Building Soils for Better Crops, Third Edition

Organic Matter Management

SARE Outreach
Fred Magdoff and Harold van Es | 2010 | 294 pages
PDF (6.8 MB)

This title is temporarily out of print. We expect to publish an updated edition in the spring/summer of 2021.

As we discussed in chapter 3, 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). 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, 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 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 buildup 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 in the past or decrease the rate of organic matter loss from soils. In addition, practices to build organic matter 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 remain 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 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.

TABLE 9.1: Effects of Different Management Practices on Gains and Losses of Organic Matter, Beneficial Organisms, and Pests
Management PracticeGains increaseLosses decreaseEnhance beneficials (EB), Stress Pests (SP)
Add materials (manures, composts, other organic materials) from off the fieldyesnoEB, SP
Better utilize crop residueyesnoEB
Include high-residue-producing crops in rotationyesnoEB, SP
include sod crops (grass/legume forages) in rotationyesyesEB, SP
Grow cover cropsyesyesEB, SP
Reduce tillage intensityyes/no*yesEB
Use conservation practices to reduce erosionyes/no*yesEB
*Practice may increase crop yields, resulting in more residue.

Using Organic Materials

Graph illustrating winter weat grain yield and soil water relationship
Graph illustrating winter weat grain yield and soil water relationship

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 10–20% 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 root cells, as well as exudates from the roots during the season. This may actually increase the plant’s belowground 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 aboveground 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 surfacederived 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 the soil.

Some farmers remove aboveground 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.

Table 9.2: Estimated Root Residue Produced by Crops
CropEstimated Root Residues (lbs/acre)
Native prairie15,000-30,000
Italian ryegrass2,600-4,500
Winter cereal1,500-2,600
Red clover2,200-2,600
Spring cereal1,300-1,800
Corn3,000-4,000
Soybeans500-1,000
Cotton500-900
Potatoes300-600
Sources: Topp et al. (1995) and other sources.
 

Burning of wheat, rice, and other crop residues in the field still occurs, although it is becoming less common in 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, and wood are not available. 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 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, 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)
  • reduced infestations of certain pests (Colorado potato beetles on potatoes 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.

CROP RESIDUES: FUEL VS. SOIL ORGANIC MATTER

There is currently a huge effort under way to more efficiently convert structural plant material (cellulose) into fuel. As we write this, it is not commercially feasible yet—but this may change in the future. One of the dangers for soil health is that if the conversion of plant structural material (not grain) to ethanol becomes commercially viable, there may be a temptation to use crop residues as an energy source, thus depriving the soil of needed organic inputs. For example, most aboveground corn residue needs to return to the soil to maintain the soil’s quality. It is estimated that between 2 and 5 tons of corn residue are needed to maintain a soil’s favorable properties. A long-term study in New York indicated that, at least for that particular soil, modest removal of cornstalks did not cause a deterioration of soil. However, we must be very cautious when considering removing crop residue as a routine practice. As the legendary soil scientist Hans Jenny put it in 1980, “I am arguing against indiscriminate conversion of biomass and organic wastes to fuels. The humus capital, which is substantial, deserves being maintained because good soils are a national asset.”

If a perennial crop such as switchgrass is harvested to burn as an energy source or to convert into liquid fuel, at least soil organic matter may continue to increase because of the contributions of extensive root systems and the lack of tillage. On the other hand, large amounts of nitrogen fertilizer plus other energy-consuming inputs will reduce the conversion efficiency of switchgrass into liquid fuel

ABOVEGROUND CROP RESIDUES
The amount of aboveground residue left in the field after harvest depends on the type of crop and its yield. The top table contains the amounts of residues found in California’s highly productive, irrigated San Joaquin Valley. These residue amounts are higher than would be found on most farms, but the relative amounts for the various crops are interesting.

Crop Residues in the San Joaquin Valley (California)
CROPTONS/ACRE
Corn (grain)5
Broccoli3
Cotton2.5
Wheat (grain)2.5
Sugarbeets2
Safflower1.5
Tomatoes1.5
Lettuce1
Corn (silage).5
Garlic.5
Wheat (after baling).25
Onions.25
CropTONS/ACRE
Corn (120 bu.)3.5
Sorghum (80 bu.)2.5
Wheat (35 bu.) 2
Soybeans (35 bu.)less than 1
-From Various Sources

Residue management in arid and semiarid regions.

In arid and semiarid regions water is usually the most common limitation to crop yields. For winter wheat in semiarid 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. Standing residue allows more snow to be maintained in the field after being deposited, significantly increasing available soil water in spring— 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.

Effects of Residue Characteristics on Soil

Decomposition rates and effects on aggregation.

