The fate of nitrogen fertilisers added to red gradational soils at Batlow, New South Wales, and its implications with respect to soil acidity

Abstract

Red gradational soils at Batlow, in New South Wales, which are used for apple growing, have acid subsoils with exchangeable aluminium (Al) frequently in excess of exchangeable calcium (Ca). There is often inadequate Ca in the fruit cortex of post-harvest apples to maintain good fruit quality and this can lead to losses in cool-store. It is possible that Al in these acid subsoils has interfered with Ca uptake by the trees. The excessive use of nitrogenous fertilisers leads to soil acidity, and it was thought likely that this was exacerbating the subsoil acidity common in the district.

In October 1992, soil analysis detected considerable ammonium in the surface 0·3 m at orchard sites at Batlow monitored for mineral nitrogen (N). This probably came from heavy spring dressings of fertiliser. One site examined in detail showed that about half of the ammonium had disappeared by January 1993, but a large nitrate envelope appeared with a peak at 0·6 m which in turn disappeared by April that year. This establishes that heavy applications of ammonium are nitrified, leached into the subsoil, and lost.

Under such a high N regime, orchard soil profiles should be more acid than adjacent forest soils. However, it was found that the acidity of the surface soil was less, and the exchangeable Ca greater in the orchard soils, compared with soil profiles in the adjacent eucalypt forest, although amelioration of the subsoils had not occurred.

Samples taken from representative sites at Batlow, at the 0–0·1, 0·1–0·2, and 0·3–0·4 m depths, were dosed with ammonium sulfate and leached with water in the laboratory for 23 days in a free-draining environment. Nitrate and ammonium were determined in the leachates. At the end of the experiment, the pH and exchangeable Ca, Mg, and Mn were determined in the leached samples. Only the neutral surface soils were able to nitrify ammonium effectively and nitrification was positively correlated with pH, and with exchangeable Ca and Mg.

From this it is argued that the acidity produced by the addition of ammonium sulfate or urea will be nitrified in the surface but the acidity produced will be neutralised, provided it is accompanied by an adequate dressing of lime. Ammonium tends to remain in the surface soil, but if leached, it will not be nitrified in the subsoil. Nitrate leached into the subsoil will not be acid-forming but, if denitrified, may help to reduce acidity.

For this work, the soil pH was measured in 1 KCl. So that readers can refer this to the pH in 0·01 CaCl₂, a relationship was established between the two measures.