On a series of eight nitrogen side‐dressing experiments in 1950, a study was conducted to determine the effectiveness of side‐dressed nitrogen on corn as shown by yield response and by the chemical composition of the corn leaf and grain. A wide range in response to nitrogen occurred. The application of nitrogen significantly increased the percent of nitrogen in the leaf on all experiments, whereas, the nitrogen content of the grain was significantly increased on only five of the eight experiments. Phosphorus percentage in the leaf was significantly increased due to nitrogen application on certain experiments; and these increases were associated with yield responses which proved to be independent of the nitrogen effect. That a definite relationship between yield and the nitrogen and phosphorus content of the leaf existed in these experiments was shown by determining the regression of yield on the content of these nutrients. Both of the regression coefficients were significant and the multiple correlation coefficient was highly significant. When the nitrogen percentage in the leaf reached the level of approximately 2.8 or above, further increases did not appear to affect corn yields. A very striking linear relationship was found to exist between increases in yield resulting from nitrogen fertilization and associated increases in the nitrogen content of the leaf, the correlation coefficient being 0.96. The results of plant analysis suggest that phosphorus deficiency was a factor limiting yields on four of the experiments. Chemical soil tests for available phosphorus did not distinguish between five of the experiments which, however, appeared to differ in phosphorus status on the basis of phosphorus content of the leaf. In these experiments, leaf analysis provided a clearer indication of the nutrient status of the plants than did grain analysis for all three nutrients.

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.

Surface samples of 21 diverse Iowa soils representing a wide range in pH, texture, and organic matter content were studied to determine the relationships between soil urease activity and other soil properties.Simple correlation analyses showed that urease activity was correlated very highly significantly with organic C (r = 0.72***), total N (r = 0.71***), and cation‐exchange capacity (r = 0.67***). Urease activity also was significantly correlated with clay (0.53*), sand (−0.47*), and surface area (0.45*), but was not significantly correlated with pH, silt, or CaCO3 equivalent. Multiple regression analyses showed that organic matter content accounted for most of the variation in urease activity.

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The relationship between corn yields and the N and P contents of corn leaves was determined in 93 fertilizer experiments by multiple curvilinear regression. The objective was to investigate critical levels and nutrient balance. Equal yields occurred at varying concentrations of N and P in the corn leaf within certain limits. The N or P content in the corn leaf at 95% of maximum yield varied with the concentration of the other nutrient because of their significant interaction on yield. Hence, the critical N or P level is not a point nor narrow range of values but includes a wide range of values depending on how it is defined and on the level of the other nutrient in the leaf. The N‐P nutrient balance appeared to be critical only at or near the maximum yield. Nutrient unbalance may occur when the leaf N or P content extends beyond a certain limit in relation to the other one.