Building Soils for Better Crops, 2nd Edition

	Why Is Organic Matter So Important? Why are soils which in our father's hands were productive now relatively impoverished? J. L. Hills, C. H. Jones, and C. Cutler, 1908 Organic matter functions in a number of key roles to promote crop growth. It also is a critical part of a number of global and regional cycles. A fertile soil is the basis for healthy plants, animals, and humans. Soil organic matter is the very foundation for healthy and productive soils. Understanding the role of organic matter in maintaining a healthy soil is essential for developing ecologically sound agricultural practices. It's true that you can grow plants on soils with little organic matter. In fact, you don't need any soil at all! [Although gravel or sand hydroponic systems without soil can grow excellent crops, large-scale systems of this type are usually neither economically or ecologically sound.] It's also true that there are other important issues aside from organic matter when considering the quality of a soil. However, as soil organic matter decreases, it becomes increasingly difficult to grow plants, because problems with fertility, water availability, compaction, erosion, parasites, diseases, and insects become more common. Ever higher levels of inputs fertilizers, irrigation water, pesticides, and machinery are required to maintain yields in the face of organic matter depletion. But if attention is paid to proper organic matter management, the soil can support a good crop without the need for expensive fixes. The organic matter content of agricultural topsoil is usually in the range of 1 to 6 percent. A study of soils in Michigan demonstrated potential crop-yield increases of about 12 percent for every 1 percent organic matter. In a Maryland experiment, researchers saw an increase of approximately 80 bushels of corn per acre when organic matter increased from 0.8 to 2 percent. What Makes Topsoil? Having a good amount of topsoil is important. But what gives topsoil its beneficial characteristics? Is it because it's on TOP? If we bring in a bulldozer and scrape off one foot of soil, will the exposed subsoil now be topsoil because it's on the surface? Of course, everyone knows that there's more to topsoil than its location on the soil surface. Most of the properties we associate with topsoil good nutrient supply, tilth, drainage, aeration, water storage, etc. -- are there because topsoil is rich in organic matter and contains a huge diversity of life. You might wonder how something that's only a small part of the soil can be so important for growing healthy and high-yielding crops. The enormous influence of organic matter on so many of the soil's properties biological, chemical, and physical makes it of critical importance to healthy soils (figure 4.1). Part of the explanation for this influence is the small particle size of the well-decomposed portion of organic matter the humus. Its large surface area-to-volume ratio means that humus is in contact with a considerable portion of the soil. The intimate contact of humus with the rest of the soil allows many reactions, such as the release of available nutrients into the soil water, to occur rapidly. However, the many roles of living organisms make soil life an essential part of the organic matter story.     Figure 4.1 Adding organic matter results in many changes.     Modified from Oshins, 1999.  Plant Nutrition Plants need 18 chemical elements for their growth carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), boron (B), zinc (Zn), molybdenum (Mo), nickel (Ni), copper (Cu), cobalt (Co), and chlorine (Cl). Plants obtain carbon as carbon dioxide (CO2) and oxygen partially as oxygen gas (O2) from the air. The remaining essential elements are obtained mainly from the soil. The availability of these nutrients is influenced either directly or indirectly by thepresence of organic matter. The elements needed in large amounts carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur are called macronutrients. The other elements, called micronutrients, are essential elements needed in small amounts. Nutrients from decomposing organic matter. Most of the nutrients in soil organic matter can't be used by plants as long as they exist as part of large organic molecules. As soil organisms decompose organic matter, nutrients are converted into simpler, inorganic, or mineral forms that plants can easily use. This process, called mineralization, provides much of the nitrogen that plants need by converting it from organic forms. For example, proteins are converted to ammonium (NH4+) and then to nitrate (NO3-). Most plants will take up the majority of their nitrogen from soils in the form of nitrate. The mineralization of organic matter is also an important mechanism for supplying plants with such nutrients as phosphorus and sulfur, and most of the micronutrients. This release of nutrients from organic matter by mineralization is part of a larger agricultural nutrient cycle (see figure 4.2). For a more detailed discussion of nutrient cycles and how they function in various cropping systems, see chapter 7.     