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Garden, wherein he cultivated various plants, some grown conti- 

 nuously on the same plot and others in a rotation. Afterwards he 

 compared the amount of plant food removed by the crops with 

 that remaining in the soil. Daubeny obtained the results with 

 which we are now familiar, that any normal soil contains the ma- 

 terial for from fifty to a hundred 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 by 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 contain about 

 3000 Ib. an acre; yet, unless to the soil about 50 Ib. of phosphoric 

 acid in the shape of manure is added, hardly any turnips at all 

 will be grown. 



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 the soil in some 

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

 time to time became soluble and available for food. He 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 carbn dioxide, therefore 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 avai- 

 lable for the plant. In this way he attacked his Botanic Garden 

 soils, and compared the materials so dissolved with the amount 

 taken away by his crops. The results, however, were inconclusive, 

 and did not -hold out 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 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 were 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. 



