G A 11 D E X P E S T S I X X E \V Z E A L A X D 



residual complex. Apart from the activities of all these organisms, 

 however, it is the earthworms, protozoa, fungi, algae and bacteria that 

 have the most fundamental influence upon soil fertility. 



Earthworms may be Correctly called the great soil builders; they 

 burrow through it, allowing the free passage of ah? and water; they 

 swallow large quantities, which they eject on the surface in the form 

 of "worm-casts," the soil materials being well mixed in the process; 

 they pull underground leaves and other parts of plants from the surface 

 and so increase the supply of organic matter for the action of the micro- 

 organisms that bring about decomposition. Further, by depositing their 

 "casts" on the surface, earthworms soon cover the accumulations of dead 

 vegetable matter, as has been illustrated by Darwin in his classic work 

 on these animals. Without the aid of earthworms e.g., in sour soils in 

 which they do not abound the plant residues accumulate on the 

 surface, to form a partially decomposed, peaty mass, which only a 

 limited number of plants can tolerate. 



The protozoa, fungi, algae and bacteria are . all microscopic 

 organisms, and are the agents responsible for the decomposition of the 

 organic residues in the soil; they do not act as independent units, the 

 processes of one group being dependent upon and intimately related 

 with those of the others. During the activities of these organisms 

 various organic and mineral substances are decomposed or transformed 

 into materials, such as humus and the inorganic compounds of nitrogen, 

 phosphorus, potassium, etc., necessary or helpful for the growth of 

 plants. 



The protozoa (see Chapter I.) are the lowest and simplest forms 

 of animal life, being mere specks of living matter. Three different 

 groups of soil protozoa occur (Fig. 2). Some, like the amoeba, progress 

 by streaming movements,, extruding temporary extensions of their sub- 

 stance in the form of finger or thread-like processes; the bodies of 

 such protozoa may be naked, or enclosed in a shell-like covering secreted 

 by the organism itself, or protected by an accumulation of particles 

 of foreign matter. Some have a body of more definite shape and 

 progress by means of the whip-like action of one or two thread-like 

 processes, or flagella, arising from one end of the body. Such forms 

 are the most numerous in the soil. Others, also of definite shape, control 

 their movements by means of short, hair-like processes, OT cilia, either 

 distributed over the body or restricted to definite regions. 



The protozoa are widely distributed, being most abundant in the 

 richer types of soil, especially during the spring and autumn. A great 

 amount of research has been undertaken at Eothamsted, England, and 

 elsewhere, on the part played by protozoa in soil fertility ; the evidence 

 thus secured points to the probability that some of these organisms may 

 be detrimental in that they devour certain kinds of bacteria responsible 

 for the production of nitrates and other substances of nutritive value 

 to plants. The extent of this may be realised from the fact that in a 

 definite weight of soil (about l-28th of an ounce) the micro-population 

 was calculated to include not only about 1,550,000 protozoa, of which 

 430,000 were amoebae (Fig. 2), but also some 6,000,000,000 bacteria. 

 Observations showed that a single bacteria-destroying amoeba required 

 about 400 organisms for its nourishment, so that the amoebae, to say 



15 



