Erosion—How Big a Threat?

Spurred by crop demand, especially for export, and relatively slow growth in crop yields, American farmers brought an additional 60 million acres under crops between 1972 and 1982. According to the U.S. Soil Conservation Service, this added land suffers more erosion than does already-producing land. Moreover, under the pressure of rising demand, many farmers sought to squeeze more production out of the land by abandoning such soil-conservation practices as strip cropping and windbreaks. The combination of additional, more erosive land and fewer conservation practices convinced many in the conservation community that soil erosion poses a major threat to the productivity of the nation's cropland. Anticipating a steadily increasing demand for crops, both at home and abroad, many conclude that the nation's ability to feed future generations at reasonable cost is at serious risk.

Concern about erosion, of course, is not new, but in the 1970s and early 1980s it acquired an intensity not felt since the Dust Bowl years of the mid-thirties. Reacting to this concern, RFF researchers undertook a major study of the erosion problem in the United States, and the first results were published late in 1983.

What do we know?

Curiously, until recently little has been known about U.S. soil erosion. Tens of billions of dollars (in 1980 prices) were spent on soil-conservation programs over the last five decades, but reliable information about the extent of erosion was not compiled until the National Resources Inventory (NRI) of 1977. Nor was much known about erosion effects on soil productivity. Soil science demonstrated that sustained erosion could reduce productivity by carrying away soil nutrients, lessening the capacity of the soil to make water and air available to plants, and diminishing the space in which crop roots could spread. And studies on experimental plots showed that stripping away topsoil would reduce crop yields by amounts varying from 10 to 40 percent, depending primarily on characteristics of the subsoil. But because these studies were on a small scale and highly specific to particular soils and climates, they provided no useful information about erosion effects on average U.S. crop yields—the kind of information needed for judgments about the threat of erosion to future capacity for crop production.

Assessing T values

For these judgments soil conservationists depend on measures of "tolerable" soil loss—T values—defined as the maximum amount of annual soil loss per acre consistent with indefinite maintenance of the productivity of the soil. T values vary from 5 tons per acre per year on deep soils to 1 ton on shallow soils.

The 1977 NRI indicated that erosion exceeded 5 tons per acre on about one-third of the nation's cropland, with erosion by water exceeding the limit on 23 percent and wind erosion on an additional 10 percent. These numbers were taken as a measure of erosion threat to the productivity of the land and a challenge, both to individual farmers and to those responsible for soil-conservation policy.

But T values are a faulty guide to farmers and to policymakers. Little scientific backing exists for the 5-ton limit, a fact widely recognized by soil scientists. Experience shows that on deep soils with favorable subsoils—for example, those in western Iowa—erosion may exceed 5 tons per year for many years with no perceptible effect on productivity. T values also are misleading because they ignore the vital issue of the timing of erosion-control efforts. For the farmer, the proper time is when the present value of future productivity losses exceeds the present value of erosion control costs. On deep soils the present value of the losses is likely to be low for a long time, even though annual erosion is well in excess of 5 tons per acre. Eventually the present value of the losses will begin to rise, perhaps because erosion threatens to shrink the crop-rooting zone, and when that happens, farmers will begin to pay serious attention to erosion control. By contrast, the T-value criterion requires the farmer to bring erosion down to T "now," even though the present value of the productivity losses may be well under the present value of erosion control costs. Not surprisingly, conservationists often express frustration at the refusal of many farmers to follow the T-value criterion.

Erosion in excess of T often is taken as a measure of the amount of soil permanently lost to agriculture. It is the basis for dramatic statements picturing billions of tons of topsoil being washed each year down the nation's rivers to the sea. Yet most eroded soil is deposited somewhere on land, not in waterways; and therefore it is not necessarily lost forever to agriculture. A key question concerns the effect of the eroded soil on productivity where it is deposited. It is astonishing how little is known about this, considering its importance for judging net productivity effects of erosion. Clearly, depositing nutrient-poor material on fertile soil will diminish productivity; but just as clearly, productivity may be increased by nutrient-rich silt and clay particles. Apart from effects on fertility, deposition builds soil depth, thus putting off the day when diminishing crop-rooting zones begin to limit productivity.

Another implication of T values is that erosion in excess of T permanently reduces productivity. Yet the soil science literature leaves no doubt that farmers can repair part—and on some soils all—of damage to productivity. On deep soils with favorable subsoils, simple replacement of soil nutrients with fertilizer often will restore productivity completely. Where subsoils are unfavorable, especially in water-holding capacity, fertilizers typically will not suffice. Even on these soils, however, practices that put back some of the lost soil organic matter will improve soil structure and often restore much of the productivity loss.

