Chemical tests for potentially available nitrogen in soil

Previous work on the distribution of radioactivity in the organic matter of soils incubated with 14C labelled ryegrass suggested that the amount of organic matter extracted by barium hydroxide might serve as an index of potentially available N in soil. This suggestion was tested on a set of soils for which the amounts of N mineralised during incubation, and the amounts of N taken up by ryegrass grown in the soils, were known. The amounts of organic C and non‐nitrate N extracted by barium hydroxide were correlated fairly closely with the amounts of N released by the soils; polysaccharide (measured as the glucose equivalent) extracted by barium hydroxide was more closely correlated with release of N. The amount of ‘glucose’ extracted was significantly correlated with the yield of unfertilised barley in 36 field experiments (r = 0.83**); the correlation with response of barley to N was smaller (r = −0.54**).

The amount of ‘glucose’ extracted from soil by barium hydroxide increased during air‐drying, but once air‐dry no further change occurred, even on prolonged storage.

Barium hydroxide‐extractable ‘glucose’ is proposed as an index of potentially available N in soil. The correlation coefficients between uptake of N by ryegrass grown in pots (14 soils) and soil measurements were 0.70**, 0.65**, 0.70**, 0.67**, and 0.28, respectively, for barium hydroxide‐extractable ‘glucose’, N extractable by boiling water, or sodium bicarbonate, ammonia released by hot aqueous calcium hydroxide, and ammonia released by alkaline permanganate. None of the chemical tests were correlated as closely with uptake of N by grass, as the N mineralised when the re‐wetted air‐dried soil was incubated.