Abstract

The leaching of inorganic nitrogen and phosphate was studied at field sites that utilised subsurface drip irrigation (SDI) for commercial production of processing tomatoes. Irrigation expended for the establishment of crops, at emergence, accounted for 34% of the seasonal total in a Rochester clay (site 1) and 25% in a Binabbin clay (site 2). In the period after emergence and up to crop harvests, irrigation and rainfall amounted to 88% and 105% of calculated crop evapotranspiration for site 1 and site 2, respectively. Drainage between the commencement of irrigation and crop harvests amounted to 66 mm below 2 m at site 1 and 60 mm below 0.9 m at site 2. Total irrigation caused a 2- and 5-fold depletion of soil chloride at site 1 (within 2 m) and site 2 (within 0.9 m), respectively. The drainage and chloride depletion showed that irrigation regimes at these sites were conducive to the leaching of fertiliser inputs. It was calculated that ~32 kg NO^–₃-N + NH⁺₄-N/ha and 7 kg HPO^2–₄-P/ha was leached below 2 m during irrigation at site 1. At site 2, ~75 kg NO^–₃-N + NH⁺₄-N/ha was leached below 0.9 m during irrigation.

Post harvest amounts of inorganic nitrogen and phosphate at site 1 and site 2 indicated that fertiliser applications exceeded total crop uptake of both compounds. Post harvest quantities of inorganic nitrogen and phosphate at several other commercial locations (sites 3, 4, 5, 6) showed that excessive fertiliser use was not restricted to site 1 and site 2. At site 1, between an early autumn harvest and early spring, when vegetable beds were fallow, the leaching losses of post harvest amounts of both compounds below 2 m were 137 kg NO^–₃-N + NH⁺₄-N/ha and 11 kg HPO^2–₄-P/ha. The irrigation and fertiliser management practices at all sites were similar to those reported for studies of SDI production of other vegetable and grain crops. Altogether, these results indicated that the effects of SDI combined with excessive fertiliser inputs in current commercial practice may cause the leaching of significant amounts of nitrogen and phosphate to groundwater depths.