Adding fertilizer and adapting practices to build organic matter increase production costs, but so does controlling erosion. For the farmer, therefore, the issue is whether control costs are more or less than the costs of repairing damage to productivity. This "real world" choice, of crucial importance to the farmer, is ignored by strict application of the T-value criterion.

The rationale for T values is fairness to the future—intergenerational equity. And for soil conservationists, intergenerational equity requires that land should be managed to avoid imposing higher costs of food and fiber on future generations. But the T-value criterion is unnecessarily restrictive for achieving this objective. It implicitly assumes that any loss of soil productivity means higher future production costs, despite the fact that it may be less costly for farmers—and for society—to accept some productivity loss now and repair it later. Beyond this, investing in research to develop new technology can more than offset erosion-induced losses of productivity. At the margin, technology is a substitute for land in the production process.

An alternative to T values

We may accept the imperative of intergenerational equity, therefore, but reject the T-value criterion as a guide to achieving it: allowing for technological advance broadens the options. To be sure, technological advance should not be taken for granted, and achieving it inevitably imposes costs. In general, the slower the expected development of new technology and the more it depends on costly factors, such as fossil fuels, the greater the value of the soil as a productive resource and the more important its preservation to achieving intergenerational equity. This way of viewing obligations to the future clearly retains a place for soil conservation policies. But it puts them in the context of a broader set of policies, all aimed at avoiding rising costs of producing food and fiber.

What the record shows

Part of the RFF study examined the relationship between erosion, as measured in the NRI, and the growth of yields of corn, soybeans, and wheat between 1950 and 1980. Data were for counties in the Corn Belt, Northern Plains, and the Palouse region of the Pacific Northwest. The results showed that erosion had no effect the growth of wheat yields. For corn and soybeans, the effect was small, but significant: for both crops, yields between 1950 and 1980 increased about 4 percent less because of erosion than they would have otherwise.

Despite the effect of erosion on yields of corn and soybeans, the prices of both crops, adjusted for inflation, were less at the end of the 1970s than at the beginning of the 1950s. The decline reflected the fast pace of technological advance over the period, and was all the more impressive because demand for both crops increased substantially and real prices of farm inputs also rose. The cost-increasing tendencies of erosion-induced productivity loss, rising demand, and higher inputs prices were more than offset by the cost-decreasing tendency of technological progress. Compared to the cost effects of technology and demand growth, those of erosion were small.

Work by others, reviewed in the RFF study, indicates that continuation of 1977 rates of erosion for one hundred years would reduce national average crop yields by 5 to 10 percent. Even a modest rate of technological advance—say, half that achieved since the 1930s—would far outweigh the erosion effect on productivity, suggesting that current erosion presents no serious threat to intergenerational equity. But no one really understands the process of technological change, so that even a modest rate of advance should not be taken for granted, especially where the interests of future generations are at stake. Moreover, the prospective erosion-induced productivity loss, although small, is not negligible.

To illustrate, assume that erosion reduces corn and soybean yields by 10 percent over one hundred years in equal annual increments, that "normal" yields are 110 bushels per acre for corn and 32 bushels for soybeans, that corn is priced at $3 per bushel and soybeans at $7, that there are 72 million acres of land in corn and 70 million acres in soybeans, and that the rate of discounting future productivity losses is 10 percent. Under these assumptions, the productivity loss in the first year is 33 cents per acre for corn and 22 cents per acre for soybeans, apparently trivial sums. For the 142 million acres in the two crops the first-year loss is somewhat more impressive—almost $40 million—but still small relative to the total value of corn and soybean production.

This way of calculating the cost of erosion-induced productivity loss understates the true cost of erosion because it does not account for costs farmers may incur in order to restore some of the lost productivity, for example, by putting on more fertilizer. The amount of these uncounted costs may be significant.

Moreover, the cost of lost productivity is cumulative, each year's cost being added to that of the previous year. Even if the incremental loss were no more than $40 million annually, the undiscounted value of the loss would be $4 billion in the hundredth year. Of course, discounting at 10 percent annually greatly reduces the present value of the more distant losses. But even so, the sum of the discounted losses over one hundred years is about $4.3 billion at 10 percent, assuming the annual incremental loss is $40 million. Discounted at 5 percent, its present value is almost $17 billion.

Thus, the cumulative loss of productivity is more impressive than the first-year loss would suggest. Whether the loss would be consistent with intergenerational equity is uncertain. It probably would be, unless the nation enjoys markedly less technological progress in the next one hundred years than the record documents for the last fifty. Until this is settled, erosion-induced productivity loss, while perhaps not a major threat, deserves a place on the nation's policy agenda.