NxP AND NxPxK INTERACTIONS IN RELATION TO YIELD, WEED COVER AND FUNGI INFECTION IN SOYBEAN

The experimental calcareous chernozem loamy soil had in its ploughed layer 3% humus, around 5% CaCO3and 20% clay and it was originally found to be poorly supplied with P and Zn, moderately well supplied with N and K. The trial involved 4N × 4P = 16 × 4K = 64 treatments with 2 replications giving a total of 128 plots. The groundwater depth was 13-15 m and the area was prone to drought. The fertilizers were applied in the form of calcium ammonium nitrate, superphosphate and potassium chloride. In the case of excessive NxP fertilisation the weed cover increased from 5 to 21%, while the soybean cover dropped from 78% to 56%. In this treatment the air-dry weed shoot mass on August 15thwas 4 times that on the control plots, reaching a value of 2.5 t ·ha-1, chiefly due to the spread ofChenopodiumandAmaranthusspecies. The degree of infection withMacrophonia phaseolinaat harvest was 100% on the withered plants growing on unfertilized soil, while this value was below 5% with maximum NxPxK nutrition, where some of the plants were still green. The late ripening induced by overnutrition was responsible for the resistance toMacrophonia, since forced physiological ripening did not occur even in a dry year.