8 io Sterilisation of Soil for Glasshouse Work, [jan., 



matter to feed the plant. The straw manure that plays so 

 large a part in commercial greenhouse work is not itself a 

 food of plants, but has to be worked up into various suitable 

 substances. The process can, of course, be effected artifici- 

 ally at a certain price, but it is done for nothing in the soil. 

 The active agents are bacteria visible only under powerful 

 microscopes, but so numerous that a thimbleful of soil may 

 contain from five to fifty millions of them. Great as they 

 are these numbers do not represent the whole possibility, for 

 the bacteria no more have a clear field than anything else, 

 and never work at their maximum efficiency under natural 

 conditions. Thus neither of the two living things the grower 

 really wants — the crop and the food-making bacteria — can 

 attain to their full development under the competition 

 ordinarily going on in a greenhouse soil. 



Recent experiments have shown that it is possible to 

 simplify very considerably the population of the soil. The 

 different inhabitants show very different degrees of suscepti- 

 bility to heat and to poisons, the most resistant, fortunately, 

 being the spores of some of the food-making bacteria, so that 

 it is possible by selecting one's poisons to kill off various 

 groups — not very sharply, of course, because there is con- 

 siderable overlapping — and finally to kill everything, leaving 

 a perfectly sterile soil. The same object may be attained by 

 using various degrees of heat. Certain other effects are 

 produced as well : the poison has some chemical or other 

 action on the soil, and the heat causes some decomposition, 

 but the net result of treatment with the less violent poisons 

 and degrees of heat is that the food-making bacteria now 

 have a clearer field, and can multiply and make much more 

 food than before. Such treated soils are therefore we.ll suited 

 for glasshouse work because they are free from harmful and 

 competing organisms. This differential treatment, whereby 

 some organisms are killed and others are spared, is called 

 Partial Sterilisation. 



The effects of partial sterilisation are: — 



(1) the food-making processes in the soil are geared up 

 so that the plant gets a larger supply of food than before ; 



(2) the food is not quite the same as usual because some of 

 the bacteria have been killed; there are also some new sub- 



