The relationship between yield response of spring wheat (Triticum (aestivum L.)sp.) and barley (Hordeum vulgare L.) to fertilizer nitrogen 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 NH4-N plus NO3-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 NH4-N plus NO3-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 stored available soil moisture at seeding to 122 cm or a dry zone; stored NO3-N to 61 cm at seeding; precipitation from seeding to 5 days before harvest and degree days above 21C from 5 to 60 days before harvest. Using stored NO3-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.
