COMMERCIAL FERTILIZERS CONTINUED 2OI 



mus. It should not be applied oftener than once in 

 four or five years. 



Gypsum, a sulphate of lime, is similar to lime in 

 its action on the soil. Its most important effect is 

 the setting free of potash from its compounds. 



Gas lime should be used with great care as it 

 contains substances that are poisonous to plant 

 roots. It is best to let it lie exposed to the weather 

 several months before using. 



Marl is simply soil containing an amount of lime 

 varying from five to fifty per cent. It has value in 

 the vicinity of marl beds but does not pay to haul 

 very far. 



