Seasonal levels of available nutrients under grassed down, Cultivated and zero tilled orchard soil management practices

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Abstract
The effects of zero-tillage, clean cultivation or the maintenance of a sward on the seasonal levels of available nutrients in a silt loam were examined during the third season of an orchard soil management trial. Total nitrogen and sulfur and oxidizable carbon were highest in the surface soil of grassed plots, intermediate in untilled plots and lowest in cultivated plots. The seasonal pattern in levels of nitrate nitrogen and sulfate sulfur showed similar trends in both the surface soil down the profile. Levels of both nitrate nitrogen and sulfate sulfur in the top 5 cm of soil remained high for a longer period into autumn under zero-tillage than under cultivation. During the summer and autumn period, concentrations of nitrate nitrogen in the surface soil from non-grassed treatments were notably higher than those from grassed plots. There was an accumulation of sodium bicarbonate extractable phosphorus and total phosphorus in the surface soil of untilled plots and a phosphorus fractionation indicated the accumulation of phosphorus in the aluminium and iron phosphate fractions. To a lesser extent exchangeable phosphate also accumulated at the surface of the untilled soil. Levels of exchangeable calcium and magnesium in the surface soil under the grassed treatment were markedly higher than those from the non-grassed treatments, indicating less leaching of cations in the former treatment. The possible relevance of the results is discussed in terms of orchard nutrition and maintenance of soil fertility.