52 THE SOIL SOLUTION 



phoric acid in the soil solution, since the great majority of ag- 

 ricultural soils contain vastly more ferric oxide (more or less 

 hydrated) than is equivalent to any amount of phosphoric acid 

 that will ever be brought into the soil ; and ferric phosphates are 

 less soluble relatively than lime phosphates. Investigation of 

 the relation of ferric oxide to solutions of phosphoric acid shows 

 that the system is quite similar in many respects to the basic 

 lime phosphates and water just described. When the ratio of 

 iron to phosphoric acid in the solid is greater than that required 

 by the formula of the normal phosphate, FePO 4 , the aqueous 

 solution will have an acid reaction and contain a mere trace of 

 iron and an amount of phosphoric acid determined by the com- 

 position of the solid and by the proportion of solid to water. 

 The basic ferric phosphates seem to be solid solutions which 

 yield a very dilute aqueous solution when brought into contact 

 with water. What the concentration will be under soil condi- 

 tions is shown by the percolation experiments cited above. 



The addition of other substances will in many cases affect 

 more or less the solubility of the soil minerals. If these sub- 

 stances be electrolytes, they will generally, but not always, affect 

 the solubility of the minerals as would be anticipated from the 

 hypothesis of electrolytic dissociation. Thus, the addition of 

 potassium sulphate lessens the solubility and hydrolysis of a 

 potash feldspar or a potash mica. Contrary, however, to the 

 indications of the hypothesis, sodium nitrate decreases the solu- 

 bility of a ferric phosphate. While appreciable solubility effects 

 take place with sufficiently high concentrations, laboratory ex- 

 periments indicate that the addition of such substances, even in 

 a liberal application of fertilizers, is not sufficient to produce 

 any great effect on the concentration of the soil solution. Simi- 

 larly, it has often been supposed that the ammonia, and nitrous 

 and nitric oxides of the atmosphere carried into the soil by rain, 

 or formed in the soil by bacterial action, affect the solubility of 

 the soil minerals, but it is highly improbable that the concentration 

 with respect to these agents ever becomes sufficiently high, as 

 laboratory investigations show to be necessary to affect appreci- 

 ably the solubility of the ordinary rock- or soil-forming minerals. 



