" 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 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 sup- 

 pose 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, phos- 

 phoric acid, whether the soil contains 1000 or 3000 Ib. per acre 

 of the comparatively insoluble calcium and iron salts of phosphoric 

 acid which occur in the soil, the plant must be under equal con- 

 ditions as regards phosporic acid, whatever the soil in which it 

 may be grown. 



This argument is, however, a little more suited to controversy 

 than to real life; it is too fiercely logical for the things themselves, 

 and depends upon various assumptions holding rigourously, whereas 

 we have more reason to believe that they are only imperfect 

 approximations to the truth. Still, this view does merit our ca- 

 reful 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 maintain 

 a good supply of water than in the amount of plant food they 

 contain. 



Even in a climate like that of England, 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 supposition, that the fertility of soil is 

 very often determined by excretions from the plants themselves 

 which thereby poison the land for a renewed 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. Schreiner has, indeed, isolated a number of or- 

 ganic substances from soils dihydroxystearic acid and picoline- 

 carboxylic acid were the first examples -- which he claims to be 

 the products of plant growth and toxic to the further growth of 

 the same plants. The evidence of toxicity as determined by water- 

 cultures requires, however, the greatest care in interpretation, and 



