Nitrification in soils after different periods of dryness

Summary

An experiment is described in which soils with carbon contents ranging from about 1 to 7 per cent (Walkely-Black, uncorrected) were air dried for 3, 6, 9, 12, and 15 weeks and then remoistened. The amounts of carbon mineralised and ammonia and nitrate-nitrogen subsequently produced during 19 days were then determined and compared with the amounts mineralised from a non-dried soil. For a given drying period the amounts of carbon and nitrogen mineralised were proportional to the carbon content of the soil while, for a given soil, they were found to be a significant linear function of the log of the time the soil was in an air-dry state prior to moistening. With high-humic soils drying produced extra nitrogen on moistening sometimes equivalent to over 1 ton of sulphate of ammonia per acre. Even with low-humic soils values of about 300 pounds were common.

The results of the drying effect are tabulated both in conventional units and as pounds of sulphate of ammonia per acre 6″. In the latter units the table should more readily serve as a general guide in applying the results. The application of the results to nitrogen fertiliser trials, irrigation, rundown of soil fertility and soil carbon, the effect of shadeetc. is then discussed. The material and mechanism possibly involved is also considered. The results should be of fairly wide application since a basic principle operating on material (humus) of more or less uniform composition and common to all soils is involved.