This primitive form of vegeculture (cultivation of root and tree crops) may have been the economic tradition onto which the cultivation of summer rainfall cereal crops was grafted as it came into use south of the grassland areas on the Saharas southern borders.
as是怎么翻译成因为还是当什么时候，use在into后面是名词词性吗？use后面接了一个south怎么理解？