ~ ~ |(no soil-cover, with tree-canopy (5 cm. rainfall)
4~ ~ ~ |(soil+ plant litter+ tree-canopy (5 cm. rainfall)
3~ ~ ~ |(soil + plant litter, no canopy (5 cm. rainfall)
0~ ~ ~ |(soil + plant litter and under-growth *#
*# with or without canopy

A primary reason why croplands erode so much faster than other lands is that the amount of vegetative cover on croplands is significantly less than the amount on other lands. The phytomass of cultivated lands = 4.5 Gt. carbon in 1981 (3.3 Gt. carbon more than in 1860). The phytomass of the former natural vegetation which was replaced would have been 137+3.3 = 140.3 Gt. carbon (87E1).

Land in fallow is particularly vulnerable to soil erosion and salinization. Introduced in the early 1900s in Canada's prairie provinces, the excessive tillage associated with summer fallow has contributed substantially to reduced organic-matter levels, reduced soil-tilth, nitrogen loss, and less-efficient crop-use of available water (Ref. 7 of (87M1)).

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Part [Hf] ~ Positive Feedbacks (Instabilities) ~
A USDA position paper (1981) is cited as supporting the claim that, where erosion is by water, erosion rates accelerate with increasing erosion. This is attributed to erosion reducing water-infiltration which causes more runoff and hence more erosion. Also, erosion reduces plant-cover and plant-residue cover, and this promotes higher erosion rates (p. 27 of (83C1)). Comments: Storm severity is known to have a very large effect on soil erosion, because after soil becomes water-saturated, water runoff increases dramatically. Note that the effects discussed in (83C1) are positive feedback effects.

Difficulties in detecting soil productivity losses are compounded by the non-linear nature of erosion processes. Erosion increases runoff because of reduced infiltration. Increased runoff reduces available soil water and thus reduces plant growth. Reduced plant growth reduces plant-residue that increases erosion. The process thus advances exponentially, and reversing it may quickly become economically impossible (81N1).

Part [Hg]~ Wind Erosion ~
World-wide soil degradation mechanisms: water 56%; wind 28%; chemical degradation 12, physical degradation 4% (from L. R. Oldeman et al, 1990, World Map of the Status of Human-Induced Soil Degradation, An Explanatory Note., revised 2nd Edition, International Soil Reference and Information Center, Wageningen, The Netherlands).

Small particles carried down-wind in suspension account for only 10% (by weight) of wind erosion (89P1). Salinization (lifting and dropping repeatedly) accounts for 50-80% of total wind erosion (Distances moved are under one mile.) (89P1). Surface creep (rolling) represents 7-25% of total wind-eroded soil (89P1).

Most of the soil picked up by wind in one place must come down in another place much like the place of origin (83C1). Comments: or in woodlands or well-vegetated areas that are much better at retaining the deposited dust than a bare, wind-blown field.

Cropland expansion into climatically marginal areas is occurring in the US, Australia, the former Soviet Union, Sahel, Ethiopia, India and elsewhere. As a result, wind erosion has become a greater problem in recent years, even in the absence of droughts (85D2). Comments: Wind erosion predominates on arid lands. These are typically lands that would not be used as cropland unless there was some pressure to do so, e.g. the lack of moister lands.

Comparison of Wind- to Water-Erosion by comparing Areas of Moderate- to Excessive Soil Degradation.
Sources: (90O1) (Table 4 of (92N1)) (Areas are in units of millions of km2)
Region- - - - | Water |Wind ~ |Chemical|Physical|Total
- - - - - - - |Erosion|Erosion|Degrad. |Degrad. |~.
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~ ~ |12.14
*# Probably a reflection of a smaller inventory of arid land.

