68 K. GREIG-SMITH. 



the destruction of the moulds and algae, and it is quite 

 possible that this may be their function. We do know that 

 the actual numbers of bacteria are not influenced, but we 

 do not know if some bacteria are absorbed by the amoebae 

 and others ignored. The selective action upon the soil 

 bacteria is a point that requires elucidation. We know 

 that certain amoebae select certain nutrients, as for example, 

 A. nitrophylla, and it may be that they select or refuse 

 certain kinds of bacteria, just as do the phagocytic leuco- 

 cytes of the animal body. They will certainly withdraw 

 nutrients from the soil water, and by converting them into 

 organised proteid, will prevent them passing into the sub- 

 soil. But the by-products of digestion will be extruded, 

 and may, as Wolff suggests, by reason of their acidity, exert 

 a solvent action upon mineral fragments. By reason of 

 their simpler and more diffusible nature, they will assist 

 the growth of the soil micro-flora and of plants. 



Sewage-sick Soils. 



It has been claimed by Russell and Golding 1 that the 

 sickness of sewage soils is due to the activity of protozoa, 

 because treatment with volatile disinfectants brings about 

 a cure. I have shown that the protozoa are not destroyed 

 by the treatment, possibly because the soil was air-dried in 

 order to obtain a uniform condition. Russell and Pettier- 

 bridge say that partial sterilisation may fail in the case of 

 soils rich in organic matter, and it is under this category 

 that sewage soils must be classed. Personally I believe 2 

 that the sickness is caused by the slime that accumulates 

 in such soils, and any substance or treatment that destroys 

 the slime will cure the trouble. The soils are akin to a 

 slimy bath-sponge, which may be able to be used if the 

 water is hard, for lime salts coagulate the bacterial slime. 



1 Jour. Soc. Cheni. Ind., 30, 471. 

 a Proc. Linn. Soc. New South Wales, 1912, 238. 



