Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 8

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

Building high-quality soils 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?

There are three different general management approaches to enhancing soil health. 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.

It is difficult to be sure exactly why problems develop when organic matter is depleted in a particular soil. 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). Close to 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 humus can continually accumulate. This does not mean that organic materials must be added to each field every year. However, it does mean that a field cannot go without additions of organic residues 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.

Soil Organic Matter Management Strategies

Increase Additions of organic residues to soils.
Use varied sources of organic materials.
Decrease losses of organic matter from soils.

There are three 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, be sure 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, implement practices that decrease the loss of organic matter from soils because of accelerated decomposition or erosion.

Raising and maintaining soil organic matter levels. It is not easy to dramatically increase the organic matter content of soils or even to maintain good levels once they are reached. Improving organic matter content requires a sustained effort that includes a number of approaches to return organic materials to soils and minimize soil organic matter losses. It is especially difficult to raise the organic matter content of soils that are very well aerated, such as coarse sands, because added materials are decomposed so rapidly. Soil organic matter levels can be maintained with less organic residue in high clay-content soils with restricted aeration than in coarse-textured soils.

All practices that help to build organic matter levels do at least one of two things -- add more organic materials than was done in the past or decrease the rate of organic matter loss from soils (table 8.1). 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 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 9 through 15, we will evaluate management options and issues associated with their use.

Table 8.1 Effects of different management practices on gains and losses of organic matter

How much organic matter is enough? Unlike the case with plant nutrients or pH levels, there are no accepted guidelines for organic matter content. We do know some general guidelines. For example, 2 percent organic matter in a sandy soil is very good, but in a clay soil, 2 percent indicates a greatly depleted situation. The complexity of soil organic matter composition, including biological diversity of organisms as well as the actual organic chemicals present, means that there is no simple interpretation for total soil organic matter tests.

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. For a crop of corn, roots may account for over a ton of dry weight per acre (thus more than 4 ½ tons of surface residues plus roots about 60 percent of the total plant remain following a Midwest grain harvest of about 120 bu. per acre). The estimated root residues (from Prince Edward Island in Canada) give some idea of the differences that you might find (table 8.2).

Some farmers remove above ground residues 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 renewed interest in using crop residues as a wood substitute to make a variety of products, such as particleboard. This activity could cause considerable harm because residues are not returned to soils.

Crop Residues

The amount of residue left in the field after harvest depends on the type of crop and its yield. The table on the left 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 Residue in the
San Joaquin Valley (California)

Residues of Common Crops in the
Midwest and Great Planins

Crop	TONS/ACRE
Corn (grain)	5
Broccoli	3
Cotton	2.5
Wheat (grain)	2.5
Sugarbeets	2
Safflower	1.5
Tomatoes	1.5
Lettuce	1
Corn (silage)	.5
Garlic	.5
Wheat (after baling)	.25
Onions	.25
-- Mitchell et. al., 1999

Crop	TONS/ACRE
Corn (120 bu.)	3.5
Sorghum (80 bu.)	2.5
Wheat (35 bu.)	2
Soybeans (35 bu.)	less than 1
-- From various sources

Burning of wheat, rice, and other crop residues in the field is a common practice 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. There, crop residues and manures frequently serve as fuel for cooking or heating when gas, coal, oil, or 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 alternative 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 a 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.

enhanced water availability to crops (better infiltration into the soil and less evaporation from the soil);
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 beetle on potatoes is less severe when potatoes 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. 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.

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 hard-to-degrade hemicellulose and lignin, such as cover crops when still very green and soybean residue, decompose rapidly (figure 8.1) 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 and lignin), are decomposed more slowly and tend to have more long-lasting effects on total soil organic matter than crop residues or manures without bedding. Also, cows because they eat a diet containing lots of forages, which they do not completely decompose have manure with longer lasting effects on soils than non-ruminants, such as chickens and hogs, that are fed exclusively a high-grain/low-fiber diet. Composts contribute little active organic matter to soils, but add a lot of well decomposed materials (figure 8.1).

Figure 8.1 Different types of residues have varying effects on soils (thicker lines indicate more material, dashed line indicates very small percent of that type). Modified from Oshins, 1999.

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 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 is needed to provide food to a diverse population of organisms, 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 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 to 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 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 8.2). A high C:N residue has a low percentage of nitrogen. Low C:N residues have relatively high percentages of nitrogen. Crop residues are usually pretty close to 40 to 45 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.

Figure 8.2 Nitrogen release and immobilization with changing nitrogen content. Based on data of Vigil and Kissel, 1991.

If you want crops 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.

Mature plant stalks and sawdust that have C:N over 40:1 (table 8.3) may cause temporary problems for plants. Microorganisms using materials containing 1 percent 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 to 2 percent of the entire 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, temperature, and moisture conditions. If the C:N ratio of residues is in the teens or low 20s, corresponding to greater than 2 percent nitrogen, then 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 of residues. Residues with C:N in the mid-20s to low 30s, corresponding to about 1 to 2 percent nitrogen, will not have much effect on short-term nitrogen immobilization or release.

Sewage sludge on your fields? In theory, the use of sewage sludges on agricultural lands 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 other industrialized countries and they 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 are 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 need to be 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.

C:N Ratio of Active Organic Matter

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 C:N ratio for most agricultural soils is in the range of 10 to 12:1, the different types of organic matterwithin a soil have different C:N ratios. The larger particles of soil organic matter have higher C:N ratios, indicating that they are less decomposed than smaller fractions. Microscopic evidence also indicates that the larger fractions are less decomposed than the smaller particles.

$figure 8.3 C:N ration of different size fractions of organic matter$

Figure 8.3 C:N ratio of different size fractions of organic matter.
Magdoff, F., unpublished data, average for three soils.

