vii.] THE PROTOZOA OF THE SOIL 237 



practical limitations still remain to be overcome. The 

 task of heating the layer of cultivated soil to the 

 temperature of a little below the boiling point of water 

 is a very great one, the heat from the combustion of 

 three tons of coal being required to raise the temperature 

 of an acre of soil, to the depth of a foot, by 70 . This 

 is only the lowest possible estimate, when the whole of 

 the heat reaches the soil and none is wasted. To realise 

 the temperature in practice, some two or three times as 

 much coal would be consumed and considerable labour 

 would also be spent. The heating process has, however, 

 been applied, with considerable practical success, to the 

 treatment of soil from greenhouses when only small 

 quantities of soil are involved, where heating mechanisms 

 are at hand, and where the value of the produce is so 

 large in relation to the soil employed that it can repay 

 a heavy expenditure. Greenhouse soils, because of the 

 great enrichment they are given, the high and regulated 

 water supply and the high temperatures, very rapidly 

 become " sick " through the extensive development of 

 protozoa within them. It was customary to renew them 

 at considerable expense, though the material to be 

 replaced was still loaded with plant food. The heating 

 processes to which they are now subjected not only 

 renew and enhance their fertility, but at the same 

 time get rid of nematodes and many other organisms 

 which were injurious to the plants growing in them, so 

 that, from the commercial point of view, the process is 

 doubly remunerative. For the treatment of soils on 

 a field scale, search is being made for an antiseptic that 

 will be effective, and yet cheap enough for application 

 on a large scale. Toluene, carbon bisulphide, etc., are 

 too volatile to be of use in the open ; what is needed is 

 something that is volatile enough to be effective against 

 the protozoa, and yet capable of retention for a sufficient 



