INTRODUCTION

Fifty per cent, more or less, of the soil organic matter from most cultivated lands has been lost. The remainder is perhaps more resistant to loss and therefore is stable but that which has been lost was perhaps the most important half--it resisted erosion, it made soils permeable, it increased water-holding capacity and it produced healthy plants. The 50 per cent that has been lost is via two major mechanisms. One is loss per unit weight of soil by decomposition (mineralization) induced by cultivation, and the other is loss by erosion--loss by wash away and blow away of the surface soil which contains the most soil organic matter.

The loss of organic matter has important implications for landscaping work. Soil organic matter is the component of mineral soils that makes it possible for successful growth of most plants. Soil organic matter adds water-holding capacity and cation-exchange capacity to sandy soils. It adds structure, stability and permeability to soils high in clay. For all soils it is a source of nutrients, usually slow release, as organic matter is decomposed. Few if any plants could grow in clay soil without at least a modest amount of soil organic matter, and it has been a great mistake in the past to think that management of plant residues and soil organic matter for conservation is not important.

In Australia it is stated that six times as much productivity is lost due to loss of soil structure than due to more visible soil salinity. Compaction and many other soil problems are due primarily to loss of soil organic carbon. Decreasing percent of soil organic carbon was a better indication of soil degradation than was tons of soil lost by erosion.

We know at least 20 reasons why soil organic matter is important: It stops soil erosion, it supplies nutrients, it is a buffer against pH change, it holds water, it increases the cation exchange capacity which protects against leaching loss of nutrients, it decreases compaction, it stores nutrients from season to season, it makes soil warmer in the spring, it makes soils easier to till especially when slightly too wet, it makes inputs more valuable, it protects against plant diseases, it gives better aerated more permeable soil, it protects against heavy metal and salt toxicities, it detoxifies pesticides and prevents their leaching, it is a storage mechanism for excess atmospheric CO2 to decrease global warming, it gives high yields, it promotes microbial breakdown of toxic substances, it makes it possible to grow acid loving plants, it supports microorganisms that recycle nutrients, and it promotes soil formation.

Soil that has never been tilled can easily contain from 5 to 10 per cent soil organic matter, especially in non-acid areas. Continued cultivation can rapidly deplete these levels. Repeated stirring of the soil mixes in extra air and promotes oxidation or burn-out of the soil organic matter. Unless properly managed, adding supplies of organic matter under some conditions can actually "fuel the fire" and result in little or no addition to the supply of soil organic matter.

MORE VALUES ATTRIBUTED TO SOIL ORGANIC MATTER

Some of the values of soil organic matter include serving as an energy source for soil microorganisms and other organisms. Any list is not all inclusive. Soil organic matter is essential to the chemical, physical and biological health of the soil; it is essential to sustainable growth. Youngberg edited the special issue on soil quality published by the American Journal of Alternative Agriculture. One major focus was that the biological health of the soil has been neglected with more emphasis having been given to chemical health first and physical health second. Sustainable agriculture requires emphasis on all three.

There are many very compelling reasons for increasing levels of soil organic matter. The more soil organic matter that can be accumulated, the less there will be problems such as soil erosion, inefficient use of water, inefficient use of plant nutrients that may end up in groundwaters, and poor physical properties of soil that cause poor plant growth. Levels of soil organic matter decrease rapidly with cultivation unless steps are taken to prevent loss. Present levels are much lower generally than when soils were first used for cultivation. In certain areas, 100 and more years of growing the same plant continuously decreased soil organic matter levels to much less than half of previous levels. Resulting effects include decreased productivity of the land and severe wind and water erosion of the land. Today's concern for the environment asks the pertinent questions: Is it not time to reverse this trend? And can it be done?

Use of cover and green manure crops can help improve levels of soil organic matter. It has been reported that the turning under of a green-manure crop increases the content of organic matter and decrease the rate of erosion. A plot of corn without stable or green manure lost 77.4 tons of soil, while a corresponding plot on which tall sweet clover had been turned under lost only 12.5 tons. The authors of that work add, "The addition of organic matter in the form of green manure, stable manure, etc. considerably increases the (water) infiltration rate."

