Abstract
Amato and Ladd showed that the amount of ninhydrin-reactive nitrogen (ninhydrin-N) extracted from soil by 2 m KCl following chcl 3 fumigation is a reliable and sensitive indicator of the amount of soil microbial biomass. However, when we attempted the assay in 0.5 m k 2so 4 soil extracts, serious analytical problems (e.g. precipitation of CaSO 4 and K 2SO 4) occurred, which invalidated the ninhydrin-N determinations. As measurement of biomass C (and probably biomass N) in soils by fumigation-extraction is more reliable when K 2SO 4, rather than KCl, is used as the extractant, we modified the ninhydrin-N assay procedure to permit measurements in 0.5 m K 2SO 4. Our modified procedure is described. A strong relationship ( r = 0.99) between ninhydrin-N measured in KCl and K 2SO 4 soil extracts was obtained (KCl-ninhydrin-N = 0.90 K 2SO 4-ninhydrin-N). There were also strong linear relationships ( r = 0.91−95) between biomass C, biomass N and biomass ninhydrin-N (all extracted with 0.5 M K 2SO 4). We conclude that reliable ninhydrin-N measurements are obtained in 0.5 m K 2SO 4 soil extracts by our modified procedure.
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