LIMITATIONS TO GROWTH AND YIELD OF CEREAL AND LUPIN CROPS ON DUPLEX SOILS

The combination of soil physical and chemical properties in duplex soils can depress crop growth and yield, and cause considerable variability in yield within a paddock and from year to year. At an intensively monitored site at East Beverley, Western Australia, the sandy A horizon was found to be nutrient deficient, low in pH, and contained hard layers which impeded root growth. The clayey B horizon had a higher pH, and variable permeability which influenced waterlogging.in winter, and root growth and water availability from the subsoil in spring. Growth of wheat and lupin crops reflected response to these stresses. Wheat yields in particular were very variable (0.7-4.2 t/ha in 1988), and as all components of yield were affected, this suggests that the crop in low yielding areas experienced stress throughout the growing season. Lupin yields varied more widely from year to year than wheat yields, but were less variable within a year, possibly because of a more limited exploration of the subsoil by roots. Treatments to overcome the chemical constraints to growth raised yields and reduced variability; wheat and lupin yields were increased by about 25% after application of potassium, and wheat yields increased by about 10% after application of lime. Ripping lowered soil strength in the A horizon, but had little effect on yield, suggesting that the subsoil plays a larger role in determining yield on this soil. Attempts to improve the permeability of the subsoil using gypsum were inconclusive in the year of application. Similar physical and chemical problems have been identified on other duplex soils throughout Australia. Responses to ameliorative treatments have been variable, and there have often been interactions between treatments. Further improvements in yield on these difficult soils requires a better understanding of the interaction of soil physical, chemical and biological properties, and their interactions with crop physiology.