Figure 4.2 The cycle of plant nutrients. Addition of nitrogen. Bacteria living in nodules on legume roots convert nitrogen from atmospheric gas (N2) to forms that the plant can use directly. There are a number of free-living bacteria that also fix nitrogen. Storage of nutrients on soil organic matter. Decomposing organic matter can feed plants directly, but it also can indirectly benefit the nutrition of the plant. A number of essential nutrients occur in soils as positively charged molecules called cations (pronounced cat-eye-ons). The ability of organic matter to hold onto cations in a way that keeps them available to plants is known as cation exchange capacity (CEC). Humus has many negative charges. Because opposite charges attract, humus is able to hold onto positively charged nutrients, such as calcium (Ca++), potassium (K+), and magnesium (Mg++) (see figure 4.3a). This keeps them from leaching deep into the subsoil when water moves through the topsoil. Nutrients held in this way can be gradually released into the soil solution and made available to plants throughout the growing season. However, keep in mind that not all plant nutrients occur as cations. For example, the nitrate form of nitrogen is negatively charged (NO3-) and is actually repelled by the negatively charged CEC. Therefore, nitrate leaches easily as water moves down through the soil and beyond the root zone. Figure 4.3 Cations held on organic matter and clay. Clay particles also have negative charges on their surfaces (figure 4.3b), but organic matter may be the major source of negative charges for coarse and medium textured soils. Some types of clays, such as those found in the southeastern United States and in the tropics, tend to have low amounts of negative charge. When these clays are present, organic matter may be the major source of negative charges that bind nutrients, even for fine textured (high clay content) soils. Protection of nutrients by chelation. Organic molecules in the soil may also hold onto and protect certain nutrients. These particles, called "chelates" (pronounced key-lates) are byproducts of active decomposition of organic materials and are smaller than those that make up humus. In general, elements are held more strongly by chelates than by binding of positive and negative charges. Chelates work well because they bind the nutrient at more than one location on the organic molecule (figure 4.3c). In some soils, trace elements, such as iron, zinc, and manganese, would be converted to unavailable forms if they were not bound by chelates. It is not uncommon to find low organic matter soils or exposed subsoils deficient in these micronutrients. Other ways of maintaining available nutrients. There is some evidence that organic matter in the soil can inhibit the conversion of available phosphorus to forms that are unavailable to plants. One explanation is that organic matter coats the surfaces of minerals that can bond tightly to phosphorus. Once these surfaces are covered, available forms of phosphorus are less likely to react with them. In addition, humic substances may chelate aluminum and iron, both of which can react with phosphorus in the soil solution. When they are held as chelates, these metals are unable to form an insoluble mineral with phosphorus. Beneficial Effects of Soil Organisms Soil organisms are essential for keeping plants well supplied with nutrients because they break down organic matter. These organisms make nutrients available by freeing them from organic molecules. Some bacteria fix nitrogen gas from the atmosphere, making it available to plants. Other organisms dissolve minerals and make phosphorus more available. If soil organisms aren't present and active, more fertilizers will be needed to supply plant nutrients. A varied community of organisms is your best protection against major pest outbreaks and soil fertility problems. A soil rich in organic matter and continually supplied with different types of fresh residues is home to a much more diverse group of organisms than soil depleted of organic matter. This greater diversity of organisms helps insure that fewer potentially harmful organisms will be able to develop sufficient populations to reduce crops yields. Soil Tilth When soil has a favorable physical condition for growing plants, it is said to have good tilth. Such a soil is porous and allows water to enter easily, instead of running off the surface. More water