LOSS OF SOIL ORGANIC MATTER

The author prepared for a class a statement concerning the history of the productivity of the Texas Blacklands. Part of that report is repeated here because of its relevance for today. "What has happened to the Blacklands area, the waxy Blacklands of Texas? We are familiar with the stories of the fabulous crop production of this area of quite a number of years ago. Old-timers said there was no limit to the richness of this soil, and year after year of row crops without attention to return of organic matter to the soil reflected their attitude. What was the situation after 100 years?

Here is the Texas Research Foundation's analysis: 'the Blacklands of Texas occupy 26 million acres of land, extending as a belt of variable width from the Red River on the northern state border, across the central part of the state, almost to Brownsville at the southern tip. Until about 1900, practically all of the agriculture of Texas was found in these prairies, and a long list of towns and cities grew up in this area. The soils of this great Blacklands region have lost much of their fertility in the course of 50 to 100 years of continuous cropping to cotton, corn and small grains, and the entire area now is producing about one-half as much total harvested crops as was produced in the 1920s.

The soils have lost a major portion of the original soil humus; the soil structure has deteriorated with low capacity to absorb and retain moisture, and is difficult to till; the available supply of the major plant nutrients is at a low level; and most of the broadleafed plants suffer from inadequate supplies of trace elements--boron, iron, copper, manganese, zinc and perhaps other elements.

Besides low yields and nutrient deficiencies, the effect of loss of soil organic matter includes extreme sensitivity to rain-drop erosion and tendency for crops to wilt under slight water stress.

The soil organic matter content of Houston black clay had decreased from 5.2 to 2.8 per cent in the first 6 inches of soil. For a soil high in clay, this is devastating so far as the physical properties are concerned, relating to such things as water penetration into the soil and oxygen and carbon dioxide movement in the soil. Soils such as these are easy to compact, especially when cultivated or plowed when too wet. The loss of the soil organic matter has hastened the process. Studies found with the Houston black clay that virgin soil (never cultivated) had 50 to 100 per cent more air space at 30 cm tension than soil cultivated for 50 to 90 years.

The land can be improved. In order to maintain productivity it is essential that all crop residues produced in a farming system be returned to the soil. The residues should never be burned or otherwise removed from the land." He found that the addition of at least 4.0 MT/ha (3600 pounds/acre) of crop residue a year was necessary to maintain the level of soil organic matter. To achieve this level or to exceed it, it was necessary to fertilize crops with nitrogen and phosphorus fertilizers. Barnyard manure and crops in a rotation helped maintain the organic matter. Grasses in general are exceptionally suited not only to providing sufficient crop residue to maintain the level of soil organic matter, but also to maintain and increase the aggregation of soil particles into the larger sizes which are conducive to tilth and productivity. The value of nitrogen fertilizer in protecting against loss of soil organic matter cannot be overemphasized.

A SEVERE NATIONAL AND WORLD PROBLEM

The conservation of soil organic matter has caused great national excitement. The Soil Conservation Service, now the National Resource Conservation Services, was started in 1936. Progress needs to continue. As consequence of this loss in organic matter, the soil structure was modified to an extent that might be represented by reducing the number of granules that were the size of particles of sand by 11 per cent and increasing the number that were the size of clay particles by 5.5 per cent." The loss of organic matter represents soil compaction, which hampers the circulation of air and water and hinders tillage operations at the same time that the function of the soil in plant nutrition is disturbed. Thus in but 60 years, more than one-third of the organic matter, representing centuries of accumulation, was destroyed, and the efficiency of the soil for crop production was reduced."

And, "The nation should be made aware of the rapid rate at which the organic matter in the soil is being exhausted. Practices should be adopted that will at least maintain, and in as many cases as possible even increase, the supply of this natural resource in the soil. The maintenance of soil organic matter might well be considered a national responsibility."

CAN SOIL ORGANIC MATTER LEVELS BE MAINTAINED?

In general, the more clay in a soil, the greater the need for more soil organic matter. Soil texture appears to impose both an upper and a lower limit under a given climate to maintain soil organic matter. Bear and Price said, "By good management, the organic matter content of a soil that has a normal value of 2 per cent can be raised fairly readily to 2.5 per cent, but any further rise will be difficult to effect. Under poor management, it may fall as low as 1.5 per cent, but further loss of organic matter will be very slow." We hope that good research can show how to overcome this upper-limit barrier.

