Roots cannot take up plant food unless it is pro- 

 vided in solution, and different forms of fertilizers 

 respond differently to these dissolving agents. Fine 

 grinding is very important. As a rule, organic forms 

 are most available. There are some exceptions, such 

 as sulphate of ammonia, nitrate of soda and the acid 

 phosphates. The latter act best in soils that do not 

 contain enough lime or iron or other bases to cause 

 rapid reversion to insolubility. 



If the farmer knows the source and form of the 

 nitrogen and phosphoric acid, he has a guide to their 

 availability. 



All forms of potash as usually purchased in fertil- 

 izers are readily dissolved and there is no danger oi 

 buying this ingredient in an insoluble form. 



Some substances, as barnyard manure and lime, 

 make all fertilizers more available, but they do not add 

 plant food to the soil in the amounts, nor as fast as 

 required, and their use alone will in time exhaust a 

 soil. Especially is this true of lime. Better results 

 will be obtained by using commercial fertilizers with 

 manure than by using either one alone, because the 

 conditions of availability will be increased. (See also 

 "Cultivation and Fertilizers. ") 



INSOLUBILITY DESIRABLE. 



It is well known that the nitrates may easily be 

 lost by leaching, because they are soluble. This is 

 not the case with the phosphates, or phosphorous 



17 



