This soil, however, could not, for various reasons, be regarded as a typical sub-soil.
Kofoid records the presence of Nægleria gruberi in clay and rock talus taken from the sides of excavations of over 20 ft. depth, but the possibility of external infection does not appear to have been excluded.
The presence of protozoa is not peculiar to British soil since they have been found by various workers in Germany, France, the United States, and elsewhere. In view of their probable importance in the soil economy there has been instituted a survey of the protozoan species of soil from all parts of the world.
This work is in charge of Mr. Sandon, to whom I am indebted for the following summary of his as yet unpublished research.
“The majority of soil protozoa (like the fresh-water forms) appear to be quite cosmopolitan, for the species found in such widely separated localities as England, Spitsbergen, Africa, West Indies, Gough Island (in the South Atlantic) and Nauru (in the Pacific) are, with few exceptions, identical. This distribution indicates an ability to withstand an extremely wide range of conditions, for the same species occurring in Arctic soils, which are frozen for the greater part of the year, are found also in soils exposed to the direct rays of the tropical sun. Even sand from the Egyptian desert contains protozoa, though it seems probable that in such cases they must be present only in the encysted condition for the greater part of the time.
“Not every sample of soil, however, contains all the species capable of living in soil, but the local conditions determining the presence or absence of any species are at present unknown. In general the numbers, both of species and of individuals present, follow the number of bacteria. They are consequently most numerous in rich moist soils. The statement sometimes made that protozoa are most numerous in peaty soils is based solely on the number of Rhizopod shells found in such localities; but as most of these shells are empty, their abundance is probably due simply to the slowness with which they disintegrate in these soils where bacterial activity is low, they do not indicate a great protozoal activity. Active protozoa do occur even in extremely acid soils, but their numbers in such cases are low. The common soil protozoa, in fact, appear to be as tolerant of differences in soil acidity as they are of differences in climate, for many of the same forms which occur in acid soils are found also in soils containing high percentages of chalk. It is possible that some of the less common species may be confined within closer limits of external conditions but the information available on this point is inadequate. All the species, however, which in Rothamsted soils occur in the highest numbers (e.g. Oicomonas termo, Heteromita spp., Cercomonas crassicauda, Nægleria gruberi, Colpoda cucullus, C. steinii) occur in practically every soil which is capable of supporting vegetation, though, of course, in very varying numbers.”
It is evident, therefore, that the protozoa must be regarded as constituting part of the normal micro-organic population of soils, and as such are probably playing an important rôle. Unfortunately our knowledge of the physiology of these organisms is extremely scant, and much of future research must be directed towards elucidating their functions and their responses to varying environmental conditions.
CHAPTER V.
PROTOZOA OF THE SOIL, II.
In the preceding [chapter] an outline has been given of the development of the study of soil protozoa, with especial reference to its qualitative aspects.
Here it is proposed to deal with the quantitative methods which have been devised for studying these organisms and the results obtained.