Organic Matter in Soils

by A. G. NORMAN

SOIL ORGANIC matter is a product of its environment. Climate more than anything else determines the amount present. Climate, temperature, and rainfall together affect the rate of growth of the vegetation and consequently the amount of residues entering the soil. They affect also the activities of the micro-organisms that utilize the vegetation in the soil, and consequently the rate at which decomposition of the residues proceeds.

Some years ago H. Jenny, working at the Missouri Agricultural Experiment Station, showed that there is a clear relationship between mean annual temperature and the amount of organic nitrogen in the grassland soils of the Mississippi Valley or of the Great Plains. The lower the mean annual temperature, the higher the nitrogen and organic matter content of the soil and the wider the ratio between nitrogen and carbon. Similarly, when soils of similar origin along an annual isotherm are examined, the organic matter and total nitrogen content are found to rise with increasing rainfall and humidity.

R. H. Fuller and L. C. Wheeting more recently examined the factor of rainfall in certain prairie soils in western Washington that are relatively uniform in all climatic features except precipitation. Samples were taken from areas on which the annual rainfall varied from 16 to 120 inches. While there was generally a direct relationship between precipitation and organic matter content, there was also clear indication of a narrowing of the carbon-nitrogen ratio with a decrease in mean annual rainfall. In other words, the nitrogen content of the Organic matter is low under high rainfall and increases with less rain.

Because of the operation of climate on the renewal and decomposition of organic matter, there is for every soil a stable equilibrium value under virgin conditions that becomes unstable as soon as the land is brought under cultivation.

Ordinarily, cultivation lowers the content of organic matter by speeding up microbial processes and reducing the amount of residues of vegetation that enter the soil. Rarely is the new level higher than the old. For each land use the new equilibrium value will be slowly attained, but, by his choice of rotations and management practices, a farmer can influence the new level at which a balance is reached between the annual additions from the vegetation on the land and that used by the soil micro-organisms. That equilibrium is generally much lower than that attained under virgin conditions. The decline is not abrupt. It is not necessarily a matter for great concern. It must be accepted as inevitably accompanying the use of the land.

There are indications that in the Northern States at least the establishment of a new equilibrium level may take 70 to 100 years; it is probably true, therefore, that much of our farm land is only now approaching stability in content of organic matter. East of the Alleghenies, where the soils have been longer cultivated, and in the South, where both temperature and humidity are such that the rate of microbial utilization of the organic matter is higher, it is likely that the new equilibrium has already been attained.

Information on the effects of cropping practices on the changing level of organic matter in the soil is slowly accumulating. In Missouri, for instance, Dr. Jenny found a reduction in organic matter and nitrogen content of about one-third in 60 years of cultivation. In Pennsylvania, J. W. White and others compared the organic matter in plots that had been in a 4-year rotation of corn, oats, wheat, and mixed hay for 72 years with the organic matter in adjacent plots that had been continuously in grass for 72 years. The whole area previously had been farmed for at least 30 years. The differential that developed in the 72 years was substantial. The unfertilized grassland was found to have a content of organic matter 61 percent above that of the unfertilized cropped plots. Applications of complete commercial fertilizer or manure and lime to the grass caused only a slightly higher equilibrium level to be reached. Similar treatments applied to the cropped plots, however, did reduce the differential to 41 and 23 percent, respectively. The carbon-nitrogen ratios of the cultivated plots and the grass plots were quite similar; where any difference occurred those of the cultivated plots were wider.

The cropping system affects not only the final equilibrium value but also the rate of decline to this value. Under conditions at Manhattan, Kans., a 16-year rotation (including alfalfa for 4 years), a 3-year rotation ( corn, soybeans, wheat), and continuous wheat were accompanied by similar losses of nitrogen and carbon. The average annual losses of carbon from the soil under those rotations were respectively 0.59, 0.68, and 0.56 percent. Continuous alfalfa, on the other hand, increased the supply of soil nitrogen and organic matter at the rate of 0.71 and 0.43 percent per year, respectively. The depletion during the corn years in the rotations was at a rate two to three times as great as under continuous wheat. In a 30-year period the loss in organic matter on Webster and Clarion soils in Iowa was greater under continuous corn or a corn-oats rotation than in longer rotations. The decreases in organic matter and nitrogen where manure had been applied consistently were much less, even than where all crop residues were returned.

Under more arid conditions microbial activities may sometimes be impeded. Comparisons were made of the changes in organic matter within a 22-year period at several places in Kansas. The losses were relatively low (about 0.5 percent a year), under systems of continuous grain, or alternate grain and fallow, but were appreciably larger when row crops were grown, or row crops and fallow were alternated.

Again the experience was that manure offset in part the depletion of soil carbon and nitrogen. Twelve tons an acre every third year cut losses by 50 percent or more, without concurrently increasing crop yields.

Irrigation adds another factor, in that the moisture status of the soil is completely changed. This might be expected to speed up microbial processes. Studies in Utah over 20 years revealed great differences in the changes in organic matter in response to cropping systems. Where alfalfa or beets were grown continuously, the latter with a manure application equal to 30 tons an acre annually, organic matter increased about 0.7 ton an acre yearly. At the other extreme, under continuous fallow or continuous oats, the annual loss was 1.6-1.7 tons of organic matter, which was halved in an alternate oats-fallow system. Sugar beets without manure caused an annual loss of 0.5 ton, but the application of 10 tons of manure annually reduced the loss only to 0.2 ton a year.