622 



SCIENCE 



[N. S. Vol. XXVIII. No. 723 



enormous quantities of plant food, wMcli 

 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 0.1 

 per cent., or 2,500 pounds per acre, of phos- 

 phoric acid and yet yield a very indifferent 

 swede crop unless it be supplied with an 

 additional dressing of 50 pounds 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 fertilizer, because of which the ad- 

 ditional plant food is directly available for 

 the crop. But a little consideration of the 

 reaction set up in the soil will show how 

 insufficient such a theory must be; the 

 phosphoric acid is very rapidly precipi- 

 tated 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 insoluble condition, Whitney 

 and Cameron proceed to argue that previ- 

 ous to the addition of the fertilizer a cer- 

 tain amount of phosphoric acid exists in 

 solution in the soil water, this amount be- 

 ing in equilibrium with the various phos- 

 phates of calcium, iron, aluminium, etc., 

 making up the great store of phosphates in 

 the soil. This particular state of equilib- 

 rium would be but little disturbed by the 

 addition of the soluble fertilizer in quanti- 

 ties which are small compared with the 

 great mass of undissolved phosphates in 

 contact with the soil water; the added 

 phosphoric acid would only displace an 

 almost equivalent amount of the phos- 

 phoric acid already in solution, and the 

 concentration of the new solution would 

 only differ from the old in the same degree 

 as the ratio the phosphoric acid in the soil 

 plus fertilizer (2,500 -\- 50 pounds of phos- 

 phoric acid) bears to the phosphoric acid 

 originally in the soil (^. e., 2,500 poiinds 



phosphoi-ic acid). In other words, before 

 the fertilizer was added the soil water was 

 as fully saturated with phosphoric acid as 

 the amount of calcium, iron, aluminium 

 and other bases would permit, and as these 

 bases are present in enormous excess, the 

 soil water must remain at the same satura- 

 tion point after the fertilizer has been 

 added, just as water will only hold 35 per 

 cent, of the common salt in solution with 

 however large a quantity of salt it may be 

 in contact. Just in the same way the soil 

 contains certain double silicates of which 

 potassium is a constituent and these 

 hydrolize to a slight extent in contact with 

 the soil water, to yield a solution contain- 

 ing potassium ions. The addition of a 

 soluble potassium salt, as in a fertilizer, 

 will diminish the dissociation and there- 

 fore the solubility of the double silicate, 

 the potassium of which is thrown out of 

 solution until, as Whitney and Cameron 

 argue, no more potassium ions remain in 

 solution than were present before the addi- 

 tion of the fertilizer. 



According to this point of view, the con- 

 centration of the soil water for a given 

 plant food, such as phosphoric acid, must 

 be approximately constant for all soils of 

 the same type, however much or little phos- 

 phatic fertilizer may have been applied, 

 and since water-culture experiments show 

 that this low limit of concentration at- 

 tained by the soil water is more than suffi- 

 cient for the needs of the plant, no soil can 

 be regarded as deficient in this or any 

 other element of plant food. It therefore 

 follows that the action, if any, of a fertil- 

 izer must be due to some other cause than 

 the direct supply of plant food, with 

 which the soil water must always be satu- 

 rated to a degree which is quite unaffected 

 by the supply of fertilizer. 



This view of the interactions between the 

 sparingly soluble phosphates of the soil, the 

 soil water and the added soluble fertilizer. 



