To a large extent the results of a farmer's efforts to get higher crop yields will be determined by the prevailing environmental conditions, i.e. by the existing complex of physical, chemical and biological factors. The possibilities of an efficient use of these factors are enlarged by our knowledge of their effects on agricultural production. Such knowledge is in first instance mostly gained from practical experience or field experiments often giving, however, only empirical information of local value.To apply local results with success to analogous problems in other regions a fundamental treatment of research data is necessary, by which the occurring phenomena and processes can be analyzed more objectively.In this paper attention has mainly been paid to the effects of the physical environmental conditions on plant development. As local research object, part of the economically important horticultural region of Geestmerambacht was taken situated in the Netherlands in the northern part of the province of North-Holland.This area is partly covered with heavy sea clay soils under insufficient water management, resulting in a one sided cropping pattern. This together with a fragmented parcelling and a poor accessibility makes cultivation of profitable intensively managed crops difficult.To investigate, in connection with an integrated land consolidation program for the region, the possibilities of improving the soil profile and the water management a groundwater level experimental field was laid out (Chapter II). On the experimental field ten different constant groundwater table depths were realized in three soil profiles, namely the original heavy clay soil and two improved clay soils: a sandy loam and a clay on sandy loam. Previous investigations on this field had the purpose of finding relationships between yield of a number of vegetable crops and mean optimum groundwater table depth during the growing season. Due to changes in weather conditions the results differed from year to year. It was therefore decided to ...