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THE RESIDUAL EFFECT OF K FERTILIZATION IN A HUNGARIAN CORN MONOCULTURE LONG-TERM FIELD TRIAL, 1990–1999

Hungarian agriculture is still living on the residues of previous K (and P) applications, but these reserves are becoming smaller and smaller. It is essential to know how long the previous K application provides adequate K for crop as a function of residual effects. Corn (Zea mays L.) is considered as a potassium-demanding row crop. The K-exhaustion long-term field trial with 0, 240, 480 and 960 and 1440 kg initial build-up K2O doses/ha was established in the autumn of 1989, on a light loam calcareous Mollisol, having 23% clay content, with Illite clay mineral dominance. The soil is moderately supplied K. Corn as test plant was sown in monoculture in the trial. The first 10 years are evaluated in the paper. Soil K-test/AL-K/values did not decrease in the first 6 years, but diminished in the 7–10th years. The light loam soil did not fix the added fertilizer-K strongly. Yield losses, due to K-shortage, started when AL-K values dropped below 150 mg K kg−1 soil. The 1440 kg K2O/ha initial dose provided appropriate corn yield levels for 10 years, the 960 kg dose for 6 years, the 480 kg dose for 4 years, and the 240 kg dose for 1 year, on this soil, originally moderately supplied with potassium. The residual effect of build-up K doses was measurable even after the 10th year. Weather conditions, i.e., the amount and distribution of rainfall during the vegetation period, affected strongly the magnitude of residual K effects. Both the AL-K test values, and flowering stage leaf K concentrations indicated the soil and plant K supply in the different K levels and years similarly. Residual K effects can be hindered by different reasons in the first years: due to previous K application in the experimental area, or drought period, or efficient utilization of soil-K by hybrids. Soil and plant K analyses data are useful tools in adapting the results of long-term field trials for developing fertilizer recommendations.