Factors affecting the accumulation and loss of nitrogen and organic carbon in cropped soils

Data are presented showing the changes that have occurred in the nitrogen and organic carbon contents of fertility plat soils devoted to both continuous and rotative cropping at the Ohio Experiment Station. These data are analyzed and inferences drawn regarding the comparative effects of different crops and the influence of the size of crop. An attempt is made to differentiate between the effects of cultural practices and those of crop residues. The effects of manure are divided into those arising from the residues of the larger crops grown and those representing residues from the manure itself. It is estimated that a single year's cropping to the various crops has increased or decreased the organic carbon content of the soil by the following percentages of the total amount present in the soil: corn, -3.12; wheat, 1.44; oats, -1.41; hay in 5-year rotation (timothy predominating), +1.36.; hay in 3-year rotation (clover), + 3.25. The corresponding values for nitrogen are: corn, -2.97; wheat, -1.56; oats, -1.45; hay in 5-year rotation, +0.64; hay in 3-year rotation, +2.87. The amounts of both organic carbon and nitrogen in fertilized plats of the 5-year rotation experiment were found after 32 years cropping to be highly and positively correlated with the total crop production of these plats. The regressions were apparently linear and from the corresponding equations the amounts of nitrogen and carbon calculated for zero crop production were interpreted as the quantities that would have been found had the same cultural practices been employed but no residues whatever returned. The data of White for fertility plats of the Pennsylvania Experiment Station were found to show a similar relation, indicating a fairly constant contribution of organic matter per unit of crops produced irrespective of the yield. By comparing the composition of fertilized and unfertilized plat soils of the Ohio experiments, it is concluded that residues from the corn crop were of little value in conserving soil nitrogen or organic matter, those from oats were notably effective, and those from wheat intermediate in value. Of the nitrogen and organic carbon conserved during 32 years in the soil of a liberally manured plat in the 5-year rotation, it was estimated that about one-half on the limed end and three-fifths on the unlimed end was attributable to residues from the larger crops grown, the remainder being residual from the manure itself.