Summary:

In this studies, a plant water needs index was defined as an amount of rainfall, in the period of plant increasing water demands, which didn't increase
a yield of an irrigated plants. Such defined optimal rainfall was calculated on the basis of results of long-term field experiments carried out on two kinds of soil in
a region of Bydgoszcz. Irrigation needs understood as rainfall insufficiency in the period of plant increasing water demands, were calculated as a difference be-tween optimal and actual rainfall. Homogeneous rainfall data from years 1971-2005 in Mochełek (placed near Bydgoszcz) were used. Calculations of irrigation needs were made in the period of increasing water demands of the followed plants: spring cereals, medium-early potato and maize cultivated for grain. All of them were grown on two kinds of soil: a very light soil of permeable subsoil and a light soil of compact subsoil. Obtained results enabled to classify the evaluated irrigation needs. The criterions were: a number of essential individual water doses and a quantity of forecasted productive effects of the irrigated plants. Re-sults of the studies show that the irrigation needs calculated on a very light soil in the region of Bydgoszcz occurred in 87,7-97,1% (it depended on the plant). In 40,0-62,8% of the years the water needs were high and their cover by irrigation had increased significantly the agricultural production. On a light soil in the re-gion in 77,1-
-88,6% of years irrigation was considered as a suplementary (interventional), nevertheless in 14,3-25,7% of the years the water needs were high in the period of plant increasing water demand which had required at least five water doses of
irrigation. In conclusion, the plant irrigation needs in the region were character-ised by a large-scale variability in the time (the years 1971-2005) which was sta-tistically confirmed by not significant trends.