THE NUTRITION OF THE ROOT 41 



(a.) Potassium. / 



Of the nutritive substances which have to be added to the 

 soil, potassium is most readily absorbed by the soil ; the next 

 in order are ammonia, magnesium, sodium, and calcium. Of 

 the acids, tlie very essential phosphoric acid is very energe- 

 tically retained. This is not so much the case with carbonic 

 acid, and still less with nitric acid. Sulphuric acid and 

 hydrochloric acid are not fixed at all by the soil, which will, 

 however, absorb silicic acid. 



The absorptive power of the soil depends very much on 

 the quantity of basic hydrated silicates which are soluble 

 in hydrochloric acid (zeolithic). They contain the necessary 

 lime, sodium, magnesium, and potassium, but are almost use- 

 less to the roots on account of their insolubility. They are, 

 however, decomposed by the carbonic acid contained in the 

 soil or by compounds of nitric or sulphuric acid with alkalies 

 or alkaline earths, and the resulting carbonates, sulphates, 

 and chlorides are readily soluble, and can therefore be easily 

 absorbed. During the decomposition of certain silicates 

 hydrated iron oxide and hydrated aluminium oxide are formed, 

 and these compounds as well as the hydrated silicates enable 

 the soil to separate the potassium from its most stable com- 

 bination with hydrochloric, nitric, and sulphuric acid and to 

 retain it. These iron and aluminium compounds absorb very 

 little potassium, however, without the presence of such silicates, 

 but the soil absorbs it very actively if aluminium phosphates is 

 present. Potassium is, however, scarcely absorbed at all by the 

 humic acid compounds, by carbonate of lime and of magnesium. 



The potassium salts which are absorbed by the soil are 

 able to form combinations with other nutritive substances, 

 and therefore to fix important substances in the soil. Thus 

 kainit (a sulphate and chlorate of magnesium) can be used 

 to fix nitrogen. The actual potassium salt (potassium sul- 

 phate) which takes part in this process plays a quite sub- 

 sidiary part. As soon as ammonium carbonate begins to 

 form in the soil, the acid of the magnesium sulphate combines 

 with the ammonia ; the ammonium sulphate so formed is still 

 very soluble. But if the application of kainit was preceded 



