Laboratory methods of estimating potentially mineralizable nitrogen in organic potting mixes. I. Development of incubation and chemical methods

Organic potting mixes supply plant nutrients through the mineralization of organic matter. However, laboratory methods of estimating mineralization in soilless potting mixes have not been developed. Using 100 potting mix form ulations, this study investigated whether an anaerobic incubation method or two chemical methods (alkaline KMnO4 and HCl) could be used to predict pot entially mineralizable N (PMN) in organic potting mixes, using N uptake by ryegrass herbage as the benchmark method of estimation of PMN. The anaerobic incubation method failed to produce NH4+ from many mixes, and was considered unsuitable as a PMN test. The alkaline KMnO4 and HCl method s produced NH4+ in all cases, and correlation between net NH4+ production a nd ryegrass herbage N was good (r = 0.66, P < 0.001 and r = 0.80, P < 0.001 , respectively). However, when initially available N was included with the NH4+ produced by the tests, correlations with ryegrass herbage N were impro ved (r = 0.95, P < 0.001 and r = 0.92, P < 0.001 respectively). It was conc luded that the two chemical methods, modified to include initially availabl e N, have potential as laboratory methods of estimating PMN in organic pott ing mixes.