Building Soils for Better Crops, 2nd Edition

	Healthy Soils All over the country [some soils are] worn out, depleted, exhausted, almost dead. But here is comfort: These soils possess possibilities and may be restored to high productive power, provided you do a few simple things. C.W. Burkett, 1907 It should come as no surprise that many cultures have considered soil central to their lives. After all, people were aware that the food they ate grew from the soil. Our ancestors who first practiced agriculture must have been amazed to see life reborn each year when seeds placed in the ground germinated and then grew to maturity. In the Hebrew bible, the name given to the first man (Adam) is the masculine version of the word earth or soil (adama). The name for the first woman (Eve, or Hava in Hebrew) comes from the word for living. Soil and human life were considered to be intertwined. A particular reverence for the soil has been an important part of the cultures of many other civilizations, including American Indian tribes. Although we focus on the critical role soils play in growing crops, it's important to keep in mind that soils also serve other important purposes. Soils govern what percent of the rainfall runs off the field, as compared to the percent that enters the soil and eventually helps recharge underground aquifers. When a soil is denuded of vegetation and it starts to degrade, excessive runoff and flooding are more common. Soils also absorb, release, and transform many different chemical compounds. For example, they help to purify wastes flowing from the septic system drain in your back yard. Soils also provide habitats for a diverse group of organisms, some of which are very important such as with those bacteria that produce antibiotics. Soil organic matter stores a huge amount of atmospheric carbon. Carbon, in the form of carbon dioxide, is a greenhouse gas associated with global warming. We also build roads and buildings on soils; some are definitely better than others for this purpose. What kind of soil do you want? Farmers sometimes use the term soil health to describe the condition of the soil. Scientists usually use the term soil quality, but both refer to the same idea how good is the soil in its role of supporting the growth of high yielding, healthy crops? How would you know a high quality soil from a lower quality soil? Most farmers or gardeners would say that they know one when they see one. Farmers can certainly tell you which of the soils on their farms are of low, medium, or high quality. They know high quality soil because it generates higher yields with less effort. Less rainwater runs off and fewer visible signs of erosion are seen on the better quality soils. Less power is needed to operate machinery on a healthy soil than on poorer, compacted soils. Soil scientists are working together with farmers and agricultural extension personnel to try to come up with a widely accepted definition of soil health and to determine what factors (pH, bulk density, aggregate stability, etc.) need to be measured to estimate a soil's quality. The first thing many might think of is that the soil should have a sufficient supply of nutrients throughout the growing season. But don't forget, at the end of the season there shouldn't be too much nitrogen and phosphorus left in highly soluble forms or enriching the soil's surface. Leaching and runoff of nutrients are most likely to occur after crops are harvested and before the following year's crops are well established. We also want the soil to have good tilth so that plant roots can fully develop with the least amount of effort. A soil with good tilth is more spongy and less compact than a soil with poor tilth. A soil that has a favorable and stable soil structure also promotes rainfall infiltration and water storage for plants to use later. For good root growth and drainage, we also want a soil with sufficient depth before there's a restricting layer. We want a soil to be well drained, so it dries enough to permit timely field operations. Also, it's essential that oxygen is able to reach the root zone to promote optimal root health and that happens best in a soil without a drainage problem. (Keep in mind that these general characteristics do not hold for all crops. For example, flooded soils are important for paddy rice production.) We want the soil to have low populations of plant disease and parasitic organisms so plants grow better. Certainly, there should also be a low weed pressure, especially of aggressive and hard-to-control weeds. Most soil organisms are beneficial and we certainly want high amounts of organisms that help plant growth, such as earthworms and many bacteria and fungi. A high quality soil is free of chemicals that might harm the plant. These can occur naturally, such as soluble aluminum in very acid soils or excess salts in arid region soils. Potentially harmful chemicals also are introduced by human activity, such as fuel oil spills or application of sewage sludge with high concentrations of toxic elements. A high quality soil should resist being degraded. It also should be resilient, recovering quickly after unfavorable changes like compaction. The Nature and Nurture of Soils Some soils are exceptionally good for growing crops and others are inherently unsuitable; most are in between. Many soils also have limitations, such as low organic matter content, texture extremes (coarse sand or heavy clay), poor drainage, and layers that restrict root growth. Iowa's loess-derived prairie soils are naturally blessed with a combination of silt loam texture and high organic matter contents. By every standard for assessing soil health, these soils in their virgin state would rate very high. We can compare them with a person who is naturally very healthy and has great athletic abilities. Many of us are not quite so lucky and Nature has given us qualities that may never make us great baseball players, swimmers, or marathon runners, even if we tried very hard. The way we care for, or nurture, a soil modifies its inherent nature. A good soil can be abused through years of poor management and turn into one with poor health, although it generally takes a lot of mistreatment to reach that point. On the other hand, an innately challenging soil may be very "unforgiving" of poor management and quickly become even worse. For example, a heavy clay loam soil can be easily compacted and turn into a dense mass. Both the naturally good and poor soils can be productive if