3. Organic nitrogen approaches new steady state under cultivation of continuous corn.

The steady state largely had been attained under virgin conditions. Cultivation made a new set of environmental conditions. Changes were made in the kinds and amounts of plant and animal residues that were returned to the soil. Tillage operations and the growth of cultivated crops altered the environment for microbiological activity. The tendency for mechanical or erosion losses increased. In consequence, the soil organic matter contents began adjustments toward new equilibrium levels consistent with the new rates of organic matter formation, mineralization, and mechanical loss.

Although the changes in some soils have been marked in the past, further declines in the future are likely to be small. The adjustments from virgin conditions, brought about by cultivation, largely have been made. Although the time necessary to reach complete equilibrium is very long, in view of the rates of change observed in longtime experiments, we can conclude that more than 90 percent of the total adjustment in the content of organic matter from the virgin condition occurs during the first 80 to 100 years of cultivation. Sometimes a steady state appears to have been established in 50 to 70 years.

WHEN THE MAJOR ADJUSTMENTS in soil organic matter accompanying present-day management have been made, some soil areas and systems of soil management will be found to have higher amounts of organic matter than others, although the differences among regions will not be so marked as they were under virgin conditions. The steady state levels will be highest in the Prairie soil regions, just as the native Prairie soils contained initially the highest quantity of organic matter of the normal, arable mineral soils.

Organic matter levels at equilibrium with current cultivation generally will be low in semiarid regions and in the Red and Yellow soils of the Southern and Southeastern States.

Heavy-textured soils will be higher in organic matter than comparable light-textured soils, partly because of higher fertility and of greater water-holding capacity and partly because of the stabilizing influence of mineral soil colloids on decomposition of the soil organic matter.

Levels of organic matter will be higher when meadow crops occur frequently in the cropping system than where row crops predominate. Because the first year of meadow makes a larger contribution to organic matter than succeeding years, the frequent use of meadow in short rotations with non-meadow crops will be more effective in maintaining organic matter than an equal proportion of meadow in longer rotation cycles.

Culture of intertilled crops, particularly those requiring extensive cultivation, will result in less organic matter in the soil than the growth of crops requiring little or no cultivation.

Soils less subject to erosion will be higher in organic matter than those susceptible to erosion.

The losses that have occurred in the contents of organic matter in soils since cultivation began--unless caused by excessive erosion were the natural consequence of the changes in rates of organic matter addition and decomposition associated with crop production. These losses of soil organic matter represent a depletion of a valuable substance, but the depletion has not been the result of a wanton misuse of soil.

The losses of soil organic matter that have occurred in the past have not resulted from any substantial difference between present-day and earlier practices of crop production and soil management. Current cropping systems and soil management, except for somewhat better control of erosion, if practiced in the past would have altered the rate of decline but little.

Decomposition of soil organic matter provided most of the nitrogen and part of the other plant nutrients that made many of our soils productive for nonleguminous crops in earlier years. In the past, the absence of this source of nitrogen would have required more dependence on fixation of nitrogen by frequently recurring plantings of legumes. With modern developments of technology and facilities for fertilizer production, we are approaching a position in which fertilizer nitrogen can substitute for a large part of the nitrogen once supplied by the decomposition of organic matter.

The maintenance of very high levels of soil organic matter is not compatible with economic use of much of our land. Maintenance at levels adequate for high productivity, however, can be continued or achieved. Some buildup in organic matter can be attained in soils that are below the steady state for the area. Good management of soils that are now above the equilibrium level will arrest the rate of the decline of organic matter.

Organic matter under cropping will be highest in fields where the largest residue returns are made to the soil and mechanical losses and microbial decomposition are kept at a minimum.

To MAINTAIN maximum amounts of organic matter in cultivated soils, the following principles should be followed:

Keep some crop growing on the land whenever circumstances permit. Meadow crops are most effective in maintaining organic matter. Inter-tilled crops are the least effective.

Use frequent short stands of meadow because they make the best use of the time the land is in meadow. The first year the land is in meadow is the most effective in the maintenance of soil organic matter as well as the fixation of nitrogen if the meadow crop is leguminous. Lesser contributions of organic matter and nitrogen result in subsequent years of meadow.

Follow good soil-management practices to produce high yields of crops. High yields permit the return of large amounts of crop residues and manures.

Return all residues to the soil. Mature residues and animal manures are most effective.

Cultivate no more than necessary, because tillage tends to hasten the biological decomposition process.

Control losses by wind and water erosion, which carries away the lighter organic materials from soil.