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, three or more tons per acre of leaf, stalk, and cob residues remain in the field when corn is harvested for grain. 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 ground- water 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 these locations, 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 percent 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 each year. After some years, you may need only half of the amount used to supply all the nitrogen needs in the first year.

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 air-tight 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, 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 where crops are grown on strips for the purpose of chopping them and spraying the residues onto an adjacent strip.

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. Also, leaves can be raked up in the fall and applied to the garden. Cover crops can also be used on small size 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 over-seeding after the last crop of the year is well established. Another source of organic materials grass clippings are probably in short supply compared with the needs of cropped areas, but are 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 composted first. As with home gardeners, market gardeners can purchase manures, composts, and straw mulch, but should get volume discounts on the amounts needed for an acre or two.

The role of diversity is critical to maintaining a well functioning and stable agriculture. Where many different types of organisms coexist, there are fewer disease, insect, and nematode problems. There is more competition for food and more possibility that many types of predators will be found. This means that no single pest organism will be able to reach a population high enough to cause a major decrease in crop yield. We can promote a diversity of plant species growing on the land by using cover crops, intercropping, and crop rotations. However, don't forget that diversity below the soil surface is as important as diversity above ground. Growing cover crops and using crop rotations help maintain the diversity below ground, but adding manures and composts and making sure that crop residues are returned to the soil are also critical for promoting soil organism diversity.

Managing Soils and Crops to Minimize Pest Problems

Many of the practices discussed in this chapter and the other chapters in Part 2 help to reduce the severity of crop pests. It is now known that plants have very sophisticated defense mechanisms against insects and diseases. When plants are under environmental stresses caused by compact soils, droughty conditions, or excess nitrogen, they are less able to combat pests and may be even more attractive to them. On the other hand, healthy plants growing on soils with good biological diversity are able to mount a strong defense against many pests. For example, when attacked by insects they may emit chemicals that attract beneficial insects that are predators of the pest. In addition, good soil management decreases levels of pests that live in the soil.

It is well established and known by most farmers that crop rotation can decrease disease, insect, nematode, and weed pressures. A few other examples are given below.

Insect damage can be reduced by avoiding excess nitrogen levels in soils through better nitrogen management.
Root rots and severity of leaf diseases can be reduced with composts that contain low levels of available nitrogen, but still have some active organic matter.
Fungal diseases of roots and insect damage are decreased by lessening soil compaction. 3 Many pests are kept under control by competition for resources or direct antagonism (including the beneficials feeding on them). Good quantities of a variety of organic materials help maintain a diverse group of soil organisms.
Root surfaces are protected from fungal and nematode attack by high rates of beneficial mycorrhizal fungi. Most cover crops help keep mycorrhizal fungi spore counts high and promote higher rates of infection by the beneficial fungi.
Parasitic nematodes can be suppressed by cover crops.
Residues of some cover crops, such as winter rye, reduce weed seed germination.
Weed seed numbers are reduced in soils with a lot of biological activity, with both microorganisms and insects helping the process.

Developing and maintaining an optimum physical environment. Plants thrive in a physical environment that allows roots to actively explore a large area, gets all the oxygen and water needed, and maintains a healthy mix of organisms. Although the soil's physical environment is strongly influenced by organic matter, the practices and equipment used from tillage to planting to cultivation to harvest have a major impact. If a soil is too wet whether it has poor internal drainage or it receives too much water some remedies are needed to grow high yielding and healthy crops. Also, erosion whether by wind or water is an environmental hazard that needs to be kept as low as possible. Erosion is most likely when the surface of a soil is bare and doesn't contain sufficient medium- to large-size water-stable aggregates. Practices for management of soil physical properties are discussed in chapters 13 to 15.

Many of the practices that build up and maintain soil organic matter also help enrich the soil with nutrients or make it easier to manage nutrients in ways that satisfy crop needs and are also environmentally sound. For example, a legume cover crop increases a soil's active organic matter and reduces erosion, but it also adds nitrogen that can be used by the next crop. Cover crops and deep-rooted rotation crops help to cycle nitrate, potassium, calcium, and magnesium that might be lost to leaching below crop roots. Importing mulches or manures onto the farm also adds nutrients along with the organic materials. However, specific nutrient management practices are needed, such as testing manure and checking its nutrient content before applying additional nutrient sources. Other examples of nutrient management practices not directly related to organic matter management include applying nutrients timed to plant needs, liming acidic soils, and interpreting soil tests to decide on the appropriate amounts of nutrients to apply (see chapters 16 to 19). Development of farm nutrient management plans and watershed partnerships also improve soil while protecting the local environment.

Sources
Barber, S.A. 1998. Chemistry of soil-nutrient interactions and future agricultural sustainability. In Future Prospects for Soil Chemistry (P.M. Huang, D.L. Sparks, and S.A. Boyd, eds.). SSSA Special Publication No. 55. Soil Science Society of America. Madison, WI.

Brady, N.C., and R.R. Weil. 1999. The Nature and Properties of Soils. 12th ed. Macmillan Publishing Co. New York, NY.

Cavigelli, M.A., S.R. Deming, L.K. Probyn, and R.R. Harwood (eds.). 1998. Michigan Field Crop Ecology: Managing Biological Processes for Productivity and Environmental Quality. Michigan State University Extension Bulletin E-2646. East Lansing, MI.

Mitchell, J., T. Hartz, S. Pettygrove, D. Munk, D. May, F. Menezes, J. Diener, and T. O'Neill. 1999. Organic matter recycling varies with crops grown. California Agriculture 53(4):3740.

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, and K.T. Webb. 1995. Changes in Soil Structure. In The Health of Our Soils: Toward Sustainable Agriculture in Canada (D.F. Acton and L.J. Gregorich, eds.). Center 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