Continuous, alternate and double crop systems on a Vertisol in subtropical Australia

Abstract

Three cropping systems using 5 crop species were compared over a 10-year period on a cracking clay soil (Vertisol) in the sub-humid subtropics of eastern Australia. The 3 cropping systems were continuous (the same crop every year), alternate (the same crop every second year) and double (a winter and summer crop in the one year). There were 2 cereal crops (sorghum and wheat) and 3 grain legumes (chickpea, green gram and 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 below average in 8 and 5 years out of 10, respectively. Grain yield of cereal monocultures was about twice that of legume monocultures. The potential for double cropping, despite the generally below-average rainfall, was clearly shown with the highest grain and protein yields coming from the combination of green gram (summer) and wheat (winter). Averaged over 10 years, wheat yield (1460 kg/ha. year) was identical in the continuous and alternate cropping systems. Sorghum yields were marginally 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, respectively. Black gram gave half the average yield of either green gram or chickpea (about 300 v. 600 kg/ha). This was attributed to the indeterminate nature of the crop in an environment with variable rainfall and 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.