On the basis of exact field experiments from a Station of Cultivar Valuation in north-east Poland, performed in 1985-2004, the authors elaborated, separately for good wheat and very good rye complexes, a model of the relation between yield of spring wheat and the following factors: number of days with precipitation in the period March-July, rainfall in the months March-April, May, June, July, soil richness in phosphor and potassium, nitrogen fertilization and year of investigation. The investigation showed that the precipitation factor has a significant effect on the size of yield. On soils of good wheat complex, the rainfall conditions that favoured high yields of spring wheat were: large number of days with rainfall in the period March-July (90 days), relatively low rainfall in March and April (42 mm), high rainfall in June (96 mm) and low in July (34 mm). On soils of very good rye complex, the decisive factor for high yields was a high number of days with rainfall in the period March-July (100 days), a relatively low rainfall in March and April (42 mm) and a higher rainfall in March (80 mm) and June (100 mm). A large effect was found on yielding of spring wheat of soil richness in phosphor and potassium and also nitrogen fertilization, the reaction of yield to those factors being greater on soils of very good rye complex than good wheat complex.