| Two experiments were conducted on a Cecil sandy loam to determine the effect of P fertilization on N(,2) fixation (acetylene reduction) in soybeans and on the nutrient status and yield of double-cropped wheat and soybeans. In Exp. I, P was applied in 1977 and the effects of residual soil P were evaluated in 1980 and 1981 under conservation tillage management. In Exp. II, time and rate of P fertilization were evaluated for double-cropped wheat and soybeans under conservation and conventional tillage.;Results in Exp. I indicate that a one-time application of 128 kg P/ha can maintain soil P levels in the medium range (11 to 20 ppm) for three years in a soil with a high P-fixing capacity such as a Cecil sandy loam. Application of 256 and 384 kg P/ha maintained soil P levels in the medium to high range (21 to 37 ppm) for four years after application. The 128 kg P/ha treatment resulted in soybean yields equal to or greater than two annual P rates in both years of the study; however, soil P may have been a limiting factor in wheat yields in 1981, since all residual P soil treatments resulted in lower yields than either of the two annual P applications. There was a curvilinear relationship between N(,2) fixation in soybeans and soil P at the V9 and R2 stages of growth. Nitrogen fixation rates increased rapidly as soil P increased from 2 to 15 ppm, but increased little from 15 to 39 ppm. There was a linear relationship between N(,2) fixation rates and total plant N, with highest fixation rates giving the greatest amount of total N.;In Exp. II, average soil P levels were greater under conservation tillage (31.8 ppm P) than conventional tillage (21.0 ppm P). Plant P levels were greater in soybeans grown under conservation tillage (0.28% P) than under conventional tillage (0.25% P) at the V9 stage of growth. Phosphorus application did not affect soybean yields, but yields were increased by conservation tillage in 1981. Applied P significantly increased N(,2) fixation only at the V9 stage of growth in 1981. There was, however, a significant P x tillage interaction at podfill (R5) in both years, with greater N(,2) fixation with conservation tillage. In 1980, wheat yields were greater with conventional tillage; but in 1981, yields were greater with conservation tillage than conventional tillage. Time of P application did not significantly affect N(,2) fixation in soybeans or yield of soybeans or wheat. |
