FERTILIZERS 29 



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. (33) (34). 



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 of 

 buying this ingredient in an insoluble form. 



45. Some substances, as lime carbonate and 

 gypsum, make all fertilizers more available but they 

 do not add plant food to the soil, and their use alone 

 will in time exhaust a soil. Better results will be 

 obtained by using commercial fertilizers with some 

 humus material than by using either one alone, 

 because the conditions of availability will be in- 

 creased. 



46. Probable order of availability, nitrogenous 

 substances : 



Nitrate of soda 



Dried blood 



Tankage (high in nitrogen) 



(medium in nitrogen) 

 " (low in nitrogen ) 

 Guanos, same as tankage, unless greatly 



nitrified. 



Bone meal (finely ground) 

 " " (coarse) 



