8 THE FERTILITY OF THE SOIL 



soil contains the material for 50 to 100 field crops. If 

 then the growth of the plant depends upon the amount 

 of this material it can get from the soil, why is that 

 growth so limited, and why should it be increased by 

 the supply of manure which only adds a trifle to the 

 vast stores of plant food already in the soil? For 

 example, a turnip crop will only take away about 30 Ib. 

 per acre of phosphoric acid from a soil which may con- 

 tain about 3,000 Ib. per acre; yet unless to the soil 

 about 50 Ib. of phosphoric acid in the shape of manure 

 be added, hardly any turnips at all will grow. DAUBENY 

 then arrived at the idea of a distinction between the 

 active and dormant plant food in the soil ; the chief stock 

 of these materials, he concluded, was combined in some 

 form that kept it away from the plant, and only a small 

 proportion from time to time became soluble and avail- 

 able for food. DAUBENY took a further step and 

 attempted to determine the proportion of the plant food 

 which can be regarded as active. He argued that since 

 plants only take in materials in a dissolved form, and as 

 the great natural solvent is water, percolating through 

 the soil more or less charged with carbon dioxide, there- 

 fore in water charged with carbon dioxide he would find 

 a solvent which would extract out of a soil just that 

 material which can be regarded as active and available 

 for the plant. In this way he attacked his Botanic 

 Garden soils and compared the materials so dissolved 

 with the am ount taken away by his crops. The results, 

 however, were inconclusive, and did not afford much 

 hope that the^ fertility of the soil can be measured by 

 the amount of available plant food so determined. 

 DAUBENY'S paper was forgotten, but exactly the same 

 line of argument was revived again about twenty years 



