AVAILABLE NITROGEN IN SOILS AND ITS DETERMINATION BY THE NMIN-METHOD AND BY ELECTROULTRAFILTRATION (EUF)

Excess nitrogen fertilizer rates are an environmental hazard. To avoid excess rates, the level of available nitrogen in the soil must be known and considered for assessing the nitrogen fertilizer rate. In arable soils nitrate and exchangeable ammonium in the rooting depth of a crop are directly available to plant roots. These two nitrogen forms are recovered with the 'Nmin method' and they are considered in assessing the nitrogen fertilizer rate for arable crops. Besides nitrate and ammonium recovered by the 'Nmin method' from soil samples taken in early spring, a considerable amount of organic soil nitrogen may be mineralized during the growth period and contribute to crop nutrition. The nitrogen mineralization potential can be determined in incubation tests. The absolute quantities of mineralized nitrogen thus obtained, however, are much higher than the mineralization rates under field conditions. It is for this reason that incubation tests have not obtained a major importance for assessing nitrogen fertilizer rates. With the electro-ultrafiltration (EUF) method nitrate, ammonium, and a minor portion of organic soil nitrogen are extracted from a soil suspension. There is experimental evidence that this organic nitrogen (= Norg) is easily mineralizable. Based on field experiments with cereals and maize formulae were established by which the nitrogen fertilizer rate can be calculated. In these formulae nitrate, ammonium and organic nitrogen extracted by EUF are considered. Advantages and drawbacks of the 'Nmin method' and 'EUF method' are discussed.