Building Soils for Better Crops, 2nd Edition

	Amount of Organic Matter In Soils The depletion of the soil humus supply is apt to be a fundamental cause of lowered crop yields. J.H. Hills, C.H. Jones, and C. Cutler, 1908 The amount of organic matter in any particular soil is a result of a wide variety of environmental, soil, and agronomic influences. Some of these, such as climate and soil texture, are naturally occurring. Human activity also influences soil organic matter levels. Tillage, crop rotation, and manuring practices all have profound effects on the amount of soil organic matter. Pioneering work on the effect of natural influences on soil organic matter levels was carried out in the U.S. more than 50 years ago by Hans Jenny. The amount of organic matter in soil is a result of all the additions and losses of organic matter that have occurred over the years (figure 5.1). In this chapter, we will look at why different soils have different organic matter levels. Anything that adds large amounts of organic residues to a soil may increase organic matter. On the other hand, anything that causes soil organic matter to decompose more rapidly or be lost through erosion may deplete organic matter. Figure 5.1 Additions and losses of organic matter from soils. If additions are greater than losses, organic matter increases. When additions are less than losses, there is a depletion of soil organic matter. When the system is in balance, and additions equal losses, the quantity of soil organic matter doesn't change over the years. NATURAL FACTORS Temperature In the United States, it is easy to see how temperature affects soil organic matter levels. Traveling from north to south, average hotter temperatures lead to less soil organic matter. As the climate gets warmer, two things tend to happen (as long as rainfall is sufficient): more vegetation is produced because the growing season is longer, and the rate of decomposition of organic materials in soils also increases, because soil organisms work more efficiently in warm weather and for longer periods of the year. This increasing decomposition with warmer temperatures becomes the dominant influence determining soil organic matter levels. Rainfall Soils in arid climates usually have low amounts of organic matter. In a very dry climate, such as a desert, there is little growth of vegetation. Decomposition may be very low when the soil is dry and microorganisms cannot function well. When it finally rains, a very rapid burst of decomposition of soil organic matter occurs. Soil organic matter levels generally increase as average annual precipitation increases. With more rainfall, more water is available to plants and more plant growth results. As rainfall increases, more residues return to the soil from grasses or trees. At the same time, soils in high rainfall areas may have less soil organic matter decomposition than well-aerated soils decomposition is slowed by restricted aeration. Soil Texture Fine textured soils, containing high percentages of clay, tend to have naturally higher amounts of soil organic matter than coarse textured sands or sandy loams. The organic matter content of sands may be less than 1 percent; loams may have 2 to 3 percent; and clays from 4 to more than 5 percent. The strong bonds that develop between clay and organic matter seem to protect organic molecules from attack and decomposition by microorganisms. In addition, fine textured soils tend to have smaller pores and have less oxygen than coarser soils. This also causes reduced decomposition of organic matter. The lower rate of decomposition in soils with high clay contents is probably the main reason that their organic matter levels are higher than in sands and loams. Soil Drainage and Position in the Topography Some soils have a compact subsoil layer that doesn't allow water to drain well. Decomposition of organic matter occurs more slowly in poorly aerated soils, when oxygen is limited or absent, than in well-aerated soils. For this reason, organic matter accumulates in wet soil environments. In a totally flooded soil, one of the major structural parts of plants, lignin, doesn't decompose at all. The ultimate consequence of extremely wet or swampy conditions is the development of organic (peat or muck) soils, with organic matter contents of over 20 percent. If organic soils are artificially drained for agricultural or other uses, the soil organic matter will decompose very rapidly. When this happens, the elevation of the soil surface actually decreases. Some homeowners in Florida were fortunate to sink corner posts below the organic level. Originally level with the ground, those homes now perch on posts atop a soil surface that has decreased so dramatically the owners park under their homes. Soils in depressions at the bottom of hills are often wet because they receive runoff, sediments (including organic matter), and seepage from up slope. Organic matter is not decomposed as rapidly in these landscape positions as in drier soils farther up slope. However, soils on a steep slope will tend to have low amounts of organic matter because the topsoil is continually eroded. Type of Vegetation The type of plants that grow on a soil over the years affects the soil organic matter level. The most dramatic differences are evident when soils developed under grassland are compared with those developed under forests. On natural grasslands, organic matter tends to accumulate in high amounts and to be well distributed within the soil. This is probably a result of the deep and extensive root systems of native grasses. Their roots have high "turnover" rates, for root death and decomposition constantly occurs as new roots are formed. The high levels of organic matter in soils that were once in grassland explains why these are some of the most productive soils in the world. By contrast, in forests, litter accumulates on top of the