The Composition and Use of Fertilizers. 209 



not of the chlorine, that is desired. An excess of compounds of 

 chlorine in soils renders them barren. 



(d) Specific Functions in Plant Growth. — The function of 

 chlorine in connection with the development of plants is not satis 

 factorily settled. There are some reasons for believing that some 

 of the compounds of chlorine, especially the potassium chloride 

 (muriate of potash), are instrumental in transferring starch from 

 the leaves, where it is formed, to the flower and fruit. There are 

 also some suggestive experiments which favor the belief that chlo- 

 rides favor the formation of cellulose (woody fiber) from sugar. 



Sodium. 



(a) Source. — Sodium occurs in nature mostly in combination 

 with the element chlorine in the form of sodium chloride or com- 

 mon salt. It is found everywhere in the soil, but usually in small 

 quantities. Sodium and nitric acid form sodium nitrate or nitrate 

 of soda, commonly known as Chili saltpeter. 



(&) Relation to Fertilizers. — Sodium in the form of sodium 

 chloride (common salt) is found in small quantities in all soils. 

 While it appears as a regular, though small, constituent of plants, 

 it is a disputed question whether it is a necessary constituent of 

 plant food, but the weight of evidence appears to show that it is 

 not necessary and that the requirements of plants do not call 

 for the addition of sodium compounds to fertilizers. In the case 

 of nitrate of soda, it is not the sodium, but the nitrogen in the 

 form of nitrate, which gives the compound its value as plant food. 

 It merely happens that the nitrate can be furnished most cheaply 

 in this form. The application of sodium chloride as an indirect 

 fertilizer has been found, under some conditions, to be attended 

 with beneficial results. Sodium cannot take the place of potas- 

 sium in plant nutrition. 



Silicon - . 



(a) Source. — Silicon, next to oxygen, is the most abundant 

 element in soils. ' It does not occur uncombined with other ele- 

 ments. Silicon combined with oxygen forms a compound com- 

 monly called silica. Quartz and sand are nearly pure silica. 



Silicon combined with oxygen and several of the metallic ele- 

 ments, such as sodium, potassium, calcium, etc., forms compounds 

 which are called silicates. The feldspars are silicates; clay is 

 a silicate. Ordinary glass consists of a mixture of silicates. 

 14 