Soil organic matter levels depend partly upon vegetation as well as other factors. It varies widely among the great soil groups. A vast belt of black soil is found in a region that extends southward from the Dakotas to Texas. The native vegetation in that region was short prairie grasses. These grasses grew luxuriantly during late fall and early spring, but the summers were so dry that the accumulated organic matter decomposed relatively slowly. Over the centuries, large amounts accumulated sometimes from 6 to 10 per cent in the top foot or more of soil. Higher rainfall areas climatic conditions favor luxuriant growth of trees, but they also favor rapid decomposition of the organic matter derived from the trees. In addition, heavy rains leaches much more nitrogen and mineral matter from the soil than in the black-soil areas farther west to give lower soil organic matter.

The organic matter content of a farmed soil fluctuates around a normal level for that soil. If the soil is sandy, this level will usually be relatively low. In proportion as there is more clay in the soil, the content of organic matter tends to be greater. Soil in low-lying areas contains much more organic matter than does soil in surrounding higher areas. Under conditions of very poor drainage, deep deposits of peat may accumulate.

Conservation tillage and soil amendments can result in increases in the soil organic matter. In arid regions, irrigation and cropping can provide more plant residues from which soil organic matter can be developed by soil microorganisms. In New Jersey, liming, fertilization, vetch winter cover crop, barnyard manure allover a long time period increased the soil organic matter in an additive manner. The first step in restoring soil organic matter is to apply fertilizer and lime or gypsum, if necessary, to the soil. Nitrogen is needed to build stable soil organic matter as well as to properly grow plants. Without proper management, these two activities can be mutually exclusive. With good management, better soil and plant growth can be accomplished simultaneously.

With the present national concern for keeping nitrogen fertilizer from reaching groundwaters, there is a tendency to withhold nitrogen because the soil organic matter will supply some. This is a mistake. Soils generally contain from 2,000 to 5,000 and sometimes more pounds per acre top six inches of nitrogen in the soil organic matter. It is best kept as soil organic matter. If a farmer looks to the stable soil organic matter for much of the plant nitrogen needs, the soil will be mined of both nitrogen and soil organic matter. About two percent of this nitrogen will be mineralized in a year if the mineral nitrogen levels in soil are low. This will give from 40 to 100 or more pounds per acre of available nitrogen per year which could supply an entire crop. A fallacy in letting this happen is that, simultaneously, from 400 to 1,000 or more pounds per acre of carbon would also be lost which represent from 700 to 1,700 pounds per acre of precious soil organic matter. It is easy to see that mining a soil for 50 to 100 or more continuous years can deplete the soil of most of its non-passive soil organic matter and nitrogen. This has been done all too much in the past and is a trap that many organic and conventional farmers get into. Poor soil and erosion always results when this happens.

If soil organic matter is to be increased for the good of the land which, indirectly, can improve plant production, attention to its biological mechanisms is essential. If stable soil organic matter has a stable carbon:nitrogen ratio, it will take both carbon and nitrogen to build up the supply of soil organic matter. The carbon can be grown as plants or waste sources as amendments can be applied. If the residual sources have carbon:nitrogen ratios of 20, 30, 50, or 100, soil microorganisms, if managed, may proceed to fix nitrogen from the air so that they can use the carbon sources to grow some microbes. When crops are not being grown this is an excellent way to increase levels of soil nitrogen with possibility of increasing levels of soil organic matter. If overused, however, the procedure can cause nitrogen deficiencies in plants, so the management must be careful.

Until such time as there are adequate supplies of soil organic matter to create soil with excellent physical and biological properties, the use of water-soluble polymers (PAM) as soil conditioners can help tremendously. Small amounts of the polymers enhance pore space in soil when properly applied and the combination of some organics with the polymers results in synergistic improvement of plant quality on most soils. A rational program would be to spread the available supply of organic amendments over all the land at low rates like 1, 2, 3, 4, 5, tons per acre and additionally supply very low levels of the soil conditioner. Needed research is being done. Available supplies of organic matter can be magnified this way. Lime or gypsum or both are important for organic matter and PAM to be effective.