THE FERTILITY OF THE SOIL 9 



ago, and all over the world investigators began to try to 

 measure the fertility of the soil by determining as 

 available plant food the phosphoric acid and potash that 

 could be extracted by some weak acid. A large number 

 of different acids was tried, and although a dilute 

 solution of citric acid is at present the most generally 

 accepted solvent, I am still of opinion that we shall 

 come back to the water charged with carbon dioxide as 

 the only solvent of its kind for which any justification 

 can be found. Whatever solvent, however, is employed 

 to extract from the soil its available plant food, the 

 results fail to determine the fertility of the soil, because 

 we are measuring but one of the factors in plant pro- 

 duction, and that often a comparatively minor one. In 

 fact, some investigators, WHITNEY and his colleagues in 

 the American Department of Agriculture, have gone so 

 far as to suppose that the actual amount of plant food 

 in the soil is a matter of indifference. They argue that 

 as a plant feeds upon the soil water and as that soil 

 water must be equally saturated with, say, phosphoric 

 acid whether the soil contains 1,000 or 3,000 pounds per 

 acre of the comparatively insoluble calcium and iron 

 salts of phosphoric acid which occur in the soil, the 

 plant must be under equal conditions as regards phos- 

 phoric acid whatever the soil in which it may be grown. 

 This argument is, however, a little more suited to con- 

 troversy than to real life; it is too truly logical, and 

 depends upon various assumptions holding rigorously, 

 whereas we have more reason to believe that they are 

 only imperfect approximations to the truth. Still, this 

 view does merit our careful attention, because it insists 

 that the chief factor in plant production must be the 

 supply of water to the plant, and that soils differ from 



