The Composition and Use of Fertilizers. 205 



siiim compounds. As potash is taken up by vegetation, most 

 soils under constant cultivation sooner or later become deficient 

 in potash, and this loss must be supplied by means of fertilizers. 

 Therefore, potassium (potash) compounds are regarded as essen- 

 tial constituents of direct fertilizers. 



(c) Specific Functions in Plant Growth. — Potassium com- 

 pounds are instrumental in performing several different func- 

 tions in the economy of plant life. 



1. Potash compounds are essential to the formation and trans- 

 ference of starch in plants. Starch is known to be first formed 

 in the leaves of plants, after which in some way it becomes solu- 

 ble enough within the plant cells to enable it to pass through the 

 cell walls gradually and later to be carried into the fruit, where 

 it accumulates and changes back to its insoluble form. It is well 

 established that potassium compounds are intimately connected 

 with the formation of. starch in the leaves and with its trans- 

 ference to the fruit. No other element can take the place of 

 potassium in performing this work. For the carrying of starch 

 a minute amount of calcium (lime) and chlorine appears to be 

 needed in addition to the potassium. 



2. Potassium compounds are important because they exercise a 

 marked influence upon the development of the woody parts of 

 stems and the fleshy portions of fruit. 



3. We have already stated that the formation and transference 

 of starch is more or less dependent upon potassium compounds 

 present in the plant. Now, this formation of starch and transfer- 

 ence to the fruit are preliminaries in the production of sugar, 

 which contributes so much to human enjoyment. 



4. Potassium compounds are present in those plant juices, 

 which are rather sour; in these cases, the potassium is combined 

 as an acid salt with such acids as citric, malic, tartaric, oxalic and 

 other acids. 



5. Potassium compounds appear to be essential in the forma- 

 tion of proteids. 



The functions of the element potassium cannot be performed 

 by the element sodium and hence sodium cannot lake the place of 

 potassium in feeding plants. 



Calcium. 



i 



(a) Source. — The metal calcium is always found in nature 

 combined with other elements. The compounds of calcium are 

 ordinarily known as lime compounds, because calcium, when com- 



