SOIL FOR GREENHOUSES 185 



that the soil conditions are as well, or perhaps a little 

 better, suited to the development of these microscopic 

 plants than to our higher plants, and they may gain 

 the ascendancy and utilize the plant-food designed for 

 the crop. This idea, too, is one of the recent develop- 

 ments in soil fertility. In England, Russell and Hut- 

 chinson have developed many facts showing that in their 

 soil and especially in soils known to be sewer-sick, and in 

 greenhouse soils that have come into a -so-called sick 

 condition, there may be the development of excessive 

 numbers of the very simplest animals, known as protozoa, 

 which compete with the plant roots and with beneficial 

 forms of bacteria. They have developed facts which 

 indicate that any treatment which kills or reduces the 

 number of these competing organisms tends to promote 

 the growth of higher plants, and in general, to create a 

 better state of fertility. It should be pointed out that 

 in the greenhouse where conditions are continually favor- 

 able for growth, such difficulties may become much more 

 acute than in the field where the variations in climatic 

 conditions and the changes in season, including freezing 

 in winter, tend to hold such processes in check. It is 

 found that partial sterilization by the use of heat or by 

 chemicals has an important influence upon the fertility 

 of such soil for some time thereafter, by killing certain 

 of these competing organisms and by effecting other 

 changes, such as those dealing with nitrogen. In green- 

 house soil, with its usually large stock of organic matter, 

 attention to lime, soil moisture, high temperature and 

 other conditions favorable to biological processes, these 

 difficulties may become exceedingly acute, and they may 

 be promoted or affected by different kinds of fertilizers 

 used. Another treatment that seems to have some bear- 



