3.3: Human Influences

Erosion loss of topsoil that is rich in organic matter has dramatically reduced the total amount of organic matter stored in many soils after they were developed for agriculture. Crop production obviously suffers when part of the most fertile layer of the soil is removed. Erosion is a natural process and occurs on almost all soils. Some soils naturally erode more easily than others, and the problem is greater in some regions (like dry sparsely vegetated areas) than others. However, agricultural practices greatly accelerate erosion whether by water, wind or even tillage practices themselves (see Chapter 16). It is estimated that erosion in the United States is responsible for annual losses of about $1 billion in available nutrients and many times more in total soil nutrients.

Unless erosion is severe, a farmer may not even realize a problem exists. But that doesn’t mean that crop yields are unaffected. In fact, yields may decrease by 5–10% when only moderate erosion occurs. Yields may suffer a decrease of 10–20% or more with severe erosion. The results of a study of three Midwestern soils (referred to as Corwin, Miami and Morley), shown in Table 3.1, indicate that erosion greatly affects both organic matter levels and water-holding ability. Greater amounts of erosion decreased the organic matter content of these loamy and clayey soils. In addition, eroded soils stored less available water than minimally eroded soils. Organic matter also is lost from soils when organisms decompose more organic materials during the year than are added. This occurs as a result of practices that accelerate decomposition, such as intensive tillage and crop production systems that return low amounts of plant biomass, directly as crop residues or indirectly as manure. Even with residue retention, cash grain production systems export 55–60% of the organic matter off the farm. Therefore, much of the rapid loss of organic matter following the conversion of grasslands to agriculture has been attributed to large reductions in plant residue annually returned to soil, accelerated mineralization of organic matter because of plowing, and erosion.

Tillage Practices

Tillage practices influence both the amount of topsoil erosion and the rate of decomposition of organic matter. Conventional plowing and disking provide multiple short-term benefits: creating a smooth seedbed, stimulating nutrient release by enhancing organic matter decomposition, and helping control weeds. But by breaking down natural soil aggregates, intensive tillage destroys large, water-conducting channels and the soil is left in a physical condition that is highly susceptible to wind and water erosion.

Table 3.1 Effects of Erosion on Soil Organic Matter and Water
Soil	Erosion	Organic Matter (%)	Available Water Capacity (%)
Corwin	slight moderate severe	3.03 2.51 1.86	12.9 9.8 6.6
Miami	slight moderate severe	1.89 1.64 1.51	16.6 11.5 4.8
Morley	slight moderate severe	1.91 1.76 1.6	7.4 6.2 3.6
Source: Schertz et al. (1985)

The more a soil is disturbed by tillage practices, the greater the potential breakdown of organic matter by soil organisms. This happens because organic matter held within aggregates becomes readily available to soil organisms when aggregates are broken down during tillage. Incorporating residues with a moldboard plow, breaking aggregates open and fluffing up the soil also allow microorganisms to work more rapidly. It’s something like opening up the air intake on a wood stove, which lets in more oxygen and causes the fire to burn hotter. Rapid loss of soil organic matter (and a burst of CO₂ pumped into the atmosphere) occurs in the early years because of the high initial amount of active (“dead”) organic matter available to microorganisms. In Vermont, we found a 20% decrease in organic matter after five years of growing silage corn on a clay soil that had previously been in sod for decades. During the early years of agriculture in the United States, when colonists cleared the forests and planted crops in the East, and farmers later moved to the Midwest to plow the grasslands, soil organic matter decreased rapidly as the soils were literally mined of this valuable resource. In the Midwest, many soils lost 50% of their organic matter within 40 years of the onset of cropping. It was quickly recognized in the Northeast and Southeast that fertilizers and soil amendments were needed to maintain soil productivity. In the Midwest, the deep, rich soils of the tall-grass prairies were able to maintain their productivity for a long time despite accelerated loss of soil organic matter and significant amounts of erosion. The reason for this was their unusually high reserves of soil organic matter and nutrients at the time of conversion to cropland.