is stored in the soil for plants to use between rains and less soil erosion occurs. Good tilth also means that the soil is well aerated. Roots can easily obtain oxygen and get rid of carbon dioxide. A porous soil does not restrict root development and exploration. When a soil has poor tilth, the soil's structure deteriorates and soil aggregates break down, causing increased compaction and decreased aeration and water storage. A soil layer can become so compacted that roots can't grow. A soil with excellent physical properties will have numerous channels and pores of many different sizes. Studies on both undisturbed and agricultural soils show that as organic matter increases, soils tend to be less compact and have more space for air passage and water storage. Sticky substances are produced during the decomposition of plant residues. Along with plant roots and fungal hyphae, they bind mineral particles together into clumps, or aggregates. In addition, the sticky secretions of mycorrhizal fungi those that infect roots and help plants get more water and nutrients are important binding material in soils. The arrangement and collection of minerals as aggregates and the degree of soil compaction have huge effects on plant growth (see chapter 6). The development of aggregates is desirable in all types of soils because it promotes better drainage, aeration, and water storage. The one exception is for wetland crops, such as rice, when you want a dense, puddled soil to keep it flooded. Organic matter, as residue on the soil surface or as a binding agent for aggregates near the surface, plays an important role in decreasing soil erosion. Surface residues intercept raindrops and decrease their potential to detach soil particles. These surface residues also slow water as it flows across the field, giving it a better chance to infiltrate into the soil. Aggregates and large channels greatly enhance the ability of soil to conduct water from the surface into the subsoil. Most farmers can tell that one soil is better than another by looking at them, touching them, how they work up when tilled, or even by sensing how they feel when walked on. What they are seeing or sensing is really good tilth. For an example, see the photo on the back cover of this book. It shows that soil differences can be created by different management strategies. Farmers and gardeners would certainly rather grow their crops on the more porous soil depicted in the photo on the right. Since erosion tends to remove the most fertile part of the soil, it can cause a significant reduction in crop yields. In some soils, the loss of just a few inches of topsoil may result in a yield reduction of 50 percent. The surface of some soils low in organic matter may seal over, or crust, as rainfall breaks down aggregates, and pores near the surface fill with solids. When this happens, water that can't infiltrate into the soil runs off the field, carrying valuable topsoil (figure 4.4). Figure 4.4 Changes in soil surface and water-flow pattern due to soil crusting. Large soil pores, or channels, are very important because of their ability to allow a lot of water to flow rapidly into the soil. Larger pores are formed a number of ways. Old root channels may remain open for some time after the root decomposes. Larger soil organisms, such as insects and earthworms, create channels as they move through the soil. The mucus that earthworms secrete to keep their skin from drying out also helps to keep their channels open for a long time. Protection of the Soil Against Rapid Changes in Acidity Acids and bases are released as minerals dissolve and organisms go about their normal functions of decomposing organic materials or fixing nitrogen. Acids or bases are excreted by the roots of plants, and acids form in the soil from the use of nitrogen fertilizers. It is best for plants if the soil acidity status, referred to as pH, does not swing too wildly during the season. The pH scale is a way of expressing the amount of free hydrogen (H+) in the soil water. More acidic conditions, with greater amounts of hydrogen, are indicated by lower numbers. A soil at pH 4 is very acid. Its solution is 10 times more acid than a soil at pH 5. A soil at pH 7 is neutral there is just as much base in the water as there is acid. Most crops do best when the soil is slightly acid and the pH is around 6 to 7. Essential nutrients are more available to plants in this pH range than when soils are either more acidic or more basic. Soil organic matter is able to slow down, or buffer, changes in pH by taking free hydrogen out of solution as acids are produced or by giving off hydrogen as bases are produced. (For discussion about management of acidic soils, see chapter 18.) Stimulation of Root Development Microorganisms in soils