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The Relationship Between Soil Tests for Available Nitrogen and Nitrogen Uptake by Various Irrigated Crops in the Western States¹

ABSTRACT

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 NO₃-N released on incubation for 4 weeks. Nitrogen uptake by corn was most highly correlated with initial NO₃-N, 6–12 inches; initial NO₃-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 NO₃-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 NO₃-N, 0–12 inches, was most highly correlated with N uptake, and combining the effect of initial NO₃-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.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Research Soil Scientists, USDA, Riverside, Calif., Brawley, Calif., and Ft. Collins, Colo., respectively.

Received for publication January 21, 1966. Accepted for publication February 28, 1966.