ANALYSIS OF THE EFFECT OF DIFFERENT FORECROPS, INCREASED N-FERTILIZATION, WEATHER IN THE YEARS AND SITES ON WINTER-WHEAT YIELDS

The results of two series of polyfactorial field trials in Praha-Ruzyne: (soil type: luvisol) and in Lukavec u Pacova (soil type: dystric cambisol) obtained in the years 1985 to 1991 were evaluated from the point of view of the effects of sites, weather conditions in the years, different forecrops and three levels of N-fertilization (including mutual interactions of these factors) on the winter wheat yields. The yield variability was affected by studied factors in this order: weather conditions, forecrops, sites, N-fertilization. Significant effect of convenient forecrop was not compensated by high doses of N-fertilization to unsuitable forecrop oats. In the mean of experimental years the yields of winter wheat grain without N-fertilization after leguminous fodder lay, clover and peas on luvisol were higher by 0.08 to 1.20 t.ha(-1), on cambisol by as much as 0.72 t.ha(-1) in comparison with oats as forecrop with the application of 100 kg N.ha(-1). Highest increments of winter wheat grain yields obtained by 1 kg N in mineral fertilizers were gained on less fertile cambisol in Lukavec u Pacova after the forecrop oats (+13,9 kg). As the result of more suitable moisture conditions and high efficiency of N-fertilization on this site, the significance of lower grain yields in comparison with more fertile luvisol in Praha-Ruzyne: was not confirmed. On the basis of regression analysis and model economic calculations the most effective doses of N for winter wheat following different forecrops were determined.