soil, and surface organic layers commonly contain over 50 percent organic matter. However, subsurface mineral layers in forest soils typically contain from less than 1 to about 2 percent organic matter. Acidic Soil Conditions In general, soil organic matter decomposition is slower under acidic soil conditions than at more neutral pH. In addition, acidic conditions, by inhibiting earthworm activity, encourage organic matter to accumulate at the soil surface, rather than distributed throughout the soil layers. HUMAN INFLUENCES Soil erosion removes topsoil rich in organic matter so that, eventually, only subsoils remain. Crop production obviously suffers when part or all 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 also greater in some regions than others. However, agricultural practices accelerate erosion. Nationwide, soil erosion causes huge economic losses. It is estimated that erosion in the United States is responsible for annual losses of $500 million in available nutrients and $18 billion in total soil nutrients. Unless erosion is very severe, a farmer may not even realize that a problem exists, but that doesn't mean that crop yields are unaffected. In fact, yields may decrease by 5 to 10 percent when only moderate erosion occurs. Yields may suffer a decrease of 10 to 20 percent or more with severe erosion. The results of a study of three midwestern soils, shown in table 5.1, indicate that erosion greatly influences both organic matter levels and water-holding ability. Greater amounts of erosion decreased the organic matter contents of these loamy and clayey soils. In addition, eroded soils stored less available water than soils experiencing little erosion. 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 such as intensive tillage and growing crops that produce low amounts of residues (see below). Tillage Practices Tillage practices influence both the amount of topsoil erosion and the rate of decomposition of soil organic matter. Conventional plowing and disking of a soil to prepare a smooth seedbed breaks down natural soil aggregates and destroys large, water-conducting channels. The soil is left in a physical condition that allows both wind and water erosion. The more a soil is disturbed by tillage practices, the greater the potential breakdown of organic matter by soil organisms. 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. In fact, the soils were literally mined of a valuable resource organic matter. In the Northeast and Southeast, it was quickly recognized 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 soil organic matter loss and significant amounts of erosion. The reason for this was their unusually high original levels of soil organic matter. Rapid soil organic matter decomposition by soil organisms usually occurs when a soil is worked with a moldboard plow. Incorporating residues, breaking aggregates open, and fluffing up the soil allows 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. In Vermont, we found a 20-percent decrease in organic matter after five years of growing corn on a clay soil that had previously been in sod for a long time. In the Midwest, 40 years of cultivation caused a 50-percent decline in soil organic matter. Rapid loss of soil organic matter occurs in the early years, because of the high initial amount of active ("dead") organic matter available to micro-organisms. After much of the active portion is lost, the rate of organic matter loss slows considerably. With the current interest in reduced (conservation) tillage, growing row crops in the future may 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 plow and disk tillage. In fact, soil organic matter levels usually increase when no-till planters place seeds in a narrow band of disturbed soil, while leaving the soil between planting rows undisturbed. The rate of decomposition of soil organic matter is lower because the soil is not drastically disturbed by plowing and disking. Residues accumulate on the surface because the soil is not inverted by plowing. Earthworm populations increase, taking some of the organic matter deeper into the soil and creating channels that help water infiltrate into the soil. Decreased erosion also results from using conservation tillage practices. Crop Rotations and Cover Crops At different stages in a rotation, different things may be happening. 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, or legume-grass forage crops tend to increase soil organic matter. The turnover of the roots of these hay and pasture crops, plus the lack of soil disturbance, allow 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 5.2) In addition, different types of crops result in different quantities of residues returned to the soil. When corn grain is harvested, more residues are left in the field than after soybeans, wheat, potatoes, 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 (figure 5.3). Figure 5.2 Organic carbon changes when growing corn silage or alfalfa. Redrawn from Angers, 1992. 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 extensive root systems of sod crops account for much of the reduction in erosion. Having sod crops as part of a rotation reduces loss of topsoil, decreases decomposition of residues, and builds up organic matter by the extensive residue addition of plant roots. Figure 5.3a Soil surface after harvest of corn silage. Photos by Win Way.   Figure 5.3b Soil surface after harvest of corn grain. Photos by Win Way.    