Influence of inorganic soil N,yield potential,and market class on irrigated spring wheat response to N

Abstract

Fertilizer N recommendations for small grains are frequently based on soil test N but data is limited for irrigated spring wheat. The relative grain yield response of irrigated spring wheat to N as affected by inorganic soil N (NO₃‐N and NH₄‐N), yield potential and market class was evaluated in thirteen Southern Idaho field experiments involving N rates. Experiments were conducted on silt loam soils from 1978 to 1986. Preplant soil NO₃‐N and NH₄‐N to a depth of 60 cm and ranging from 27 to 142 kg/ha accounted for approximately 73% of the relative yield variability. NO₃‐N and NH₄‐N were significantly correlated (r=.72). NH₄‐N with NO₃‐N did not account for more of the relative yield variability than using NO₃‐N alone.

Inorganic N in the first 30 cm and the second 30 cm were significantly correlated (r=.69) but N in the first depth increment accounted for more of the relative yield variability. The linear regression coefficient relating inorganic N in the first 30 cm to relative yield of unfertilized spring wheat was almost twice as high as the coefficient for the second 30 cm increment (.50 vs .27). Results indicate that inorganic N below 30 cm should be weighted differently than N in the first 30 cm when determining the N requirements of irrigated spring wheat.

Yield potential significantly affected the relative yield response to N. The response to N was not significantly affected by spring wheat market class (hard red vs soft white).

For estimating fertilizer N requirements, the results provide little justification for the current widespread practices of (1) using the combined NH₄‐N and NO₃‐N inorganic soil test N values when NO₃‐N alone has as much predictive value and (2) assigning equal weight to inorganic soil N at all sampling depths.