THE FERTILITY OF THE. .SO/I 17 



becomes a noxious beast of prey. Since the publication 

 of these views of the functions of protozoa in the soil, 

 confirmatory evidence has been obtained from various 

 sources. For example, men who grow cucumbers, 

 tomatoes, and other plants under glass are accustomed 

 to make up extremely rich soils for the extensive culture 

 they practise, but despite the enormous amount of 

 manure they employ, they find it impossible to use 

 the same soil for more than two years. Then they are 

 compelled to introduce soil newly taken from a field 

 and enriched with fresh manure. Several of these 

 growers have observed that a good baking of this used 

 soil restores its value again ; in fact it becomes too rich 

 and begins to supply the plant with an excessive amount 

 of nitrogen. It has also been 'pointed out that it is the 

 custom of certain of the Bourbay tribes to burn vegetable 

 rubbish, mixed, as much as possible, with the surface 

 soil, before sowing their crop, and the value of this 

 practice, though forgotten in European agriculture, is 

 still on record in the books on Roman agriculture. We 

 can go back to the Georgics again, and there find an 

 account of a method of heating the soil before sowing, 

 which has only received its explanation within the last 

 year, but which in some form or other has got to find 

 its way back again into the routine of agriculture. 



My time is out, and I fear that the longer I go on the 

 less you will feel that I am presenting you with any 

 solution of the problem with which we set out : ' What 

 is the cause of the fertility of the soil?* Evidently 

 there is no simple solution, there is no single factor to 

 which we can point as the cause; instead, we have 

 indicated a number of factors, any one of which may at 

 a given time become a limiting factor and determine the 



