10 THE FERTILITY OF THE SOIL 



one another far more in their ability to maintain a good 

 supply of water than in the amount of plant food they 

 contain. Even in a climate like our own, which text- 

 books describe as ' humid ', and we are apt to call ' wet ', 

 the magnitude of our crops is more often limited by 

 want of water than by any other single factor. The 

 same American investigators have more recently grafted 

 on to their theory another supposition, that the fertility 

 of soil is also determined by excretions from the plants 

 themselves, which thereby poison the land for another 

 growth of the same crop, though the toxin may be 

 harmless to a different plant which follows it in the 

 rotation. This theory had also been examined by 

 DAUBENY, and the arguments he advanced against it in 

 1845 are valid to this day. As, then, we have failed to base 

 a theory of fertility on the plant food that we can trace 

 in the soil by analysis, let us come back to MAYOW and 

 DIGBY and consider again the nitre in the soil, how it is 

 formed and how renewed. Their views of the value 

 of nitrates to the plant were justified when the systematic 

 study of plant nutrition began, but it was not until within 

 the last thirty years that we obtained an idea as to how 

 the nitre came to be formed. The oxidation of ammonia 

 and other compounds of nitrogen to the state of nitrates 

 was one of the first reactions in the soil which was 

 proved to be brought about by bacteria, and by the 

 work of SCHLOESING and MUNTZ, WARINGTON and 

 VINOGRADSKY we learnt that in all cultivated soils two 

 groups of bacteria exist which successively oxidize 

 ammonia to nitrites and nitrates, in which latter state 

 the nitrogen is available for the plant. These same 

 investigators showed that the rate at which nitrification 

 takes place is largely dependent upon operations under 



