The Relationship Between Soil Tests for Available Nitrogen and Nitrogen Uptake by Various Irrigated Crops in the Western States

Soil tests for available N were evaluated by determining the relationship between N uptake from a non‐N fertilized plot, as a direct measure of the amount of N available to a crop, and available N as measured by various laboratory methods. Field experiments were conducted with four irrigated crops, corn ( Zea mays ), sugar beets ( Beta vulgaris L. ), sorghum ( Sorghum vulgare ), and cotton ( Gossypium hirsutum ) on neutral to calcareous soils in California, Nebraska, or Washington. Initial nitrate N, nitrifiable N released on incubation for 2‐ and 4‐week periods, and total N in soil samples from 0‐ to 6‐ and 6‐ to 12‐inch depths were combined to give 14 measures of available N for correlation with N uptake. Highly significant correlations were obtained between N uptake from no‐N plots and all soil test values, with the exception of NO 3 ‐N released on incubation for 4 weeks. Nitrogen uptake by corn was most highly correlated with initial NO 3 ‐N, 6–12 inches; initial NO 3 ‐N plus N released on incubation for 2 weeks; and total N, 0–12 inches. The coefficient of determination for these relationships indicated that variations in soil test values were associated with approximately 50% of the variation in N uptake from no‐N plots. Combining the effects of initial NO 3 ‐N and total N in a multiple‐regression analyses slightly improved the prediction of N uptake by corn and sugar beets growing in Nebraska. With cotton growing on desert soils low in organic matter, initial NO 3 ‐N, 0–12 inches, was most highly correlated with N uptake, and combining the effect of initial NO 3 ‐N and total N did not improve the prediction of N uptake. The data for corn indicated that N test correlations for crops following legumes should be established separately.