EFFECT OF TILLAGE ON SOME SOIL PROPERTIES, PLANT DEVELOPMENT AND YIELD OF SPRING WHEAT (TRITICUM-AESTIVUM L) IN STONY SOIL

Shallow, stony soil comprises by far the largest percentage of soil on which spring wheat is grown under a mediterranean climate in the Republic of South Africa. In a long-term field experiment, the effect of shallow tine- and no-tillage on some soil properties of these soils, crop growth and yield were compared with conventional mouldboard- and disc-ploughing in a wheat monoculture system. No-tillage and tine-tillage resulted in a smaller leaf area index (LAI) in some years. These differences in LAI correlated significantly (r = 0.91 ) with differences in yield and were mainly the result of smaller plant populations, a lower rate of nitrogen mineralisation, and a higher cone resistance which reduced root development. Because of the low water storage capacity of this shallow, stony soil and well distributed rainfall during the growth period of the wheat, tillage-induced differences in soil water content had no significant effect on yield. High seeding and nitrogen application rates favoured crop growth on no-tilled soil and tended to neutralise the yield-limiting effects of this tillage system.