CONTRIBUTIONS OF CONTROLLABLE AND UNCONTROLLABLE FACTORS TO THE YIELDS OF WINTER-WHEAT AND WINTER BARLEY

International field trials were performed in Praha-Ruzyne (Luvisol), Ivanovice na Hane (degraded Chernozem) and at Lukavec u Pacova (Cambisol) to study the contributions of the site, year, gradated nitrogen application rates and different organic fertilization of the forecrop to the grain yield in winter wheat and winter barley in 1985-1988. The investigated factors ranked as follows, as to their contributions to yield variability: 1. site (46.6), 2. nitrogen rates (39.5), 3. year (9.9), 4. the subsequent effect of different applications of organic fertilizer (4.0 %); in the yields of winter barley the factors ranked as follows: 1. year (47.5), 2. nitrogen rate (36.6), 3. site (15.3) and 4. subsequent effect of application of organic fertilizer (0.6 %). 120 kg of nitrogen (p.n.) per ha was as sufficient amount to reach the maximum yields of both cereal species. The highest yield increments ascribable to nitrogen fertilization were recorded in less fertile brown soils at Lukavec u Pacova. The different types of organic manure applied to the forecrop had a significant effect on the yields of investigated subsequent crops (cereals) only in the case of winter wheat. On the Udalf in Praha-Ruzyne, the yields of both cereals were statistically significantly higher than on the degraded Ustoll at Ivanovice na Hane or on the Ochrept at Lukavec u Pacova.