In this laboratory S. M. Nasir, by unpublished work, has shown that the limiting value on the acid side for Colpoda cucullus was PH 3·3; for a flagellate (Heteromita globosus), 3·5; and for an amœba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 PH 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.
Fig. 18.—Showing the highest fixations of nitrogen above the control recorded for Azotobacter in the presence of different species of Protozoa. (From Ann. Appl. Biol., vol. ii.)
X-axis (left): Artificial Media C A F AF AC ACF