9
Managing for High- Quality Soils:
Organic Matter, Soil Physical Condition, Nutrient Availability
Because organic matter is lost from the soil through decay, washing, and leaching, and because large amounts are required every year for crop production, the necessity of maintaining the active organic-matter content of the soil, to say nothing of the desirability of increasing it on many depleted soils, is a difficult problem.
—A. F. Gustafson, 1941
Increasing the quality of a soil—enhancing it as a habitat for plant roots and beneficial organisms—takes a lot of thought and action over many years. Of course, there are things that can be done right off—plant a cover crop this fall or just make a New Year’s resolution not to work soils that really aren’t ready in the spring (and then stick with it). Other changes take more time. You need to study carefully before drastically changing crop rotations, for example. How will the new crops be marketed, and are the necessary labor and machinery available?
All actions taken to improve soil health should contribute to one or more of the following: (a) growing healthy plants; (b) stressing pests, and (c) increasing beneficial organisms. First, various practices to build up and maintain high levels of soil organic matter are key. Second, developing and maintaining the best possible soil physical condition often requires other types of practices, in addition to those that directly impact soil organic matter. Paying better attention to soil tilth and compaction is more important than ever, because of the use of very heavy field machinery. Lastly, although good organic matter management goes a long way toward providing good plant nutrition in an environmentally sound way, good nutrient management involves additional practices. In this chapter we’ll focus on issues of organic matter management.
[H1]Organic Matter Management
As we discussed in chapter 35, [chapter ref ok?CHANGED] there are no generally accepted guidelines as to how much organic matter should be in a particular soil. And it is difficult to be sure exactly why problems develop when organic matter is depleted in an individual field. However, even in the early 20th century, agricultural scientists proclaimed, “Whatever the cause of soil unthriftiness, there is no dispute as to the remedial measures. Doctors may disagree as to what causes the disease, but agree as to the medicine. Crop rotation! The use of barnyard and green manuring! Humus maintenance! These are the fundamental needs” (Hills, Jones, and Cutler, 1908). [add to Sources list DONE] A century later, these are still some of the major remedies available to us.
There seems to be a contradiction in our view of soil organic matter. On one hand, we want crop residues, dead microorganisms, and manures to decompose. If soil organic matter doesn’t decompose, then no nutrients are made available to plants, no glue to bind particles is manufactured, and no humus is produced to hold on to plant nutrients as water leaches through the soil. On the other hand, numerous problems develop when soil organic matter is significantly depleted through decomposition. This dilemma of wanting organic matter to decompose, but not wanting to lose too much, means that organic materials must be continually added to the soil. A supply of active organic matter must be maintained, so that soil organisms have sufficient food, and so that humus can continually accumulate. This does not mean that organic materials must be added to each field every year—although that happens to a greater or lesser degree if crop roots and aboveground residues remain. However, it does mean that a field cannot go without a significant quantity of organic residue additions for many years without paying the consequences.
Do you remember that plowing a soil is similar to opening up the air intake on a wood stove? What we really want in soil is a slow, steady burn of the organic matter. You get that in a wood stove by adding wood every so often and making sure the air intake is on a medium setting. In soil, you get a steady burn by adding organic residues regularly and by not disturbing the soil too often or too greatly.
There are four general management strategies for organic matter management. First, use crop residues more effectively and find new sources of residues to add to soils. New residues can include those you grow on the farm, such as cover crops, or those available from various local sources. Second, try to use a number of different types of materials—crop residues, manures, composts, cover crops, leaves, etc. It is important to provide varied residue sources to help develop and maintain a diverse group of soil organisms. Third, although use of organic materials from off- farm can be a good source for building soil organic matter and adding nutrients, some farmers overload their fields with excess nutrients by excess imports of organic materials. Crop residues (including cover crops) as well as on-farm- derived animal manures and composts help to supply organic materials and cycle nutrients without a build up of excessive levels of nutrients. Fourth, implement practices that decrease the loss of organic matter from soils because of accelerated decomposition or erosion.
All practices that help to build organic matter levels either add more organic materials than was done in the past or decrease the rate of organic matter loss from soils. In addition, practices to build organic matter building practices will usually enhance beneficial organisms and/or stress pests (table 9.1). Those practices that do both may be especially useful. Practices that reduce losses of organic matter either slow down the rate of decomposition or decrease the amount of erosion. Soil erosion must be controlled to keep organic matter-–enriched topsoil in place. In addition, organic matter added to a soil must either match or exceed the rate of loss by decomposition. These additions can come from manures and composts brought from off the field, crop residues and mulches that remaining following harvest, or cover crops. Reduced tillage lessens the rate of organic matter decomposition and also may result in less erosion. When reduced tillage increases crop growth and residues returned to soil, it is usually a result of better water infiltration and storage and less surface evaporation. It is not possible in this book to give specific soil organic matter management recommendations for all situations. In chapters 10 through 16, we will evaluate management options that enhance the soil environment and issues associated with their use. Most of these practices improve organic matter management, although they have many different types of effects on soils.
