Fallow management increases soil water and nitrogen storage

In regions where rainfall during the cropping season is low and variable, such as most parts of Australia, stored soil moisture determines the yield and sowing time of the following crop. A long-season fallow experiment was conducted in south-eastern Australia, and a biophysical simulation model, APSIM, was evaluated and applied. Stubble cover did not significantly affect fallow soil water storage; once the soil profile was filled during the winter fallow, the presence or absence of stubble cover during the summer fallow made little difference. However, weed growth during the summer period significantly affected the soil water storage. By the time of winter crop sowing, the plant available water (PAW) was depleted by 11% (18 mm) in weed free – stubble covered treatment, 14% (23 mm) in weed free – stubble free treatments, 34% (52 mm) in the weedy – stubble covered treatment, and 42% (64 mm) in weedy – stubble free treatment. The weedy (39 kg ha⁻¹) and weed free (98 kg ha⁻¹) treatments differed significantly in the amount of soil mineral nitrogen at the end of the fallow period. APSIM was able to simulate the change in soil water storage under the weedy treatment accurately (R² = 0.93, NRMSE = 4%). Long term simulation showed that there was an 88% probability of accumulating 140 mm PAW by the time of sowing, compared with only 13% probability when weeds were present. If the summer fallow period was not properly managed, the water stored during the winter season could be lost to weeds. While soil water and nitrogen storage may vary with soil type, rainfall amount, rainfall distribution, and weed pressure, fallow weeds must be controlled to ensure accumulation of fallow soil water and nitrogen for a subsequent crop.