Abstract

The effects of ammonium-based fertilisers on the soil pH of vegetable beds that utilised subsurface drip irrigation (SDI) for commercial tomato production were investigated at field sites in the southern Murray–Darling Basin region (SMDB). The soils at these sites were Vertosols (sites 1, 3, 4, and 5) and a Chromosol (site 2). At site 1, rapid transport and hydrolysis of urea occurred within the 0–90 cm soil layer of vegetable beds after a 6-mm fertigation of 30 kg urea-N/ha during cropping. Soil pH decreased by 0.2–0.4 units in individual 15-cm soil layers up to 90 cm within 12 days after the fertigation. A longer study at site 1 showed that there was severe acidification in topsoil and subsoil by the second consecutive year of SDI cropping. The rate of acidification was highest, at 52 kmol H⁺/ha.year, immediately beneath the dripline, in the 15–30 cm soil layer. Topsoil and subsoil acidification was also evident in vegetable beds at sites 2, 3, 4, and 5 after 2 consecutive years of tomato cropping using SDI. The results from the 5 sites indicated that acidification under SDI production may be widespread. A survey of 21 other sites in the SMDB under similar production showed that one-third of the sites had soil pH ≤6.0 in their 0–30 cm soil layer. Several soil types were individually represented at acid and alkaline pH levels, by 2 or more sites. This indicated that management practices influenced the change in soil pH for a given soil type. Altogether, the combined results of these studies strongly indicated that surface and subsoil acidification can occur in soils used for intensive SDI production. This may diminish their productivity in the long term.