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. A study in Vermont during the 1960s and 1970s 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 1 to 1 ½ times the amount produced by a large Holstein cow over the whole year. Different manures can have very different effects on soil organic matter and nutrient availability. They differ in their initial composition and also are affected by how they are stored and handled in the field. ORGANIC MATTER DISTRIBUTION IN SOIL In general, more organic matter is present near the surface than deeper in the soil (see figure 5.4). This is one of the main reasons that topsoils are so productive, compared with subsoils exposed by erosion or mechanical removal of surface soil layers. Much of the plant residues that eventually become part of the soil organic matter are from the above-ground portion of plants. When the plant dies or sheds leaves or branches, it deposits residues on the surface. Although earthworms and insects help incorporate the residues on the surface deeper into the soil and the roots of some plants penetrate deeply, the highest concentrations still remain within 1 foot of the surface. Figure 5.4 Examples of soil organic matter content with depth. Modified from Brady and Weil, 1999. Litter layers that commonly develop on the surface of forest soils may have very high organic matter contents (figure 5.4a). Plowing forest soils after removal of the trees incorporates the litter layers into the mineral soil. The incorporated litter decomposes rapidly, and an agricultural soil derived from a light, sandy texture forest soil in the north or a silt loam in the southeast coastal plain would likely have a distribution of organic matter similar to that indicated in figure 5.4b. Soils of the tall-grass prairies have high levels of organic matter deep into the soil profile (see figure 5.4c). After cultivation of these soils for 50 years, far less organic matter exists (figure 5.4d). ACTIVE ORGANIC MATTER The discussion for almost all of this chapter has been about amounts of total organic matter in soils. However, we should constantly keep in mind that we are interested in each of the different types of organic matter in soils the living, the dead (active), and the very dead (humus). We don't just want a lot of humus in soil, we also want a lot of active organic matter to provide nutrients and aggregating glues when it is decomposed. We want the active organic matter because it supplies food to keep a di verse population of organisms present. As mentioned earlier, when forest or prairie soils were first cultivated, there was a drastic decrease in the organic matter content. Almost all of the decline was due to a loss of the active ("dead") part of the organic matter. It is the active fraction that increases relatively quickly when practices, such as reduced tillage, rotations, cover crops, and manures, are used to increase soil organic matter. LIVING ORGANIC MATTER In chapter 3, we talked about the various types of organisms that live in soils. The weight of fungi present in forest soils is much greater than the weight of bacteria. In grasslands, however, there are about equal weights of both. In agricultural soils that are routinely tilled, the weight of fungi is less than the weight of bacteria. As soils become more compact, larger pores are eliminated first. These are the pores in which soil animals, such as earthworms and beetles, live and function, so the number of such organisms in compacted soils decreases. Different total amounts (weights) of living organisms exist in various cropping systems. In general, high populations of diverse and active soil organisms are found in systems with more complex rotations that regularly leave high amounts of crop residues and when other organic materials are added to the soil. Organic materials may include crop residues, cover crops, animal manures, and composts. Leaves and grass clippings may be an important source of organic residues for gardeners. When crops are rotated regularly, fewer parasite, disease, weed, and insect problems occur than when the same crop is grown year after year. On the other hand, frequent cultivation reduces the number of many soil organisms as their food supplies are depleted by decomposition of organic matter. Compaction from heavy equipment causes harmful biological effects in soils. It decreases the number of medium to large pores, which reduces the volume of soil available for air, water, and populations of organisms such as mites and springtails that need the large spaces in which to live. Sources Angers, D.A. 1992. Changes in soil aggregation and organic carbon under corn and alfalfa. Soil Science Society of America Journal 56: 1244­1249. Brady, N.C., and R.R. Weil. 1999. The Nature and Properties of Soils. 12th ed. Macmillan Publishing Co. New York, NY. Carter, V.G., and T. Dale. 1974. Topsoil and Civilization. University of Oklahoma Press. Norman, OK. Hass, H.J., G.E.A. Evans, and E.F. Miles. 1957. Nitrogen and Carbon Changes in Great Plains Soils as Influenced by Cropping and Soil Treatments. U.S. Department of Agriculture Technical Bulletin 1164. U.S. Government Printing Office. Washington, D.C. This is a reference for the large decrease in organic matter content of Midwest soils. Jenny, H. 1980. The Soil Resource. Springer-Verlag. New York, NY. Jenny, H. 1941. Factors of Soil Formation. McGraw-Hill. New York, NY. Jenny's early work on the natural factors influencing soil organic matter levels. Magdoff, F.R., and J.F. Amadon. 1980. Yield trends and soil chemical changes resulting from N and manure application to continuous corn. Agronomy Journal 72:161­164. See this reference for further information on the studies in Vermont cited in this chapter. National Research Council. 1989. Alternative Agriculture. National Academy Press. Washington, D.C. Schertz, D.L., W.C. Moldenhauer, D.F. Franzmeier, and H.R. Sinclair, Jr. 1985. Field evaluation of the effect of soil erosion on crop productivity. pp. 9­17. In Erosion and Soil Productivity. Proceedings of the national symposium on erosion and soil productivity. Dec. 10­11, 1984. New Orleans, LA. American Society of Agricultural Engineers Publication 8-85. St. Joseph, MI. Tate, R.L., III. 1987. Soil Organic Matter: Biological and Ecological Effects. John Wiley & Sons. New York, NY. Top