FERTILIZERS. 87 



2. It combines with, and decomposes, vegetable sub- 

 stances, and other elements of the soil, preparing them 

 for plant food. 



3. In a general way r it has a beneficial effect upon 

 various kinds of soil, rendering them better adapted for 

 agricultural purposes. 



It renders heavy, clayey soil more loose and mellow, 

 and sandy soil more compact, so that it will retain more 

 moisture. It neutralizes the injurious acids of cold, 

 peaty soil, and loosens it for the admission of warm air. 



Many varieties of soil already contain lime in abun- 

 dance, but where it is deficient, its application is often 

 of great benefit. 



MarL Marl is composed of carbonate of lime mixed 

 with other substances, as clayey or sandy soil. It some- 

 times contains some nitrogen and phosphoric acid. The 

 low places where it is found are supposed to be the beds 

 of ancient lakes which have dried up. The deposit has 

 been formed by the accumulation of shell-fish at the bot- 

 tom of these lakes, through long periods of the ancient 

 world. Marl is used as a fertilizer in regions near these 

 beds, where the expense of transportation is not too 

 great. 



Gypsum. Gypsum, or land plaster, is produced by a 

 union of lime and sulphuric acid, giving calcium sulphate 

 and water (CaS0 4 + H 2 O). 



By heating gypsum, the water is driven off, leaving the 

 calcium sulphate, or plaster of Paris. 



It is found in nature, in some localities, and is used as 

 a fertilizer where lime and sulphur are needed. It is 

 especially beneficial to clover, and other leguminous 

 crops. 



Salt (NaCl). Common salt is composed of sodium 



