With the advent of well‐fertilized corn (Zea mays L.) raonocultures with large amounts of residues returned to the soil, the question arose as to whether organic matter could be maintained at satisfactory levels in Corn Belt soils. To answer this question changes in the amounts and composition of the soil organic matter were determined in a field experiment where different types [alfalfa (Medicago sativa L.), cornstalks, sawdust, oat straw (Avena sativa L.), and bromegras(Bromus inermis Leyss)] and amounts (from 0 to 16 tons/ha/yr) plant residues were added to Marshall silty clay loam (Typic Hapludoll) for 11 consecutive years. The soil was cropped to corn and large amounts of N fertilizer were added. Organic C, N, S, and P contents of the soils increased in proportion to the amount of plant residues added. After 11 years contents of the 0‐ to 15‐cm depth of the check plots were 1.6% C, 0.15% N, 0.023% S, and 0.018% P. Average increases over the check for C, N, S, and P were 47, 37, 45, and 14%, respectively, for the 16‐ tons/ha/yr treatment. Type of plant residue when added at 8 tons/ha/yr did not influence the C or P contents of the soils differently. The organic N and S contents, however, were lower with sawdust than with the other residues. Cumulative effects of increasing quantities of organic residues on available nutrients in the soils showed that NH4‐N production, weak‐acid‐soluble P, and exchangeable K increased significantly. The amount of cornstalk residue needed to prevent loss of organic C was estimated to be 6 t/ha/yr.

The primary objective of this study was to determine how various crop species differ in their cation‐anion balance and thus in their effects on soil reaction and on the acidifying effects of N fertilizers on the base status of soils. Samples of 27 species of cereal, grass, legume, and miscellaneous field and vegetable crops were collected from a wide range of environmental conditions in the field. The plants were analyzed for total N, nitrate N, and excess‐base (EB); and calculations were made of excess‐base/organic N ratios (EB/N). Similar determinations were also made on the principal plant parts of 13 crop species. Marked differences were found in the excess‐base of the various crop species. The average EB ranged from 30 meq/100 g for wheat (Triticum aestivum L.) to 213 meq/100 g for tobacco (Nicotiana tabacum L.). In general, monocotyledons were lower in EB than dicotyledons. Likewise, crop species showed distinctive differences in EB/N ratios, ranging from 0.24 for cabbage (Brassica oleracea capitata) to 1.09 for buckwheat (Fagopyrum esculentum). Samples of a given crop species grown under a wide range of environmental conditions showed considerable range in EB and EB/N ratios. For most crop species the coefficients of variation of EB/N ranged from 10 to 20%. The principal plant parts of a given crop species showed in many instances moderate to large differences in EB and EB/N. Thus, differences in the proportion of plant parts in different samples may affect the EB and EB/N of a given species. The significance of the differences in the EB/N of various crop species in modifying the effect of N fertilizers on soil acidity is discussed.

The objective of this investigation was to determine if the N content of the grain of corn (Zea mays L.) could be used as a measure of N sufficiency of the crop for maximum yield. This was done by studying the relationship between yield, expressed as a percentage of maximum, and the N percentage in the grain. The relationship was determined from the data of 13 site‐years of six N‐rate experiments in Iowa and from data reported in the literature. Two methods, a graphical and a regression method, were developed to relate the yield of corn, expressed as a percentage of maximum, and the N percentage in the grain. Good agreement was found between the two methods for obtaining maximum yields and N percentages at various percentages of maximum yield. The average critical N percentages, or percentages at maximum yield, by the graphical and regression methods, were 1.52% and 1.54%, respectively, for the Iowa experiments. The data calculated from the experiments reported in the literature were in general agreement with the Iowa data. The standard error of the mean of the critical N percentages (graphical method) for the 13 site‐years of Iowa experiments was 0.016% and, for the 24 site‐years of the non‐Iowa experiments, it was 0.022%. Some of the variability may have been caused by differences among the 23 hybrids represented in the experiments. Moisture stress, plant density, and adequacy of other nutrients had little or no demonstrable effect on the relative yield — percent N relationship. It is concluded that the relative yield — percent N relationship offers a promising and practical basis for estimating N sufficiency and the N requirement for maximum yield, or for an economic optimum yield.

The objectives of this study were to determine the quantitative effect of symbiotic N fixation on soil acidity and to compare this effect with the theoretical amount of acidity that should be produced on the basis of the excess‐base and N content of the crops. Alfalfa (Medicago sativa L.) and soybeans (Glycine max (L.) Merr.) were grown on two soils in a greenhouse experiment. The amount of acidity that developed was determined by measuring the amounts of calcium carbonate required to bring the pH of the cropped soils back to those of the uncropped. The excess‐base and N contents of the plants were obtained by analyzing the tops, stubbles, and roots of alfalfa and the tops, roots, and nodules of soybeans. The amount of N fixed was calculated from the differences between the total N found in the crops and the nitrate accumulation that took place in the uncropped soils during the experimental period. Some information was also obtained on the uptake of N from one of the soils by growing a nonnodulating strain of soybeans. Under the intensive cropping in the greenhouse the amount of N fixed by ‘Vernal’ alfalfa, during a growth period of 167 days (3 cuttings) or by soybeans (Glycine max, T201 and T202, U. S. Regional Soybean Laboratory) during a growth period of 67 to 73 days was sufficient to lower the pH of the soils by more than 1.0 pH unit and to require the addition of up to 2000 ppm of CaCO3 to bring the soils back to their original pH. On the basis of the data obtained, N fixation from an annual yield of 10 metric tons/ha (4.6 tons/acre) of alfalfa would produce acidity in the soil equivalent to 600 kg CaCO3/ha or 535 lb/acre. Relatively good agreement was obtained between the amount of acidity developed in the soils by alfalfa and soybeans and the theoretical amounts calculated from the N and excess‐base contents of the plants. Thus, the acidity produced in soils by N fixation is of considerable significance under certain cropping conditions. It is suggested that it may also have been an important factor in soil genesis and development.

Parker for suggesting this problem and for the very helpful suggestions and criticisms rendered during the progress o£ the investigatio~i. 3Reference by number is to "Literature Cited," ~. 268.