168 



LECTURES ON 



The portion soluble in water represents* the presently available 

 stock of plant food in the soil. As already intimated, plants receive 

 their nutriment either as gas or as liquid. The fixed mineral matters 

 of the soil are taken up by the plant from solution in water. If we 

 examine the soil with sufficient care, we do not fail to find everything 

 in it in a soluble state that is needed by vegetation. 



Quite recently, Grouven and Stockhardt have given renewed proof 

 of this statement. Below is a tabular view of the matters found by 

 these chemists in three soils — one poor, the others very productive : 



That portion which comes into solution only by the use of strong 

 acids represents the reserve forces of the soil. Here we find stores 

 of plant food, which, under natural agencies, require many years to 

 become fully available to vegetation; but which are, nevertheless, 

 constantly, though very slowly, contributing to the fertility of the 

 soil. The least soluble matters, again, do not wholly escape slow 

 alteration and partial solution, and, as analyses show, often contain 

 alkalies, lime, &c. 



As to the solubility of the food of the plant in water, it may be 

 remarked that while the analyses quoted sufficiently demonstrate the 

 general fact, science enables us to comprehend, to some extent, the 

 detail of the processes which bring about this result. The chemist 

 is in the habit of considering certain bodies, viz : silica, oxide of iron, 

 and phosphate of iron or phosphoric acid in presence of oxide of iron. 

 as absolutely insoluble, and under most circumstances'' they are so, in 

 pure water, when alone. But in presence of other bodies, especially 

 when the mixture is so complicated as in the soil, they manifest 

 a very different action. Many bodies which do not yield to the 

 solvent action of pure water are very perceptibily taken up by car- 