Triangle showing residue effects on soils
Triangle showing residue effects on soils

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, cornstalks and wheat straw). Manures, especially those that contain lots of bedding (high in hemicellulose, polyphenols, and lignin), decompose more slowly and tend to have more long-lasting effects on total soil organic matter than crop residues and manures without bedding. Also, cows—because they eat a diet containing lots of forages that are not completely decomposed during digestion—produce manure with longer-lasting effects on soils than nonruminants, 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).

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 waterholding 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 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.

Table 9.3: C:N Ratios of Selected Organic Materials
MaterialC:N
Soil10-12
Poultry manure10
Clover and alfalfa (early)13
Compost15
Dairy manure (low bedding)17
Alfalfa hay20
Green rye36
Corn stover60
Wheat, oat, or rye straw80
Oak leaves90
Fresh sawdust400
Newspaper600
Note: Nitrogen is always 1 in the ratios.
 

The ratio of the amount of a residue’s carbon to the amount 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 50:1 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% carbon, and this figure doesn’t change much from plant to plant. On the other hand, nitrogen content varies greatly depending nitrogen release on the type of plant and its stage of growth.

Chart illustarting nitrogen release
Chart illustarting nitrogen release

If you want crops to grow immediately following the application of organic materials, care must be taken to make nitrogen available. Nitrogen availability from 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.

Mature plant stalks and sawdust that have C:N over 40:1 (table 9.3) may cause temporary problems for plants. Microorganisms using materials that contain 1% nitrogen (or less) need extra nitrogen for their growth and reproduction. They will take the needed nitrogen from the surrounding soil, diminishing the amount of nitrate and ammonium available for crop use. This reduction of soil nitrate and ammonium by microorganisms decomposing high C:N residues is called immobilization of nitrogen.

When microorganisms and plants compete for scarce nutrients, the microorganisms usually win, because they are so well distributed in the soil. Plant roots are in contact with only 1–2% of the soil volume, whereas microorganisms populate almost the entire soil. The length of time during which the nitrogen nutrition of plants is adversely affected by immobilization depends on the quantity of residues applied, their C:N ratio, and other factors influencing microorganisms, such as fertilization practices, soil temperature, and moisture conditions. If the C:N ratio of residues is in the teens or low 20s, corresponding to greater than 2% nitrogen, there is more nitrogen present than the microorganisms need for residue decomposition. When this happens, extra nitrogen becomes available to plants fairly quickly. Green manure crops and animal manures are in this group. Residues with C:N in the mid 20s to low 30s, corresponding to about 1–2% nitrogen, will not have much effect on short-term nitrogen immobilization or release.

As residues are decomposed by soil organisms, carbon is lost as CO2, while nitrogen is mostly conserved. This causes the C:N ratio of decomposing residues to decrease. Although the ratio for most agricultural soils is in the range of 10:1 to 12:1, the different types of organic matter within a soil have different ratios. The larger particles of soil organic matter have higher C:N ratios, indicating that they are less decomposed than smaller fractions (see figure 9.4, right). Microscopic evidence also indicates that the larger fractions are less decomposed than the smaller particles.

Sewage sludge on your fields?

In theory, using sewage sludge—commonly called biosolids—on agricultural land makes sense as a way to resolve problems related to people living in cities, far removed from the land that grows their food. However, there are some troublesome issues associated with agricultural use of sludges. By far, the most important problem is that they frequently contain contaminants from industry and from various products used around the home. Although many of these metal contaminants naturally occur at low levels in soils and plants, their high concentrations in some sludges create a potential hazard. The U.S. standards for toxic materials in sludges are much more lenient than those in some other industrialized countries and permit higher loading of potentially toxic metals. So, although you are allowed to use many sludges, you should carefully examine a sludge’s contents before applying it to your land.

Another issue is that sludges are produced by varied processes and, therefore, have different properties. Most sludges are around neutral pH, but, when added to soils, cause some degree of acidification, as do most nitrogen fertilizers. Because many of the problem metals are more soluble under acidic conditions, the pH of soils receiving these materials should be monitored and maintained at around 6.8 or above. On the other hand, lime (calcium hydroxide and ground limestone used together) is added to some sludges to raise the pH and kill disease bacteria. The resulting “lime-stabilized” sludge has extremely high levels of calcium, relative to potassium and magnesium. This type of sludge should be used primarily as a liming source, and levels of magnesium and potassium in the soil carefully monitored to be sure they are present in reasonable amounts, compared with the high levels of added calcium.

The use of “clean” sludges—those containing low levels of metal and organic contaminants—for agronomic crops is certainly an acceptable practice. Sludges should not be applied to soils when growing crops for direct human consumption unless it can be demonstrated that, in addition to low levels of potentially toxic materials, organisms dangerous to humans are absent.

Application rates for organic materials.

