X.] SOLUBILITY OF FERTILISERS IN SOIL WATER 285 



by some other factor hardly controllable by the farmer : 

 it is a truism that poor land cannot be converted into 

 good by manuring and that fertilisers give the best 

 returns when applied to a good soil. 



One fundamental difficulty still remains in consider- 

 ing the action of fertilisers ; it has already been pointed 

 out that a soil by no means notably fertile may contain 

 enormous quantities of plant food, which is however 

 combined in so insoluble a form as to reach the plant 

 in quantities insufficient for the requirements of the 

 crop. For example, a soil may contain o-i per cent, 

 or 2500 lb. per acre, of phosphoric acid, and yet yield a 

 very indifferent Swede crop unless it be supplied with 

 an additional dressing of 50 lb, per acre of soluble 

 phosphoric acid. It is usually assumed that the effect 

 of this phosphoric acid manuring is due to the soluble 

 nature of the fertiliser, because of which the additional 

 plant food is directly available for the crop. But a little 

 consideration of the reactions set up in the soil will 

 show how insufficient such a theory must be ; the 

 phosphoric acid is very rapidly precipitated within the 

 soil, as is shown by the fact that on many soils it 

 remains close to the surface for many years, and is 

 never washed out into the drains. Bearing in mind 

 this precipitation of the phosphoric acid in an in- 

 soluble condition, Whitney and Cameron argue that 

 previous to the addition of the fertiliser a certain 

 amount of phosphoric acid exists in solution in the soil 

 water, this amount being in equilibrium with the various 

 phosphates of calcium, iron, aluminium, etc., making up 

 the great store of phosphates in the soil. This particular 

 state of equilibrium would be but little disturbed by the 

 addition of the soluble fertiliser in quantities which are 

 small compared with the great mass of undissolved 

 phosphates in contact with the soil water ; the added 



