Seasonal Trends in Mineral Nitrogen Content of the Soil in a Long-Term Npk Trial on Dessert Apples

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Abstract
Summary The seasonal levels of ammonium and nitrate nitrogen were determined, over 3 years, within the top 24 inches of soil of a long-term NPK trial on dessert apples. The four treatments selected provided comparisons of cultivated and grassed plots receiving spring applications of Nitrochalk with those having received no nitrogen since 1930. The three seasons included a very dry one in 1959 and the unusually wet summer of 1960. Ammonium nitrogen did not persist as such for long periods, and nitrate nitrogen was the major form of mineral nitrogen. There was a marked contrast between the levels under the two management systems. Grass maintained a low level throughout most of the season, whereas a steady rise in nitrate nitrogen took place until July or later in the cultivated soil; thereafter, the level fell in response to rainfall. By mid-winter, very little nitrate nitrogen remained in the 24-inch profile. Applications of nitrogenous fertilizer did not markedly affect the mineral nitrogen status in the grassed plots but, in the cultivated soil, levels substantially exceeding the annual applications were found. These seasonal trends are discussed in relation to the processes involved in the cycling of mineral nitrogen. Estimates of the annual amounts mineralized in the cultivated soil receiving no added nitrogen suggest that even after 30 years this amount may be of the same order as the net uptake by the trees, thus explaining their apparently satisfactory nitrogen status. In the grassed plots, the rapid re-cycling of nitrogen from the clippings may be an important factor in maintaining an adequate supply to the tree.

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