CONTINUOUS, ALTERNATE AND DOUBLE CROP SYSTEMS ON A VERTISOL IN SUBTROPICAL AUSTRALIA

Three cropping systems using 5 crop species were compared over a 10-ye ar period on a cracking clay soil (Vertisol) in the subhumid subtropic s of eastern Australia. The 3 cropping systems were continuous (the sa me crop every year), alternate (the same crop every second year) and d ouble (a winter and summer crop in the one year). There were 2 cereal crops (sorghum and wheat) and 3 grain legumes (chickpea, green gram an d black gram). The effect of cropping system was measured in terms of grain and protein yields and changes in soil organic carbon (surface 0 -10 cm) and nitrogen concentrations. Summer and winter rainfall was be low average in 8 and 5 years out of 10, respectively. Grain yield of c ereal monocultures was about twice that of legume monocultures. The po tential for double cropping, despite the generally below-average rainf all, was clearly shown with the highest grain and protein yields comin g from the combination of green gram (summer) and wheat (winter). Aver aged over 10 years, wheat yield (1460 kg/ha.year) was identical in the continuous and alternate cropping systems. Sorghum yields were margin ally higher with alternate cropping (1340 kg/ha.year) than continuous cropping (1050 kg/ha.year). With double cropping, average wheat yields were 1081 and 698 kg/ha when combined with green and black gram, resp ectively. Black gram gave half the average yield of either green gram or chickpea (about 300 v. 600 kg/ha). This was attributed to the indet erminate nature of the crop in an environment with variable rainfall a nd to the detrimental effect of above-average rainfall during harvest time. Soil nitrogen and carbon levels, with initial values of 0.22 and 2.96%, were reduced at the end of 10 years by 16 and 27% respectively . Their rate of decline did not differ between cropping systems.