Soil Reaction.

The development of the artificial fertiliser industry has in many ways revolutionised farm practice, with the inevitable result that new problems have arisen, not the least of which are biological in character.

If, as seems to be indubitable, the micro-organisms of the soil are of importance to soil fertility, it is necessary for us to know in what way this population is affected by the application of fertilisers, and a start has been made by investigating the effects of hydrogen ion concentration on soil protozoa. Much has already been written concerning this question, but almost entirely on results obtained in artificial cultures. It is always dangerous to argue from the artificial to the natural environment of organisms and particularly so in respect to the soil. Also, as Collett has shown, the toxic effects of acids are probably not entirely a function of the hydrogen ion concentration, but that the molecules of certain acids are in themselves toxic, an action which can, however, be diminished by the antagonistic powers of many substances such as NaCl.

In this laboratory S. M. Nasir, by unpublished work, has shown that the limiting value on the acid side for Colpoda cucullus was P_H 3·3; for a flagellate (Heteromita globosus), 3·5; and for an amœba (Nægleria gruberi), 3·9.

Also Mlle. Perey, investigating the numbers of protozoa in one of the Rothamsted grass plots of P_H 3·65, found a total of 13,600 protozoa, of which 90 per cent. were active.

The tolerance, therefore, of these organisms to varying external conditions is greater than has formerly been supposed, a conclusion which is becoming more evident from the researches mentioned in [Chapter IV.] on soils from different parts of the world.