they are managed well. However, they will probably never reach parity, because some limitations simply cannot be completely overcome. The key idea, however, is the same that we wish for our children we want our soils to reach their fullest potential. How do you build a healthy, high quality soil? Some characteristics of healthy soils are relatively easy to achieve for example, an application of limestone will make a soil less acid and increase availability of many nutrients to plants. But what if the soil is only a few inches deep? There is little that can be done within economic reason, except on a very small garden-size plot. If the soil is poorly drained because of a restricting subsoil layer of clay, tile drainage can be installed, but at a significant cost. We use the term building soils to emphasize that the nurturing process of converting a degraded or low quality soil into a truly high quality one requires understanding, thought, and significant actions. This is also true for maintaining or improving already healthy soils. Soil organic matter influences almost all of the characteristics we've just discussed. For soil tilth, organic matter is one of the main influences. Organic matter is even critical for managing pests and good management of this resource should be the starting point for a pest management program on every farm. Good organic matter management is, therefore, the foundation for high quality, healthy soils. Practices that promote good soil organic matter management are, thus, the very foundation for a more sustainable and thriving agriculture. It is for this reason that so much space is devoted to organic matter in this book. However, we cannot forget other critical aspects of management such as trying to lessen compaction by heavy field equipment and good nutrient management. Although the details of how best to create high quality soils differ from farm to farm and even field to field, the general approaches are the same: Use a number of practices that add organic materials to the soil. Use diverse sources of organic materials. Reduce unneeded losses of native soil organic matter. Use practices that leave the soil surface protected from raindrops and temperature extremes. Whenever traveling on the soil with field equipment, use practices that help develop and maintain good soil structure. Manage soil fertility status to maintain optimal pH levels for your crops and a sufficient supply of nutrients for plants without resulting in water pollution. In arid regions, a combination of gypsum and leaching may be needed to reduce the amount of sodium or salt in the soil. Figure 1.1 The downward spiral of soil degradation. Modified from Topp et al., 1995.   How do Soils Become Degraded? Although we want to emphasize healthy, high quality soils, it is also crucial to recognize that many soils in the U.S. and around the world have become degraded what many used to call "worn out" soils. Degradation most commonly occurs when erosion and decreased soil organic matter levels initiate a downward spiral (figure 1.1). Soils become compact, making it hard for water to infiltrate and roots to develop properly. Erosion continues and nutrients decline to levels too low for good crop growth. The development of saline (too salty) soils under irrigation in arid regions is another cause of reduced soil health. (Salts added in the irrigation water need to be leached beneath the root zone to avoid the problem.) Evaluating Your Soils Score cards have been developed to help farmers assess their soils. They use a simple scale to rate the health of soils. You evaluate the presence of earthworms, severity of erosion, ease of tillage, soil structure, color, degree of compaction, water infiltration rate, and drainage status. Then you rate crops growing on the soils by such characteristics as their general appearance, growth rates, root health, degree of resistance to drought, and yield. It's a good idea for every farmer to fill out such a score card for every major field or soil type on their farm every few years. But even without doing so, you probably already know what a really high quality and healthy soil would be like one that would consistently produce good yields of high quality crops with minimal negative environmental impact. More on evaluating soil health in Chapter 20. Historically, soil degradation has caused significant harm to many early civilizations, including the drastic loss of productivity resulting from soil erosion in Greece and many locations in the Middle East (such as Israel, Jordan, and Lebanon). This led to either colonial ventures to help feed the citizenry or to the decline of the early cultures. Tropical rainforest conditions (high temperature and rainfall, with most of the organic matter near the soil surface) may cause significant soil degradation within two or three years of conversion to cropland. This is the reason that the "slash and burn" system, with people moving to a new patch of forest every few years, developed in the tropics. After farmers depleted the soils in a field, they would cut down and burn the trees in the new patch, allowing the forest and soil to regenerate in previously cropped areas. The westward push of U.S. agriculture was stimulated by rapid soil degradation in the East, originally a zone of temperate forest. Under the conditions of the humid portion of the Great Plains (moderate rainfall and temperature, with organic matter distributed deeper in the soil), it took many decades for the effects of soil degradation to become evident.     Sources Doran, J.W., M. Sarrantonio, and M.A. Liebig. 1996. Soil heath and sustainability. pp. 1­54 In Advances in Agronomy Vol. 56. Academic Press, Inc. San Diego, CA. Hillel, D. 1991. Out of the Earth: Civilization and the Life of the Soil. University of California Press. Berkeley, CA. Spillman, W.J. 1906. Renovation of Wornout Soils. Farmers' Bulletin No. 245. USDA, Government Printing Office. Washington, D.C. Topp, G.C., K.C. Wires, D.A. Angers, M.R. Carter, J.L.B. Culley, D.A. Holmstrom, B.D. Kay, G.P. Lafond, D.R. Langille, R.A. McBride, G.T. Patterson, E. Perfect, V. Rasiah, A.V. Rodd, K.T. Webb. 1995. Changes in soil structure. Chapter 6. In The Health of Our Soils: Toward Sustainable Agriculture in Canada (Acton, D.F. and L.J. Gregorich (eds.). Centre for Land and Biological Resources Research. Research Branch, Agriculture and Agri-Food Canada. Publication 1906/E. http://www.agr.gc.ca/nlwis-snite/index_e.cfm?s1=pub&s2=hs_ss&page=intro Top