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 In this laboratory S. M. Nasir, by unpublished work, has shown that the limiting value on the acid side for _Colpoda cucullus_ was P_{H} 3·3; for a flagellate (_Heteromita globosus_), 3·5; and for an amœba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. PROTOZOA AND THE NITROGEN CYCLE. In partially-sterilised soil from which protozoa were absent Russell and Hutchinson obtained an increased ammonia production, a result also obtained by Cunningham. Hill, on the other hand, concluded that protozoa have no effect on ammonification, but his technique is open to criticism. Lipman, Blair, Owen and McLean’s work[17] contains many figures obtained by adding dried blood, tankage, soluble blood flour, cottonseed meal, soy-bean meal, wheat flour, corn meal, etc., to soil. It is difficult to understand how accurate results could be expected when, to an already little understood complex substance, such as soil, is added a series of substances whose effects are practically unknown. Free nitrogen-fixation in soils is an important process, more especially in soils of a light sandy nature, from which crops are taken year after year without any application of manure. The effect of protozoa on the organisms causing this process has in the past received little attention. Recently, however, Nasir[20] has studied the influence of protozoa on Azotobacter, both in artificial culture and in sand. From a total of 36 experiments done in duplicate or triplicate, 31 showed a decided gain in nitrogen fixation over the control, while only 5 gave negative results.