** (= 37% of 7.48)
US wind erosion (????) = 1.00 Gt. (76P2)
US wind erosion (1982) = 1.80 Gt. (89P1) (1.1 cropland; 0.54 from grazing land)
US wind erosion (1977) = 1.33 Gt. (81B1) (2/3 croplands; 1/3 from grazing land)
US erosion is 3/4 water-erosion, 1/4 wind erosion (76P2)

US rangeland wind erosion (1982 NRI study) = 1.5 tons/ acre (84L1)
Canada's prairie region erosion is 2/3 wind-erosion, 1/3 water erosion (86D1).
Comments: Also see Section (4-D) for wind-erosion data

China had 5 major storms during the 1950s; 23 in the 1990s and 20 during 2001-2002 (03U1) (Chinese Meteorological Agency data).

Part [Hh] ~ Gully Erosion ~
Landslides have claimed 800-1000 lives in each of the last 20 years. The rise in landslide deaths and injuries will be acute in developing countries where pressure on land resources makes slope cultivation more common (
06U1).

Asia suffered 220 major landslides between 1903-2002 (06U1).

Over 25,000 people have died in landslides in the Americas during 1903-2002 (06U1).

Landslides in Europe caused an average of US$23 million (euro19 million) in damage per landslide (06U1). Comments: Severe soil erosion reduces the vegetation and the roots thereof. This is frequently a cause of landslides.

A good photo of cropland gully-erosion in Tanzania is seen in (94P3).

Sheet erosion is more important, on average, as a sediment source than is gullying (p. 644 of (56L1)). Comments: Gully erosion causes extremely high soil loss rates/ acre. However, very few farmlands are in a state of being damaged by gullies since abandonment often follows soon after gullies render a field non-plowable.

Typical gullied land erosion: 41,000 tonnes/ km2/ year (76E1) (37,000 = 1"/ year)

Part [Hi] ~ Mulching Dependence of Erosion ~
International Inst. of Tropical Agriculture (Ibadan Nigeria) has shown that applying a crop-residue mulch at 540 tonnes/ km2 can control erosion nearly completely on slopes up to 15 degrees (Ref. 51 of (89P2)). In field trials, yields increased over non-mulched plots by 83% for cowpeas, 73% for cassavas, and 23% for maize (89P5).

When corn-residue cover is increased by 10, 30 and 50%, the amount of nitrogen lost in surface runoff is reduced by 68, 90 and 99% (Ref. 99 of (95P1)).

About 60% of crop residues in China are removed and burned for fuel (Ref. 41 of (95P1)). (90% In Bangladesh (95P1)).

In West Africa, on a 10% slope under maize, mulch reduces runoff from 42% of rainfall to 6% - and reduces erosion from 2700 tonnes/ km2/ year to under 50 (Ref. 7 of (87E2)).

Manure reduces soil erosion. 16 tons/ acre onto corn land in Iowa (9% slope) reduces soil erosion from 22.1 tons/ acre to 4.7 tons/ acre (Ref. 53 of (76P2)). Other organic matter would have a similar effect (Ref. 54 of (76P2)).

SECTION (3-I) ~ Off-Farm Impacts of Soil Erosion ~
Some off-farm impacts of soil erosion:

A model developed at RFF (Peshkin and Gianessi) shows that only 40% of the 1.9 billion tons of US sheet/rill erosion in 1977 ended up in waterways (p. 37 of (83C1)). Comments: The remainder presumably winds up on lower slopes and flood plains of the land. (Statement copied to Section (4-G-b)).

Off-site costs of soil erosion are reviewed in Ref. (85C3). Off-farm economic impacts of soil erosion are estimated to be $6 billion/ year in 1980 dollars (85C2). Also see The Economics of Soil Erosion: A Handbook for Calculating the Cost of Off-Site Damage, 135 pp., illustrated, bibliography, (1986) available from AFT01.

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SECTION (3-J) ~ Reduced Tillage Cropland Management ~ [Ja]~Extent of Reduced Tillage, [Jb]~Effects of Reduced Tillage, [Jc]~Reduced Tillage Technology and Economics, ~

Part [Ja] ~ Extent of Reduced Tillage ~
Brazil is estimated to have 150,000 of its 480,000 km2 of its arable land under conservation (reduced) tillage (99B1).

