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The distribution of sweet? and sourveld in relation to climate, parent material and soil nutrients has been examined in South Africa's grassland biome. Sourveld occurs in areas with high water supply and where parent material gives rise to soils with a low base status. Sweetveld occurs in areas with low water supply and where parent material gives rise to soils with a high base status. The distribution of sweet? and sourveld does not seem to be determined by base status itself, but by the concentration of available phosphorus and the size of the readily mineralizable nitrogen pool in the soil. The difference between the temperatures in the growth (wet) and no?growth (dry) seasons also appears important, but the relation is not clear. We propose that aspects of the environment which promote carbon assimilation (water supply and temperature) in relation to nutrient supply (soil fertility, water supply and temperature) determine the forage quality of grassland vegetation. Sourveld occurs where carbon assimilation is high relative to nutrient supply, and sweetveld where nutrient supply is high relative to carbon assimilation. Mixed veld is intermediate between these two.