This is the term applied to denote that the nitrates in the soil have become changed into free nitrogen. The nitrates thus become lost. Denitrification is said to be due to certain bacteria, just as the production of nitrates is due to other bacteria. Curiously enough, some scientists say that if air is admitted to the soil nitrogen is set free from the organic matter; and, on the other hand, if air is excluded, nitrogen is set free from the nitrates; and in both cases it is lost.

These views would appear to be mutually destructive. All good cultivators know from experience the great advantages to their crops arising from allowing a free circulation of air amongst the soil particles and organic matter, and the more thoroughly these are mixed the better the results. Growers of plants in pots always make a point of thoroughly mixing the various ingredients of their special composts so as to secure as much evenness or homogeneity as possible throughout. In field and garden, however, this work, although possible, is rarely practised. And it sometimes happens that enormous quantities of manure are dug or ploughed into a soil which already contains a good and sufficient supply of organic material. In such cases it is possible that, owing to the soil being as it were surfeited or gorged with manure, certain bacteria attack the organic material with the object of releasing the superfluous supply of nitrogen. It has already been shown at p. 110 that even a soil that has been unmanured for fifty years still has a fund of 2500 lb. of nitrogen to the acre at 9 in. deep - just in the region where the nitrate-forming bacteria are most numerous.

In support of the view that overdosing the soil with manure may result in the loss of nitrogen, the following experiments at Rothamsted may be quoted: -

I. Plot 7; Receiving Ammonium Salts (Containing 86 Lb. Nitrogen And Minerals)

lb. per Acre.

Nitrogen originally present in 1865 (.1170 per cent)

3034

Nitrogen supplied in manure, 1865-93 ...

2408

Nitrogen supplied in rain, 1865-93.......

140

Nitrogen supplied in seed, 1865-93.......

56

Total nitrogen expected, 1893

5638

Nitrogen removed in crops, 1865-93......

1932

Nitrogen found in soil, 1893 (.1146 per cent) ...

2971

Total nitrogen accounted for in 1893

4903

Leaving nitrogen unaccounted for.......

735

5638