Changes in soil quality over five consecutive vegetable crops following the application of garden organics compost

A field experiment was established near Camden in south west Sydney, Australia in 2005 to evaluate the effect of garden organics compost on vegetable production and soil quality relative to conventional practice. Treatments were full compost (125 dry t/ha), half compost (62.5 dry t/ha supplemented by inorganic fertilisers), conventional farmers practice (mixture of poultry manure and inorganic fertilisers), and control (nil inputs) in a fully randomised and replicated experiment with 4 blocks. It was evident at the start of the experiment (i.e., crop 1), that compared to the conventional farmers practice treatment, the full compost treatment (120 dry t/ha) had produced significantly (P<0.05) higher soil carbon levels (2.1 vs. 1.3%), eCEC (11.5 vs. 7.5 cmol (+)/kg), bicarbonate extractable P (108 vs. 59 mg/kg), NO3-N (123 vs. 63), exchangeable K (1.2 vs. 0.5 cmol(+)/kg), exchangeable Ca (7.6 vs. 5.6 cmol (+)/kg), and pH (5.8 vs. 5.2), but also higher EC (0.38 vs. 0.16 μS/cm) and exchangeable Na (0.4 vs. 0.2 cmol (+)/kg) levels in the 0-15 cm soil layer. These soil properties were monitored over 5 successive crops to determine trends over time. The trend in soil carbon levels in the compost treatments over time fluctuated up and down partly due to the influence of variable inputs from crop residues which were incorporated into the soil, but by the fourth crop had been reduced slightly from 2.1 to 1.9 g/100 g and the ECEC from 11.5 to 11.2 cmol (+)/kg. The conventional farmers practice treatment revealed how currently recommended P application rates are excessive and lead to elevated Colwell P levels in the soil. The compost system supplied P to the crop with lower risk to the environment. The compost treatment also improved soil structure relative to the conventional practice treatment, resulting in a higher proportion of the soil as water stable aggregates >0.25 mm. This study demonstrated that a 125 dry t/ha compost application rejuvenated soil quality and maintained many soil quality benefits over the five crops, despite the high tillage associated with rotary hoe use in this system.