Phosphorus accumulation and other changes in soil properties as a consequence of vegetable production, Sydney region, Australia

A survey of 34 farms covering the major soil types used for growing vegetables within the greater Sydney metropolitan region (New South Wales, Australia) was undertaken to determine the effect of vegetable production on soil chemical and physical properties. Comparison of farmed ‘vegetable’ v. unfarmed ‘reference’ sites revealed that the soils used for vegetable production had extremely high concentrations of total P, Colwell-P, and CaCl2-extractable P (mean 1205, 224, and 4.3 mg/kg in the 0–0.30 m layer, respectively). In the 0–0.30 m soil layer, mean bicarbonate-extractable P (Colwell-P) concentrations have increased to up to 44 times that of the unfarmed reference soils and exceed that required for adequate vegetable nutrition. Concentrations of P in the soil solution (CaCl2-P) were up to 230 times that of the unfarmed reference soils. Moreover, the vegetable soils had low total soil carbon concentrations (mean 14.1 g/kg in the 0–0.10 m layer, only 57% of the mean concentration of the reference soils). These soils exhibited extremely low structural stability, which is likely to reduce soil infiltration rates and increase the potential for runoff. Marked changes in soil pH, EC, and exchangeable cations (Ca, Mg, and K) were also observed as a consequence of vegetable production. All of these changes are a consequence of current management practices used in vegetable production, which include application of high rates of inorganic fertilisers and poultry manure, as well as excessive cultivation. Excessive accumulation of P, to at least 0.30 m depth, coupled with a loss of soil structural stability, is of particular environmental concern. Options such as adopting minimum tillage, in conjunction with using alternative inputs such as low P composts and cover crops, as a means of improving soil structure and reducing the extent of P accumulation in these soils require further investigation.