Long‐term Dryland Crop Responses to Residual Phosphorus Fertilizer

Little information is available on the long‐term effects of a single P fertilizer application on grain yields of crops grown on dryland soils deficient in plant‐available P. Therefore, duplicate plots were established in 1967 and 1968 on a Williams loam (fine‐loamy mixed, Typic Argiborolls) with a NaHCO 3 −extractable P of 6 mg P/kg soil. A split‐plot, randomized complete block design was used with three levels of available N (generally 0, 45, and 90 kg N/ha each crop year) as main plots and a one time application of P fertilizer at rates of 0, 22, 45, 90, and 180 kg P/ha as subplots. During the first crop year, application of 22, 45, 90, and 180 kg P/ha raised the average soil test P levels to 9, 12, 26, and 40 mg P/kg soil, respectively. Soil test P levels 16 yr after P fertilization and 10 or 11 crops harvested avg 5, 6, 7, 9, and 13 mg P/kg soil in 1983 for these same respective P treatments. Soil test P levels declined progressively with each additional crop year until the eighth crop or 13 yr after P application, at which time a new soil P equilibrium level appeared to be developing. Grain yields generally increased with increasing residual soil P level when adequate levels of N, either residual or applied, were present. However, no response to increasing residual soil P level was observed under annual cropping without adequate N. Accumulated grain yields after 10 or 11 crops were about three times greater with both N and P fertilization than with P fertilization alone. The study verifies that adequate available N is required to derive benefit from residual P fertilizer. The long‐term (16 yr) residual benefits from P fertilization suggest a method for possibly satisfying the P needs for several crop years in reduced and notillage systems, if the P is applied and incorporated before initiation of these tillage systems.