About 25,000 km2 of India's farmlands have been converted to zero-till agriculture (06M1).

Zero-till farming is practiced on 900,000 km2 worldwide, over half of which are in the US and Brazil (05M1).

Reduced tillage is now used on 41% of US cropland (05M1).

In 1998 16% of US cropland was no-till, according to the Conservation Technology Information Center. In 2004, preliminary data shows that number has increased to 22.5% (Pittsburgh Post Gazette (11/08/04)).

Low-till Agriculture Growth in South Asia: 30 km2 in 1998-99, 1000 km2 in 2000-2001 and may rise to 40,000 km2 by 2004. The technique cuts herbicide usage by 50%, water consumption by 30-50% and significantly improves yields ("New Farming Techniques Could 'Cut Food Crises in South Asia'", Financial Times (London) (10/3/01)).

Over 100 million acres (405,000 km2) worldwide are planted under "conservation tillage" (Howard G. Buffett, Wall Street Journal 5/22/97).

US No-Till Acreage (The Conservation Technology Information Center)
Year - - - - |1990|1991|1992|1993|1994
Million acres| 17 | 20 | 28 | 34 | 38
1000 km2  ~ ~ | 69 | 81 |113 |138 |154

In 1980, 75% of American farmers were still using the moldboard plow. By 1993, a USDA survey showed that farmers used the moldboard plow on 6-9% of corn, soybean, and wheat fields (99A2).

Conservation tillage has increased 7%/ year in the US since 1989, and is now used on just over 1/3 of all planted land in the US (96G1).

US conservation tillage (mulch-till + ridge-till + no-till) was practiced on 35% of total cropland planted in 1994 (just under 100 million acres - up 4.2 million acres from 1993 (71 million acres in 1989; 79 million acres in 1991) (94H2).

Conservation tillage is performed on over 97 million acres in the US, nearly 35% of total cropland acres planted in 1993. Nearly 108 million acres (39%) are clean-tilled. Conservation tillage has been growing at 9 million acres/ year for the past 2 years (94U2).

Extent of US Conservation Tillage by Region (94U2)
(Areas expressed in millions of acres and 1000 km2)
Corn Belt ~ ~ ~ ~ |37.0|150.0|No-till~ ~ ~ ~ ~ ~ |6.3|26.0
Northern Plains ~ |24.0| 97.0|Nebraska ridge-till|1.5| 6.0
Great Lakes States| 9.5| 38.0|Minn. Ridge-till ~ |0.6| 2.4
Iowa no-till~ ~ ~ | 6.9| 28.0|

Conservation (minimum) tillage is practiced on 25% of US tilled cropland (83B1). P. R. Crosson (RFF) predicts 50-60% conservation tillage by 2010, but the added 304,000 km2 of land being tilled then will be easily erodible, so total soil erosion may not decrease (83B1).

History of Minimum Tillage in the US (p. 33 of (84B3))
Year | 1972| 1974| 1976| 1978| 1980| 1982| 1984
Area | 29.7| 46.7| 59.6| 74.8| 88.5| 111.|107.5 (106 acres)
Share| 10.0| 14.2| 17.6| 22.2| 25.1| 31.2| 32.6% (est.)

History of Minimum Tillage in the US (88S1) (Areas in millions of acres)
Year - - - - |1968|1970|1974|1978|1980|1982|1984|1986
Tot. Cropland| 299| 293| 326| 336| 356| 363| 375| 296
Min. tillage | ~6 | ~8 | 17 | 31 | 39 | 66 | 87 | 98
Share (%)~ ~ | 2.0| 2.0| 5.2| 9.2|10.9|18.2|25.3|32.9%

History of No-till Acreage in the US (Areas in million acres) (93M1)
Year|1987|1988|1989|1990|1991|1992
Area|12.5| 13.| 14.| 17.| 21.| 28.(10% of cultivated area)

In 1974, 22,300 km2 were under no-tillage cultivation in the US (Ref. 5 of (80P1)). Conservation tillage was practiced on 446,600 km2 in the US in 1987 - 26% of total croplands (89G2).