[place table 9.1 about here]
[H2]Using Organic Materials
Amounts of crop residues.Crop residues are usually the largest source of organic materials available to farmers. The amount of crop residue left after harvest varies depending on the crop. Soybeans, potatoes, lettuce, and corn silage leave little residue. Small grains, on the other hand, leave more residue, while sorghum and corn harvested for grain leave the most. A ton or more of crop residues per acre may sound like a lot of organic material being returned to the soil. However, keep in mind that after residues are decomposed by soil organisms, only about 10 to –20 percent% of the original amount is converted into stable humus.
The amount of roots remaining after harvest also can range from very low to fairly high (table 9.2). In addition to the actual roots left at the end of the season, there are considerable amounts of sloughed- off roots cells, as well as exudates from the roots during the season. This may actually increase the plant’s below ground inputs of organic matter by another 50%! %. Probably the most effective way to increase soil organic matter is to grow crops with large root systems. Compared to above ground residues, the organic material from roots decomposes more slowly, contributes more to stable soil organic matter, and, of course, does not have to be incorporated into the soil to achieve deep distribution. When no-till is used, root residues, along with root exudates given off when they were alive, tend to promote formation and stabilization of aggregates—more so than surface- derived residue. One of the reasons that the many soils of the Midwest are so rich is that for thousands of years prairie plants with extensive and deep root systems grew there—annually contributing large quantities of organic matter deep into to the soil.
[table 9.2 about here]
Table 9.2Estimated Root Residue Produced by Crops
Crop / Estimated Root Residues (lbs./acre)
Native prairie / 15,000—–30,000
Italian ryegrass / 2,600–4,500
Winter cereal / 1,500–2,600
Red clover / 2,200–2,600
Spring cereal / 1,300–1,800
Corn / 3,000–4,000
Soybeans / 500–1,000
Cotton / 500–900
Potatoes / 300–600
—Topp et al. (1995) and other sources.
Some farmers remove above ground residues such as small grain straw from the field for use as animal bedding or to make compost. Later, these residues return to contribute to soil fertility as manures or composts. Sometimes, residues are removed from fields, to be used by other farmers or to make another product. There is increasing interest in using crop residues as a feedstock for the production of biofuels. This activity could cause considerable harm to soil health if sufficient residues are not allowed to return to soils.
Burning of wheat, rice, and other crop residues in the field still occurs, although it is becoming less common in parts of the United States as well as in other countries. Residue is usually burned to help control insects or diseases or to make next year’s fieldwork easier. Residue burning may be so widespread in a given area that it causes a local air pollution problem. Burning also diminishes the amount of organic matter returned to the soil and the amount of protection against raindrop impact.
Sometimes, important needs for crop residues and manures may prevent their use in maintaining or building soil organic matter. For example, straw may be removed from a grain field to serve as mulch in a strawberry field. These trade-offs of organic materials can sometimes cause a severe soil-fertility problem if allowed to continue for a long time. This issue is of much more widespread importance in developing countries, where resources are scarce. In those countries, crop residues and manures frequently serve as fuel for cooking or heating when gas, coal, oil, or and wood and are not available for building soil organic matter. [edits ok? YES] In addition, straw may be used in making bricks or used as thatch for housing or to make fences. Although it is completely understandable that people in resource-poor regions use residues for such purposes, the negative effects of these uses on soil productivity can be substantial. An important way to increase agricultural productivity in developing countries is to find alternate sources for fuel and building materials to replace the crop residues and manures traditionally used.
Using residues as mulches.Crop residues or composts can be used as mulch on the soil surface. This occurs routinely in some reduced tillage systems when high- residue-yielding crops are grown or when killed cover crops remain on the surface. In some small-scale vegetable and berry farming, mulching is done by applying straw from off- site. Strawberries grown in the colder, northern parts of the country are routinely mulched with straw for protection from winter heaving. The straw is blown on in late fall and is then moved into the interrows in the spring, providing a surface mulch during the growing season.
Mulching has numerous benefits, including:
• enhanced enhanced water availability to crops due to better infiltration into the soil and less evaporation from the soil (approximately 1/3 of water loss in dryland irrigated agriculture is from evaporation from the soil, but, which can be greatly reduced by using a surface mulch);
• weed control;
• less extreme changes in soil temperature;
• reduced splashing of soil onto leaves and fruits and vegetables (making them look better as well as reducing diseases); and
• reduced infestations of certain pests (Colorado potato beetles on potatoes or and tomatoes are less severe when these crops are grown in a mulch system.).
