Trends in maize grain yields in a long-term fertilizer trial

Maize (Zea mays L.) (summer) was rotated with field pea (Pisum arvense Poir.) (winter) on soil of the Hutton form (loamy, mixed, thermic Rhodic Kandiudalf) over a 50-year period. Water (W), nitrogen (N), phosphorus (P), potassium (K) and farm yard manure (M) were each applied at two levels in a 2(5) factorial experiment. The lower levels of N, P, K and M were zero applications. Simple effects for N, P and K and the main effect for M were positively correlated with yield level. Over time, the simple effect of N exhibited an increased positive trend only when pea straw was removed rather than incorporated, while simple effects of P and K increased sharply during later years. The zero fertilizer treatment yielded in excess of 2 t maize grain ha(-1) during the last decade of the trial. Manure provided consistently high yields without addition of inorganic fertilizer. This was ascribed mainly to an ample nutrient supply, as soil organic matter levels were only marginally higher than in the balanced NPK treatment, which received inorganic fertilizers. The greatest depletion of soil organic matter occurred in imbalanced treatments and the control. Threshold P and K levels in topsoil for attaining yields of 4-6 t ha(-1) were ca. 5 mg kg(-1) P and 35-40 mg kg(-1) K, respectively, in P- and K-deficient treatments. Fertilizer N, P and K were utilised more efficiently in fully balanced than in imbalanced fertilizer treatments. In treatments receiving abundant P, about 60% of potential plant-available P could not be accounted for. The legume crop apparently played a major role in the more efficient utilisation of nitrogen and phosphorus by crops in treatments where these elements were not applied.