Over 287,000 km2 of US croplands are farmed by no-till, ridge-till, and mulch-till methods (89U2). (1989 National Survey of Conservation Tillage Practices).

The USDA estimates that 620,000 km2 (45% of US croplands) will be under the no-tillage system by 2000 (80P1). A somewhat different set of figures is given in (76U1). Minimum tillage began in the early 1960s (76U1).

In 1989, Kansas had 415,000** acres under no-till farming, vs. 2.3 million acres in 1998 - **11% of farmable acres in Kansas (00H1).

US History of Mulch-tillage, No-till, and other Reduced-tillage Techniques (80U2): (million acres)
Year|1963|1968|1973|1977
Area| ~4 | 15 | 30 | 55

Part [Jb] ~ Effects of Reduced Tillage ~ [Jb1]~Erosion, [Jb2]~Herbicides and pesticides, ~

Sub-Part [Jb1] ~ Effects of Reduced Tillage ~ Erosion ~
No-till farming has helped reduce water erosion by over 40% since 1982 in the US according to David Pimentel (
04K1).

The shift away from the moldboard plow (to no-till methods) has increased organic matter in the soil and has led to a looser, less erodible soil that holds more water for crops (99A2).

McGregor et al found that, on a highly erodible soil in Mississippi, erosion was reduced from 1750 tonnes/km2/ year to 180 tonnes/km2/ year after a no-tillage system was used (Ref. 10 of (80P1)). Triplett et al (Ref. 8 of (80P1)) found that a no-tillage system reduced soil erosion by as much as 50-fold. Many similar studies are noted in (80P1).

In a Nebraska study, soil erosion averaged 763 tonnes/ km2/ year with "till-planting", compared with an erosion rate of 2400 tonnes/ km2/ year for a plow/ disk/ harrow system (p. 151 of (76P2)).

Typical soil loss on conventionally cropped fields of the Palouse Region (8,000 km2) are 5600 tonnes/ km2/ year. Minimum tillage can reduce this to 1100 (82O1).

Sub-Part [Jb2] ~ Effects of Reduced Tillage ~ Herbicides, Pesticides ~
Minimum tillage requires herbicide applications of at least 740 lb/ km2 (76U1).

Conservation tillage requires 15-40% increase in pesticides for weed and insect control relative to conventional tillage practices (Refs. 21, 42 of (89S2)).

Herbicides used on Iowa corn in 1992 (lb./ km2/ year) (93M1)
Herbicide -|Conven-|Low- | No-
- - - - - -|tional |Till | Till
Atrazine ~ | 235 ~ | 232 | 366
Alachlor ~ | 467 ~ | 576 | 618
Metolachlor| 519 ~ | 543 | 541
Cyanazine~ | 457 ~ | 521 | 578

Part [Jc] ~ Reduced Tillage Technology, Economics ~

Cropland Area Under No-Till in 1998-1999, km2 (Rolf Derpsch, "Frontiers in Conservation Tillage and Advances in Conservation Practice", in D. E. Stott, R. H. Mohtar, and G. C. Steinhardt (eds.) Sustaining the Global Farm (2001) pp. 248-54) (la).
US~ ~ ~ ~ | 193,000
Brazil~ ~ | 112,000
Argentina | ~73,000
Canada~ ~ | ~41,000
Australia | ~10,000
Paraguay~ | ~ 8,000
Mexico~ ~ | ~ 5,000
Bolivia ~ | ~ 2,000
Others~ ~ | ~ 1,100
Total ~ ~ | 455,000

Comments: For comparison, total global cropland area is about 15,000,000 km2 so only about 3% of cropland areas were under No-till during 1998-1999.