produce numerous substances that stimulate plant growth. Humus itself has a directly beneficial effect on plants (figure 4.5). Although the reasons for this stimulation are not yet understood, certain types of humus cause roots to grow longer and have more branches, resulting in larger and healthier plants. In addition, soil microorganisms produce a variety of root-stimulating substances that behave as plant hormones. Figure 4.5 Corn grown in nutrient solution with (right) and without (left) humic acids. In this experiment by Rich Bartlett and Yong Lee, adding humic acids to a nutrient solution increased the growth of tomatoes and corn and increased the number and branching of roots. Photo by R. Bartlett. Darkening of the Soil Organic matter tends to darken soils. You can easily see this in coarse-textured sandy soils containing light-colored minerals. Under well-drained conditions, a darker soil surface allows a soil to warm up a little faster in the spring. This provides a slight advantage for seed germination and the early stages of seedling development, which is often beneficial in cold regions. Color and Organic Matter In Illinois, a hand-held chart has been developed to allow people to estimate percent soil organic matter. Their darkest soils almost black indicates from 3.5 to 7 percent organic matter. A dark brown soil indicates 2 to 3 percent and a yellowish brown soil indicates 1.5 to 2.5 percent organic matter. (Color may not be as clearly related to organic matter in all regions, because the differences in the amount of clay and the types of minerals present also influence soil color.) Protection Against Harmful Chemicals Some naturally occurring chemicals in soils can harm plants. For example, aluminum is an important part of many soil minerals and, as such, poses no threat to plants. As soils become more acidic, especially at pH levels below 5.5, aluminum becomes soluble. Some soluble forms of aluminum, if present in the soil solution, are toxic to plant roots. However, in the presence of significant quantities of soil organic matter, the aluminum is bound tightly and will not do as much damage. Organic matter is the single most important soil property that reduces pesticide leaching. It holds tightly onto a number of pesticides. This prevents or reduces leaching of these chemicals into groundwater and allows time for detoxification by microbes. Microorganisms can change the chemical structure of some pesticides, industrial oils, many petroleum products (gas and oils), and other potentially toxic chemicals, rendering them harmless. ORGANIC MATTER AND NATURAL CYCLES The Carbon Cycle Soil organic matter plays a significant part in a number of global cycles. People have become more interested in the carbon cycle because the buildup of carbon dioxide (CO2) in the atmosphere is thought to cause global warming. Carbon dioxide is also released to the atmosphere when fuels, such as gas, oil, and wood, are burned. A simple version of the natural carbon cycle, showing the role of soil organic matter, is given in figure 4.6. Carbon dioxide is removed from the atmosphere by plants and used to make all the organic molecules necessary for life. Figure 4.6 The role of soil organic matter in the carbon cycle. Losses of carbon from the field are indicated by the dark border around the words describing the process.   Sunlight provides plants with the energy they need to carry out this process. Plants, as well as the animals feeding on plants, release carbon dioxide back into the atmosphere as they use organic molecules for energy. The largest amount of carbon present on the land is not in the living plants, but in soil organic matter. That is rarely mentioned in discussions of the carbon cycle. More carbon is stored in soils than in all plants, all animals and the atmosphere. Soil organic matter contains an estimated four times as much carbon as living plants. As soil organic matter is depleted, it becomes a source of carbon dioxide for the atmosphere. When forests are cleared and burned, a large amount of carbon dioxide is released to the atmosphere. There is a potentially larger release of carbon dioxide following conversion of forests to agricultural practices that rapidly deplete the soil of its organic matter. There is as much carbon in 6 inches of soil with 1 percent organic matter as there is in the atmosphere above a field. If organic matter decreases from 3 percent to 2 percent, the amount of carbon dioxide in the atmosphere could double. (Of course, wind and diffusion move the carbon dioxide to other parts of the globe.) The Nitrogen Cycle Another important global cycle in which organic matter plays a major role is the nitrogen cycle. This cycle is of direct importance in agriculture, because available nitrogen for plants