THE PARTIAL STERILIZATION OF SOILS 



237 



THE PARTIAL STERILIZATION OF SOILS. 



By E. J. Russell, D.Sc, F.R.S., Director of the Rothamsted Experi- 

 mental Station. 



[Read October 21, 1919 ; Mr. A. W. Sutton, V.M.H., in the Chair.] 



Investigations in recent years have shown that the upper layer 

 of the soil is inhabited by a highly complex and very numerous popu- 

 lation of micro-organisms. Among them are bacteria, some of which 

 are only 50 * 00 inch in length ; there are flagellates, amoebae, and 

 fungi, which are much larger ; eelworms, which are larger still, being 

 as much as ^ inch in length, and finally there are numerous visible 

 inhabitants — wireworms, ants, earthworms, and others. All this 

 population is dependent in the last instance on the plant. As regards 

 part of the population the dependence is reciprocal ; the plant also 

 depends on the organisms. But as regards many members of the soil 

 population this reciprocity does not hold ; the plant gains nothing, 

 but on the contrary loses, by the activity of some of the soil 

 inhabitants. 



Under natural conditions, therefore, one has to think of the soil 

 as carrying not only surface vegetation but also a large invisible 

 population of other living forms ; there is a certain amount of mutual 

 interdependence between the various members of the flora and fauna, 

 but also some antagonism ; the plant does not grow as well as it might. 

 Under the artificial conditions of agriculture and horticulture it is 

 desired to secure the best possible conditions for plant growth, and 

 this involves the control of the soil population. 



There are two methods of procedure. The straightforward method 

 is to analyse the soil population, ascertain what members are useful 

 and how they may be encouraged, and what members are harmful 

 and how they may be eliminated. In practice this presents many 

 difficulties and it is very tedious and extremely slow ; it is, however, 

 eminently sure. 



A second method of approach has been opened up by the discovery 

 made some years ago that a considerable degree of simplification of 

 the population of micro-organisms could be obtained by mild, not too 

 drastic, killing processes. Fortunately the organisms harmful or 

 less useful to the plant tend to be killed before the more useful 

 organisms ; hence simplification of the soil population enhances soi] 

 fertility. The improvement is strikingly seen in two directions ; there 

 is an increase in the supply of nitrogen compounds available for plant 

 nutrition, and a decrease in the pests and in the organisms causing 

 plant disease.' 



The possibility of increasing the amount of plant food in the soil by 



