A wheat crop growth model (Singels, 1983) was refined and used to calculate annual grain yield from weather data for 45 seasons in the Bloemfontein area. Five different levels of profile-available water capacity were used as soil input parameter. Other input parameters were kept constant at optimum levels. The average yield and gross margin were calculated for the different hypothetical soil types as were probabilities for positive gross margins. Results indicated that successful wheat production is possible on soils with a profile-available water capacity between 92 mm and 112 mm. Below 92 mm the low water storage capacity of soils limits yields. Soils with an available water capacity of more than 100 mm m−1(clays and loams) under certain circumstances experience profile water deficits at planting as a result of climatic conditions.