Almost all people recognize that erosion is harmful, but few realize how harmful. Certainly, there are costs to society for clean-up and repair from soil and its constituents polluting the water and air. However, the greatest costs are borne by the landowner and producer. Erosion results in higher fertilizer and fuel requirements, and lower yields. The benefits of erosion control are sometimes obscure, but the costs of erosion are real.
Erosion reduces productivity by modifying soil properties and is more harmful to soils that: 1) are shallow, 2) have poor quality subsoils, and 3) do not have a thick topsoil. Any combination of these characteristics greatly increases the damage from erosion. Deep soils with more excellent subsoil properties are virtually unaffected by soil erosion. More fertilizer would be required to replace the fertility loss, but the production potential would change little, if any. However, most soils have some undesirable properties which lower production, since erosion causes more subsoil to be incorporated into the plow layer.
Soil Properties
Erosion removes a field's original topsoil, causing the subsoil to mix with the remaining topsoil during annual plowing. In mature soils (like many of those in Kentucky), this subsoil material has more clay, less organic matter, lower available water-holding capacity and lower fertility status. Also, the soil structure is likely to be coarser, less stable and subject to more damage by rainfall impact, tillage or traffic. As more and more of the subsoil is mixed into the topsoil by tillage, it adds more and more of these characteristics to the topsoil.
The clay content in the topsoil of two soils studied in Kentucky increased from 25 to 31 percent in a Maury soil and from 20 to 25 percent in a Crider soil as a result of moderate erosion. This increased clay content also increases fuel, chemical and fertilizer requirements.
Soil erosion removes the lighter and more easily dislodged particles. This means that organic matter is one of the more easily erodable constituents. Moderate erosion of a Crider soil in Western Kentucky was found to have reduced the topsoil organic matter content from 1.7 to 1.5 percent.
Soil density also increases with erosion. This increases problems associated with tillage, tilth and seedbed preparation.
Plant Available Water
The most limiting factor on crop production in Kentucky is probably water. Erosion usually reduces the plant available water holding capacity of a soil. In a study of Kentucky Maury and Crider soils, the plant available water holding capacity decreased from 29 to 24 percent in the Maury soil and from 24 to 20 percent in the Crider soil because of erosion.
Two things cause this reduction in plant available water holding capacity --a shallower soil and more undesirable soil properties. As the clay increases and organic matter decreases, the amount of water a soil can make available to the plant decreases. This is probably the single most important effect of erosion on soil productivity.
Fertility Losses
Soil erosion removes the part of the soil which is usually richest in plant nutrients. Erosion on soils which are highly fertile, naturally or by fertilizer addition, will result in greater fertility losses. All nutrients are lost during erosion but the most economically significant losses will probably be that of nitrogen, phosphorus, potassium and lime.
If the soil is naturally low in fertility, then erosion will cause a loss of the added nutrients and increase the fertility requirements of the soil. A Crider soil at Princeton, Kentucky, which is naturally low in fertility, was studied to determine the effect of moderate erosion. Although the site had been highly fertilized for years, erosion caused a difference in the fertility status of the soil (Table 1).

Table 1. Effect of Moderate Erosion on the Fertility Status and Requirement for Corn of a Crider Soil.

	Soil Test			Fertility Recommendation
	Eroded	Uneroded		Eroded	Uneroded
pH	6.8	7.0	lime (ton/A)	0	0
P (lb/A)	34	77	P2O5 (lb/A)	55	0
K (lb/A)	116	187	K2O (lb/A)	100	45

