Yield Response of Spring Wheat and Barley to Nitrogen Fertilizer in Relation to Soil and Climatic Factors

The relationship between yield response of spring wheat ( Triticum ( aestivum L.)sp.) and barley ( Hordeum vulgare L.) to fertilizer N and various soil and climatic factors was determined in a total of 66 field experiments over a 3‐year period using simple and multiple correlation and regression. Measured variables included precipitation and air temperature from seeding to harvest; available soil moisture and NH 4 ‐N plus NO 3 ‐N at seeding to a depth of 152 cm; N extracted with alkaline permanganate and total N to 61 cm; organic matter to 30.5 cm; and NH 4 ‐N plus NO 3 ‐N accumulated during 2 weeks incubation at 35C from samples retained field moist, frozen, and air‐dried before incubation, to 61 cm. The factors that showed significant relationships with yield response to nitrogen fertilizer were (i) stored available soil moisture at seeding to 122 cm or a dry zone, (ii) stored NO 3 ‐N to 61 cm at seeding, (iii) precipitation from seeding to 5 days before harvest and (iv) degree days above 21C from 5 to 60 days before harvest. Using stored NO 3 ‐N, stored available moisture, average growing‐season precipitation and temperature, and a calculated value of 8.40 kg of fertilizer N/ha to produce a yield response of 1 quintal, predicted N fertilizer rates were within 11.2 kg/ha of actual amounts required in over 50% and within 22.4 kg in 89% of the experiments.