SOIL PROTOZOA Among the biologists of the nineteenth century it was recog nized that protozoa could be isolated from soil, but it was not until the early part of the present century that the suggestion was made that such organisms might be taking a share in the general econ omy of the soil micro-population. The view that the presence of protozoa in large numbers may lead to "soil sickness" was first put forward by Russell and Hutchinson in 1909; and from that time the study of soil protozoology has been steadily pursued.
One of the first criticisms raised against the view that protozoa were of importance in the soil was that they were present only in small numbers and always in the cystic, that is quiescent state. That such an opinion was untenable was first demonstrated by Martin and Lewin who succeeded in isolating several species of flagellates and amoebae in an active condition from normal soils; and subsequent work, using more refined methods of technique, has proved the activity of these animals in many types of soil. In a recent survey by Sandon of soils obtained from all parts of the world no soil, even the most barren, was devoid of protozoa and in some cases the number of species was as great as forty-six. Alto gether about 25o species have been recorded, a few of which occur only in soil ; while the others being highly adaptable are also found in water rich in organic matter or similar habitats. All the species appear to be world-wide in their distribution, the same species occurring in arctic, temperate and tropical soils and up to the present it has not been possible to associate characteristic species with any particular geographical areas or soil types.
Protozoa are grouped into four main classes viz., the Rhizopoda or amoebae, the Mastigophora or flagellates, the Ciliophora or ciliates and Sporozoa; representatives of each of the first three classes are found living in soil.
The life history of each species has its own characteristic fea tures as regards nuclear division, etc., and in many forms, notably
the amoebae, it is impossible to identify them with certainty unless the chief stages of the life history are known. In general, however, the soil protozoa pass through very similar phases and develop in a perfectly straightforward way. Broadly speaking, there are two main phases of the life history—a period of activity often mistermed vegetative, and a period of rest. In the former the animal moves, feeds and reproduces, while in the latter there is secreted round the body a thick wall, capable of resisting adverse external influences. This condition is termed the cystic stage, and by means of it the animals are distributed from place to place by air, water, etc. Indeed, so resistant are the cysts that many of them are capable of withstanding the action of the digestive juices of the intestines of animals, through which they pass to be deposited by the faeces on fresh ground.
Investigations into the activities of soil protozoa was at the beginning hampered by the absence of a method for enumerating their numbers. This difficulty has been overcome and it is now possible to make determinations of the size of the population in a sample of soil, and also to count both the numbers of cysts and active forms. Using this method in 1920 Cutler, Crump and Sandon made an extensive study of the bacterial and protozoal population of one of the field plots at Rothamsted. (Barnfield dunged plot.) Samples were taken every day for a year and the numbers of bacteria and of six species of protozoa counted. The numbers of all the organisms showed large fluctuations of two kinds, daily and seasonal, thus in the former case three consecu tive samples gave 58.o, 14.25, and 36.25 millions of bacteria per gram respectively and the changes exhibited by any of the species of protozoa were equally marked. Such oscillations in the size of the population continued throughout the investigation and it is now known to be characteristic of all fertile soils.