Boiling Potassium Chloride-Extractable Nitrogen as an Index of Potentially Mineralizable and Plant-Available Nitrogen in Soil

Mineralization of soil organic nitrogen (N) and its contribution toward crop N uptake is central to developing efficient N-management practices. Because biological incubation methods are time consuming and do not fit into the batch-analysis techniques of soil-testing laboratories, an analytical procedure that can provide an estimate of the mineralizable N would be useful as a soil-test method for predicting plant-available N in soil. In the present studies, the ability of boiling potassium chloride (KCl) to extract potentially mineralizable and plant-available N in arable soils of semi-arid India was tested against results from biological incubations and uptake of N by wheat in a pot experiment. Mineralization of organic N in soils was studied in the laboratory by conducting aerobic incubations for 112 days at 32 degrees C and 33 KPa of moisture. Cumulative N mineralization in different soils ranged from 8.2 to 75.6 mg N kg(-1) soil that constituted 2.7 to 8.8% of organic N. The amount of mineral N extracted by KCl increased with increase in length of boiling from 0.5 to 2 h. Boiling for 0.5, 1, 1.5, and 2 h resulted in an increase in mineral-N extraction by 9.3, 12.7, 19.6, and 26.1%, respectively, as compared to mineral N extracted at room temperature. The boiling-KCl-hydrolyzable N (deltaN(i)) was directly dependent upon soil organic N content, but the presence of clay retarded hydrolysis for boiling lengths of 0.5 and 1 h. However, for boiling lengths of 1.5, and 2 h, the negative effect of clay was not apparent. The deltaN(i) was significantly (P = 0.05) correlated to cumulative N mineralized and N-mineralization potential (N0). The relationship between N0 and deltaN(i) was curvilinear and was best described by a power function. Boiling length of 2 h accounted for 78% of the variability in N0. Results of the pot experiment showed that at 21- and 63-day growth stages, dry-matter yield and N uptake by wheat were significantly correlated to boiling-KCl-extractable mineral N. Thus, boiling KCl could be used to predict potentially mineralizable and plant-available N in these soils, and a boiling time of 2 h was most suitable to avoid the negatively affected estimates of boiling-KCl-hydrolyzable N in the presence of clay. The results have implications for selecting length of boiling in soils varying widely in clay content, and this may explain why, in earlier studies, longer boiling times (viz. 2 or 4 h) were better predictors of N availability as compared to 0.5 and 1 h.