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[N. S. Vol. XXXII. No. 820 



crop will only take away about 30 pounds 

 per acre of phosphoric acid from a soil 

 which may contain about 3,000 pounds an 

 acre; yet, unless to the soil about 50 

 pounds 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 ac- 

 tive and dormant plant food in the soil. 

 The chief stock of these materials, he con- 

 cluded, was combined in the soil in some 

 form that kept it from the plant, and only 

 a small proportion from time to time be- 

 came soluble and available for food. He 

 took a further step and attempted to de- 

 termine the proportion of the plant food 

 which can be regarded as active. He 

 argued that since plants only take in ma- 

 terials in a dissolved form, and as the 

 great natural solvent is water percolating 

 through the soil more or less charged with 

 carbon dioxide, therefore in water charged 

 with carbon dioxide he would find a sol- 

 vent which would extract out of a soil just 

 that material which can be regarded as ac- 

 tive and available 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 deter- 

 mined. 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 car- 

 bon 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 fer- 

 tility of the soil, because we are measuring 

 but one of the factors in plant production, 

 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 phos- 

 phoric acid which occur in the soil, the 

 plant must be under equal conditions as 

 regards phosphoric acid, whatever the soil 

 in which it may be grown. This argument 

 is, however, a little more suited to contro- 

 versy than to real life; it is too fiercely 

 logical for the things themselves and de- 

 pends upon various assumptions holding 

 rigorously, whereas we have more reason 

 to believe that they are only imperfect ap- 

 proximations 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 one 

 another far more in their ability to main- 

 tain a good supply of water than in the 

 amount of plant food they contain. Even 

 in a climate like our own, which the 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 

 engrafted on to their theory another sup- 