is commonly deficient in soils. Figure 4.7 shows the nitrogen cycle and how soil organic matter enters into the cycle. Some bacteria living in soils are able to "fix" nitrogen, converting nitrogen gas to forms that other organisms, including crop plants, can use. Inorganic forms of nitrogen, like ammonium and nitrate, exist in the atmosphere naturally, although air pollution causes higher amounts than normal. Rainfall and snow deposit inorganic nitrogen forms on the soil. Inorganic nitrogen also may be added in the form of commercial nitrogen fertilizers. These fertilizers are derived from nitrogen gas in the atmosphere through an industrial fixation process. Bacteria and fungi convert the organic forms of nitrogen into ammonium and different bacteria convert ammonium into nitrate. Both nitrate and ammonium can be used by plants. Figure 4.7 The role of soil organic matter in the nitrogen cycle. Losses of nitrogen from the field are indicated by the dark border around the words describing the process. Nitrogen can be lost from a soil in a number of ways. When crops are removed from fields, nitrogen and other nutrients also are removed. The nitrate (NO3-) form of nitrogen leaches readily from soils and may end up in groundwater at higher concentrations than may be safe for drinking. Organic forms of nitrate as well as nitrate and ammonium (NH4+) may be lost by runoff water and erosion. Once freed from soil organic matter, nitrogen may be converted to forms that end up back in the atmosphere. Bacteria convert nitrate to nitrogen (N2) and nitrous oxide (N2O) gases in a process called denitrification, which occurs in saturated soils. Nitrous oxide (it's called a "greenhouse gas") contributes to global warming. In addition, when it reaches the upper atmosphere, it helps to decrease the ozone levels that protect the earth's surface from the harmful effects of ultraviolet (UV) radiation. So if you needed another reason not to apply excessive rates of fertilizers or manures in addition to the economic costs and the pollution of ground and surface waters the possible formation of nitrous oxide should make you cautious. The Water Cycle Organic matter plays an important part in the local, regional, and global water, or hydrologic, cycle due to its role in promoting water infiltration into soils and storage within the soil. Water evaporates from the soil surface and from living plant leaves as well as from the ocean and lakes. Water then returns to the earth, usually far from where it evaporated, as rain and snow. Soils high in organic matter, with excellent tilth, enhance the rapid infiltration of rainwater into the soil. This water may be available for plants to use or it may percolate deep into the subsoil and help to recharge the groundwater supply. Since groundwater is commonly used as a drinking water source for homes and for irrigation, recharging groundwater is important. When the soil's organic matter level is depleted, it is less able to accept water, and high levels of runoff and erosion result. This means less water for plants and decreased groundwater recharge. Sources Allison, F.E. 1973. Soil Organic Matter and its Role in Crop Production. Scientific Publishing Co. Amsterdam, The Netherlands. Brady, N.C., and R.R. Weil. 1999. The Nature and Properties of Soils. 12th ed. Macmillan Publishing Co. New York, NY. Follett, R.F., J.W.B. Stewart, and C.V. Cole (eds). 1987. Soil Fertility and Organic Matter as Critical Components of Production Systems. Special Publication No.19. Soil Science Society of America. Madison, WI. Lucas, R.E., J.B. Holtman, and J.L. Connor. 1977. Soil carbon dynamics and cropping practices. pp. 333­451. In Agriculture and Energy (W. Lockeretz, ed.). Academic Press. New York, NY. See this source for the Michigan study on the relationship between soil organic matter levels and crop-yield potential. Oshins, C. 1999. An Introduction to Soil Health. A slide set available at the Northeast Region SARE website: www.uvm.edu/~nesare/slide.html. Powers, R.F., and K. Van Cleve. 1991. Long-term ecological research in temperate and boreal forest ecosystems. Agronomy Journal 83:11­24. This reference compares the relative amounts of carbon in soils with that in plants. Stevenson, F.J. 1986. Cycles of Soil: Carbon, Nitrogen, Phosphorus, Sulfur, Micronutrients. John Wiley & Sons. New York, NY. This reference compares the amount of carbon in soils with that in plants. Strickling, E. 1975. Crop sequences and tillage in efficient crop production. Abstracts of the 1975 Northeast Branch American Society Agronomy Meetings. pp. 20­29. See this source for the Maryland experiment relating soil organic matter to corn yield. Tate, R.L., III. 1987. Soil Organic Matter: Biological and Ecological Effects. John Wiley & Sons. New York, NY. Top