On the other hand, residue mulches in cold climates can delay soil warming in the spring, reduce early- season growth, and increase problems with slugs during wet periods. When it is important to get a rotation crop in early, you might consider using a low- residue crop like soybeans the previous year. Of course, one of the reasons for the use of plastic mulches (clear and black) for crops like tomatoes and melons is to help warm the soil.
Residue management is especially important in arid and semi-arid regions. In these arid and semiarid regions water is usually the most common limitation to crop yields. For winter wheat in semi-arid regions, for example, the available water at planting often foretells final yields (figure 9.1). Thus, in order to provide more available water for crops we want to use practices that help store more water in soils and keep it from evaporating directly to the atmosphere provide more available water for crops. [fix sentence DONE] Standing residue allows more snow to be maintained in the field after being deposited in the field [wording ok -- residue allows snow to be deposited? qualify as to location?CHANGED WORDING], significantly increasing available soil water in spring—in this way sunflower stalks used in this way can increase soil water by 4 to 5 inches. And a mulch during the growing season helps both to store water from irrigation or rainfall and to keep it from evaporating.
[figure 9.1 about here]
[H2]Effects of Residue Characteristics on Soil
Decomposition rates and effects on aggregation. Residues of various crops and manures have different properties and, therefore, have different effects on soil organic matter. Materials with low amounts of harder-to-degrade hemicellulose, polyphenols, and lignin, such as cover crops (especially legumes) when still very green, and soybean residue, decompose rapidly (figure 9.2) and have a shorter-term effect on soil organic matter levels than residues with high levels of these chemicals (for example, corn and wheat). Manures, especially those that contain lots of bedding (high in hemicellulose, polyphenols, and lignin), are decomposed more slowly and tend to have more long-lasting effects on total soil organic matter than crop residues or and manures without bedding. Also, cows—because they eat a diet containing lots of forages, which are not completely decomposed during digestion—have produce manure with longer- lasting effects on soils than non-ruminants, such as chickens and hogs that are fed exclusively a high-grain/ and low-fiber diet. Composts contribute little active organic matter to soils, but add a lot of well-decomposed materials (figure 9.2).
[fig. 9.2 about here]
In general, residues containing a lot of cellulose and other easy-to-decompose materials will have a greater effect on soil aggregation than compost, which has already undergone decomposition. Because aggregates are formed from by-products of decomposition by soil organisms, organic additions like manures, cover crops, and straw will usually enhance aggregation more than compost. (However, adding compost does improve soils in many ways, including increasing the water- holding capacity.)
Although it’s important to have adequate amounts of organic matter in soil, that isn’t enough. A variety of residues are needed to provide food to a diverse population of organisms, provide nutrients to plants, and to furnish materials that promote aggregation. Residues low in hemicellulose and lignin usually have very high levels of plant nutrients. On the other hand, straw or sawdust (containing a lot of lignin) can be used to build up organic matter, but a severe nitrogen deficiency and an imbalance in soil microbial populations will occur unless a readily available source of nitrogen is added at the same time (see discussion of C:N ratios below). In addition, when insufficient N is present, less of the organic material added to soils actually ends up as humus.
C:N ratio of organic materials and nitrogen availability.The ratio of the amount of a residue’s carbon to the amount of of its nitrogen influences nutrient availability and the rate of decomposition. The ratio, usually referred to as the C:N ratio, may vary from around 15:1 for young plants, to between 50:1to and 80:1 for the old straw of crop plants, to over 100:1 for sawdust. For comparison, the C:N ratio of soil organic matter is usually in the range of about 10:1 to 12:1, and the C:N of soil microorganisms is around 7:1.
The C:N ratio of residues is really just another way of looking at the percentage of nitrogen (figure 9.3). A high C:N residue has a low percentage of nitrogen. Low C:N residues have relatively high percentages of nitrogen. Crop residues usually average 40 percent% carbon, and this figure doesn’t change much from plant to plant. On the other hand, nitrogen content varies greatly depending on the type of plant and its stage of growth.
[fig. 9.3 about here]
If you want crops to growing immediately following the application of organic materials, care must be taken to make nitrogen available. Nitrogen availability from residues varies considerably. Some residues, such as fresh, young, and very green plants, decompose rapidly in the soil and, in the process, may readily release plant nutrients. This could be compared to the effect of sugar eaten by humans, which results in a quick burst of energy. Some of the substances in older plants and in the woody portion of trees, such as lignin, decompose very slowly in soils. Materials, such as sawdust and straw, mentioned above, contain little nitrogen. Well-composted organic residues also decompose slowly in the soil because they are fairly stable, having already undergone a significant amount of decomposition.