HOW COMMON SALT ACTS. 147 



5. Theory of the action of common salt when applied alone. 



There are certain known facts which appear to throw a toler- 

 ably clear light on the theory of the action of common salt, and, 

 therefore, on the purposes for which it ought to be added to the 

 soil. Thus 



1. All plants contain chlorine in greater or less proportions, 

 and the larger number, as I have already stated, contain it 

 also in combination with sodium, forming chloride of sodium or 

 common salt. One function of salt in the soil, therefore, is to 

 enter the plant, and to supply it with the chlorine and the com- 

 mon salt which are necessary to its constitution. 



2. The leaves of plants, according to Sprengel and Meyen, 

 occasionally give off chlorine, but chiefly in the dark. This 

 must arise from a decomposition of common salt, or of some 

 other chloride. Another function of common salt, therefore, 

 probably is, after having entered the plant, to give off its 

 chlorine, and thus to yield soda, either to combine with the 

 organic acids produced in the sap, or to perform other purposes, 

 chemical and physiological, which may be necessary to the 

 healthy growth of the plant. 



3. When common salt is present in a soil which at the 

 same time contains a sensible quantity of carbonate of lime, it 

 undergoes a slow decomposition, and carbonate of soda is gra- 

 dually produced, (Berthollet.) This carbonate of soda has the 

 property of combining with silica, and of rendering it soluble in 

 water. It unites also with various organic and other substances 

 present in the soil, which are known or believed to be necessary 

 to the growth of plants. 



4. It is the result of trials in the field that common salt 

 strengthens the straw of wheat in certain soils, giving it a 

 smoother, brighter, and harder skin. In other soils this result 

 follows more surely from the use of salt and quicklime mixed 

 together. 



Two other functions, therefore, may be performed by the 

 carbonate of soda, which is produced when common salt and 

 lime are together present in the soil. It may enter directly 

 into the plant, and thus supply it with the soda it requires, or 