Some or all elements of No-Till/ Conservation Agriculture (NT/CA) have been applied by farmers so far on 50-60 million ha worldwide. Almost half of this is in the US, where the area under zero tillage tripled over the last decade to 23 million ha (USDA, 2001e), responding to government conservation requirements and to reduce fuel costs. But a considerable share of this is under monoculture and misses two essential features, namely full soil cover and adequate crop rotation, and cannot therefore be classified as NT/CA. In Paraguay about half of all cropped land is under elements of NT/CA, mainly zero tillage. The area increased from 20,000 to almost 800,000 ha between 1992-99 because the government shared part of the initial costs of conversion (03B4).

Minimum tillage technology is reviewed in (77T1).

Energy Requirements for preparing Land for Cropping by Various systems (gallons of diesel fuel/ 100-acres) (77T1)
Moldboard plowing ~ |185 | Chisel plowing ~ ~ ~ |115
One-time disking~ ~ | 70 | Conventional planting| 65
One-time cultivating| 45 | No-till planting ~ ~ | 40

No-till farming requires about 1/3 as much fuel as regular farming (00H1).

A detailed economic analysis of conservation tillage under the conservation provisions of the 1985 farm bill is in (89G3).

SECTION (3-K) ~ Aluminum Toxicity and Other Fertility Constraints ~

Aluminum contamination is high enough on 17% of farmland worldwide that it's toxic to plants (Stanley Wood et al, report released by International Food Policy Research Institute, 2/9/01 [satellite data]).

About 16% of the world's farmland is free of fertility problems, or "constraints," such as chemical contamination, acidity, salinity or poor drainage. In parts of Asia, as little as 6% of farmland is free of such problems. North America has the largest share of the best land - 29% (Stanley Wood et al, report released by International Food Policy Research Institute (2/9/01) [satellite data]).

A newly discovered a biotech solution to the problem of aluminum toxicity cuts crop yields by up to 80% on much of the tropical world's arable land. (A naturally occurring gene for citric acid production is taken from a bacterium and inserted into crop plants. The gene causes plant roots to secrete citric acid, blocking uptake of aluminum ions and allowing crops to grow unimpeded.) (99A1).

Acid soils in humid regions have relatively high aluminum contents. Soil acidity results from leaching of Ca, Mg, and K from the root zone and their replacement by H+ and Al. High levels of Al are toxic to maize, beans and other legumes. Cowpeas and cassava are relatively tolerant to high aluminum contents (88L1).

Global Data on areas of Aluminum Toxicity Hazard for Tropical Nations (Areas in 1000 km2): (Table 18.5 of (90W1))
Africa|Central| South| S.E. | S.W.| Total
~ - - |America| Asia | Asia | Asia|
5083.0|241.57 |8427.7|2413.2|41.19|16,206.7

Aluminum toxicity in Amazon Basin affects 3.15 million km2 (73% of the Amazon basin's land area) (82B1).

Section (3-L) ~ Cropping Intensity / Multiple Cropping ~

Intensification and yield growth are subject to limits for reasons of plant physiology (Sinclair, 1998) and because of environmental stresses associated with intensification (01M1) (03B4).

The overall cropping intensity for developing countries will rise by about 6 percentage points over the (1998-2030?) projection period (from 93 to 99%). Cropping intensities continue to rise through shorter fallow periods and more multiple cropping. An increasing share of irrigated land in total agricultural land contributes to more multiple cropping. About 33% of arable land in South- and East Asia is irrigated (1998?), a share which is projected to rise to 40% in the year 2030 (03B3).

During the 36-year period when world average grain yields more than doubled from 140 tonnes/ km2 in 1961/1963 to 305 in 1997/1999 and the overall cropping intensity probably increased by some 5 percentage points, the amount of arable land required to produce any given amount of grain declined by 56%. This decline exceeded the above-mentioned 40% fall in the arable land per person that occurred during the same period (03B3).

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