Laboratory and Greenhouse Evaluation of Ultraviolet Light Absorption Methods of Estimating Nitrogen Supplying Capacity of Calcareous Soils

Abstract Application of nitrogen (N) fertilizers without knowing the nitrogen supplying capacity (NSC) of soils may lead to low N use efficiency, uneconomical crop production, and pollution of environment. Twenty-five surface (0–20 cm) soil samples (pH 7.9–8.3; organic matter 7.8–35.0 g kg−1; calcium carbonate equivalent 253–648 g kg−1) from cultivated lands of Fars Province of Iran were used in laboratory and a 6-week greenhouse experiment to compare seven methods of estimating NSC of calcareous soils. The methods used were (i) determination of –N produced following the anaerobic incubation of soils; (ii) determination of –N extracted from soils by 2 M KCl; (iii) determination of –N extracted from soils by H2O; (iv), (v), (vi) measuring the amount of ultraviolet (UV) light absorption by soil extracts at 200, 205, and 260 nm; and (vii) determination of –N from the “second derivative” of light absorbance of soil extracts in the range of 200–280 nm. The soils, provided with sufficient nutrients other than N, were planted with corn (Zea mays L., cv Single Cross 704) and total N taken up by aerial parts of the plants was taken as the NSC of soils. Regression equations obtained between plant N uptake and each of the methods showed that absorbance at 205 nm was the best method, predicting 70.5% of the variabilities in NSC, when used alone. The “second derivative” method, when used along with organic matter and clay contents of the soils, predicted 84.2% of the variabilities in NSC. It is suggested that UV absorption methods be considered as a measure of NSC because they are more rapid, less laborious, and consume less chemicals than biological and conventional chemical methods.