More fertilizer and lime would be required to raise and maintain the soil test when erosion is allowed to occur. The greater the erosion, the more fertility that would be lost. It has been estimated that the value of plant available nutrients lost from a highly fertile soil in Kentucky could range from $3 to $14 per acre (considering 1981 fertilizer prices and a soil loss of 14 tons per acre). It is difficult to put a real value on the soil nutrients lost due to erosion. It depends on the soil fertility level and the availability of the nutrients in the soil. Regardless, the loss can be considerable and should not be ignored. Fortunately, the fertility can be replaced, but the cost must be recognized and taken into account.
Productivity
A number of soil properties are affected by soil erosion. Fertility, organic matter, rooting depth, soil tilth and available water holding capacity are reduced and soil texture is changed. Collectively, these properties affect the yield potential of the soil. In most cases, the potential productivity of the soil will be decreased as erosion occurs. The yield decrease is usually so slight from year to year that the farmer may not realize that it is happening. Therefore, productivity loss due to soil erosion is usually not considered to be a financial loss. As small annual losses accumulate with time, productivity is substantially reduced. Also, crop yields may actually continue to increase during and after excessive soil erosion because of advanced technology such as fertilizer management, irrigation, improved varieties and pest control. This concept is presented in Figure 1. The yield potential of most soils decreases with erosion, but actual yield may continue to rise on moderately-eroded soils. Yields do not rise as rapidly or as high as on similar soils with none to slight erosion. Another way to view this concept is that more technological inputs are necessary for crop yields on eroded soils to equal those on uneroded soils. And more technological inputs usually mean more financial inputs as well.
The corn production of a Crider soil decreased over three years from 125 to 100 bushels per acre after moderate erosion had taken place. Both lower available water supplying capacity and lower fertility status probably contributed to the lower yields. However, water supplying capacity was probably the greatest limitation since fertilizer additions were based on 150 bushels per acre yield goals.
Many of the soils in Kentucky have limited rooting volumes due to underlying fragipans, rocks or claypans. Loss of surface soil by erosion further decreases the already limiting volume of soil favorable for root growth and available water storage. Studies on a fragipan soil (Zanesville) at Princeton indicate that as the soil depth to the fragipan horizon decreases to less than 2 feet, the corn yield will decrease about 5 bushels per acre for each inch of soil lost by erosion. This effect is cumulative and should be considered in situations where erosion is allowed to occur on a shallow soil.
Lasting Effects
After erosion has occurred and the productivity of the soil has been decreased, can it be restored? Two practices which would probably be most effective in restoring productivity would be 1) high fertility amendments and 2) the planting of a sod crop to prevent further erosion and increase organic matter. In an experiment at Lexington, moderate erosion caused the corn yields of a Maury soil to be reduced from 99 to 87 bushels per acre. The erosion had taken place 60 years prior to the study and the soil was in a very high fertility state, indicating that the effects of erosion are long-lasting. Neither low intensity use nor high fertility amendments restored the soil to its original production potential after being damaged by erosion. This is not to say that the productivity of eroded soils can not be improved through management; however, the production capacity of some eroded soils may never be restored to the level of that soil in its uneroded state.
Costs
What could moderate erosion cost a producer over the period of a lifetime? It would depend greatly on the soil and its fertility status, but it would not be unusual to lose 17 tons of soil per acre per year from a sloping field in continuous cultivation in Kentucky. If this much soil were lost for 50 years, then 5 inches of soil would be removed. To maintain the soil in a high fertility status, it might cost $10 per acre per year for additional fertilizers. In addition, the annual loss in productivity could be as much as 20 bushels of corn per acre after the 50 years of erosion. Again the losses could be higher or lower. Considering these two factors, the annual loss after 50 years would be $70 per acre per year, if corn is considered to be $3 a bushel. Certainly this is a very arbitrary figure but could be considered realistic for many cases.
Conclusions
Erosion reduces the productivity of many soils by affecting the soil properties and depth. This reduction is substantial and long-lasting, and can not be reversed over a lifetime, even with conservation management, low intensity use or high fertility amendments. The most important yield-limiting effect of erosion is probably the decrease in plant available water holding capacity. The soil fertility status may be significantly decreased by erosion but can be replenished by additions of lime and fertilizer, although with greater expense and lower efficiency than for uneroded soil. The overall effect of erosion is an economic loss which accumulates with time as erosion continues. The cost of erosion, over time, will probably outweigh any costs associated with erosion control practices.