After much of the biologically active portion is lost, the rate of organic matter loss slows and what remains is mainly the already well-decomposed “passive” or “very dead” materials. With the current interest in reduced (“conservation”) tillage, growing row crops in the future should not have such a detrimental effect on soil organic matter. Conservation tillage practices leave more residues on the surface and cause less soil disturbance than conventional moldboard plows and disks. In fact, soil organic matter levels usually increase when no-till planters place seeds in a narrow band of disturbed soil, leaving the soil between planting rows undisturbed. Residues accumulate on the surface because the soil is not inverted by plowing. Earthworm populations increase because they are naturally adapted to feeding on plant residues left at the soil surface. They take some of the residues deeper into the soil and create channels that also help water infiltrate into the soil. The beneficial effects on soil organic matter levels from minimizing tillage are often observed quickly at the soil surface, but deeper changes are much slower to develop. In the upper Midwest there is conflicting evidence as to whether a long-term no-till approach results in greater accumulation of soil organic matter than a conventional tillage system when the full profile is considered. In contrast, significant increases in profile soil organic matter have been routinely observed under no-till in warmer locations.

Crop Rotations and Cover Crops

Figure 3.3. Organic carbon changes when growing corn silage or alfalfa. Redrawn from Angers (1992).

Levels of soil organic matter may fluctuate during the different stages of a crop rotation. Soil organic matter may decrease, then increase, then decrease, and so forth. While annual row crops under conventional moldboard-plow cultivation usually result in decreased soil organic matter, perennial legumes, grasses and legume-grass forage crops tend to increase soil organic matter. The high amount of root production by hay and pasture crops, plus the lack of soil disturbance, causes organic matter to accumulate in the soil. This effect is seen in the comparison of organic matter increases when growing alfalfa compared to corn silage (Figure 3.3). In addition, different types of crops result in different quantities of residues being returned to the soil. When corn grain is harvested, more residues are left in the field than after soybean, wheat, potato or lettuce harvests. Harvesting the same crop in different ways leaves different amounts of residues. When corn grain is harvested, more residues remain in the field than when the entire plant is harvested for silage or when stover is used for purposes like bioenergy (Figure 3.4). You can therefore imagine a worst case scenario when a field has continuous annual row crop production, with grain and residue harvested from the field, and is combined with intensive tillage and no other organic additions like manure, compost or cover crops.

Soil erosion is greatly reduced and topsoil rich in organic matter is conserved when rotation crops, such as grass or legume hay, are grown year round. The permanent soil cover and extensive root systems of sod crops account for much of the reduction in erosion. Having sod crops as part of a rotation reduces the loss of topsoil, decreases decomposition of residues, and builds up organic matter by the extensive residue addition of plant roots.

Cover crops help protect soils from erosion during the part of the year between commercial crops when soils would otherwise be bare. In addition to protecting organic-matter-rich topsoil from erosion, cover crops may add significant amounts of organic materials to soil. But the actual amount added is determined by the type of cover crop (grass species versus legumes versus brassicas, etc.) and how much biomass accumulates before it is suppressed/killed in order to plant the following commercial crop.

Use of Synthetic Nitrogen Fertilizer

Fertilizing nutrient-deficient soils usually results in greater crop yields. A significant additional benefit is that it also achieves greater amounts of crop residue—roots, stems and leaves—resulting from larger and healthier plants. Most crop nutrients are applied in reasonable balance with crop uptake if the soil is regularly tested. However, nitrogen management is more challenging and includes more risk to farmers. Therefore, N fertilizer is commonly applied at much higher rates than needed by plants, sometimes by as much as 50%, which is costly and also creates environmental problems. (See Chapter 19 for a detailed discussion of nitrogen management.)

Use of Organic Amendments

An old practice that helps maintain or increase soil organic matter is to apply manures or other organic residues generated off the field. This happened naturally in older farming systems where crops and livestock were raised on the same farm, and much of the crop organic matter and nutrients was recycled as manure. A study in Vermont found that between 20 and 30 tons (wet weight, including straw or sawdust bedding) of dairy manure per acre were needed to maintain soil organic matter levels when silage corn was grown each year. This is equivalent to one or one and a half times the amount produced by a large Holstein cow over the whole year. Varying types of manure—bedded, liquid stored, digested, etc.—can produce very different effects on soil organic matter and nutrient availability. Manures differ in their initial composition and are affected by how they are stored and handled in the field: for example, surface applied or incorporated, which we discuss in Chapter 12.

Figure 3.4. Soil surface after harvest of corn silage or corn grain. Photos by Bill Jokela and Doug Karlen.