The amount of residue added to a soil is often determined by the cropping system. The crop residues can be left on the surface or incorporated by tillage. Different amounts of residue will remain under different crops, rotations, or harvest practices. For example, 3 or more tons per acre of leaf, stalk, and cob residues remain in the field when corn is harvested for grain depending on yield. If the entire plant is harvested to make silage, there is little left except the roots.

When “imported” organic materials are brought to the field, you need to decide how much and when to apply them. In general, application rates of these residues will be based on their probable contribution to the nitrogen nutrition of plants. We don’t want to apply too much available nitrogen because it will be wasted. Nitrate from excessive applications of organic sources of fertility may leach into groundwater just as easily as nitrate originating from purchased synthetic fertilizers. In addition, excess nitrate in plants may cause health problems for humans and animals.

Sometimes the fertility contribution of phosphorus may be the main factor governing application rates of organic material. Excess phosphorus entering lakes can cause an increase in the growth of algae and other aquatic weeds, decreasing water quality for drinking and recreation. In locations where this occurs, farmers must be careful to avoid loading the soil with too much phosphorus, from either commercial fertilizers or organic sources.

Effects of residue and manure accumulations.

When any organic material is added to soil, it decomposes relatively rapidly at first. Later, when only resistant parts (for example, straw stems high in lignin) are left, the rate of decomposition decreases greatly. This means that although nutrient availability diminishes each year after adding a residue to the soil, there are still long-term benefits from adding organic materials. This can be expressed by using a “decay series.” For example, 50, 15, 5, and 2% of the amount of nitrogen added in manure may be released in the first, second, third, and fourth years following addition to soils. In other words, crops in a regularly manured field get some nitrogen from manure that was applied in past years. So, if you are starting to manure a field, somewhat more manure will be needed in the first year than will be needed in years 2, 3, and 4 to supply the same total amount of nitrogen to a crop. After some years, you may need only half of the amount used to supply all the nitrogen needs in the first year. However, it is not uncommon to find farmers who are trying to build up high levels of organic matter actually overloading their soils with nutrients, with potential negative effects on crop quality and the environment. Instead of reducing the amount of off-farm residue with time, they use a standard amount annually. This may lead to excess amounts of nitrate, lessening the quality of many plants and harming groundwater, as well as excess amounts of phosphorus, a potential surface water pollution problem.

MAINTAINING ORGANIC MATTER IN SMALL GARDENS

There are a number of different ways that home gardeners can maintain soil organic matter. One of the easiest is using lawn grass clippings for mulch during the growing season. The mulch can then be worked into the soil or left on the surface to decompose until the next spring. Leaves can be raked up in the fall and applied to the garden. Cover crops can be used on small gardens. Of course, manures, composts, or mulch straw can also be purchased.

There are a growing number of small-scale market gardeners, many with insufficient land to rotate into a sod-type crop. They also may have crops in the ground late into the fall, making cover cropping a challenge. One possibility is to establish cover crops by overseeding after the last crop of the year is well established. Another source of organic materials—grass clippings— is probably in short supply compared with the needs of cropped areas but is still useful. It might also be possible to obtain leaves from a nearby town. These can either be directly applied and worked into the soil or be composted first. As with home gardeners, market gardeners can purchase manures, composts, and straw mulch, but they should get volume discounts on the amounts needed for an acre or two

Organic Matter Management on Different Types of Farms

Animal-based farms.

It is certainly easier to maintain soil organic matter in animal-based agricultural systems. Manure is a valuable by-product of having animals. Animals also can use sod-type grasses and legumes as pasture, hay, and haylage (hay stored under airtight conditions so that some fermentation occurs). It is easier to justify putting land into perennial forage crops for part of a rotation when there is an economic use for the crops. Animals need not be on the farm to have positive effects on soil fertility. A farmer may grow hay to sell to a neighbor and trade for some animal manure from the neighbor’s farm, for example. Occasionally, formal agreements between dairy farmers and vegetable growers lead to cooperation on crop rotations and manure application.

Systems without animals.

It is more challenging, although not impossible, to maintain or increase soil organic matter on non-livestock farms. It can be done by using reduced tillage, intensive use of cover crops, intercropping, living mulches, rotations that include crops with high amounts of residue left after harvest, and attention to other erosion-control practices. Organic residues, such as leaves or clean sewage sludges, can sometimes be obtained from nearby cities and towns. Straw or grass clippings used as mulch also add organic matter when they later become incorporated into the soil by plowing or by the activity of soil organisms. Some vegetable farmers use a “mow-and-blow” system in which crops are grown on strips for the purpose of chopping them and spraying the residues onto an adjacent strip. When you use off-farm organic materials such as composts and manures, soil should be tested regularly to ensure that it does not become overloaded with nutrients.