Comparison of two methods for nitrogen extraction of irrigated Spanish soils and related nitrogen balance calibrations

To establish the rate of nitrogen (N) fertilizer to be applied to a soil, the quantity of available N already present should be known, which requires that the mineralizable N content of the soil be determined. This work assesses the mineralizable N content of irrigated Mediterranean soils on the basis of the N extracted by the CaCl2 and electro-ultrafiltration (EUF) methods. Nineteen soils characteristic of central Spain were selected for this study. These mainly limestone soils had been used to cultivate maize using irrigation, and are subject to a Mediterranean-influenced continental climate. The regression equation relating the N-org obtained by the two methods-N-org(EUF I+II)=-0.580 + 0.144 N-org(CaCl2)-allows EUF-N-org to be determined from CaCl2-N-org in other soils of analogous physicochemical and climatological characteristics. To determine the N balance, a field experiment was performed with maize for two years in one of the selected soils. The N balance allows the mineralized N to be determined in the soil (of which it is an intrinsic characteristic) throughout crop growth. From these results, the N-org obtained by the two extraction methods (CaCl2 and EUF) was calibrated, which allowed the available soil N to be determined three months before sowing using the equation N-available = NO3- x a + N-org x b.