140 SOILS AND FERTILIZERS 



applied to a heavy clay soil, the phosphoric acid which 

 is not utilized the first year or two may undergo fixa- 

 tion to such an extent as to become Unavailable as 

 plant food. It is not desirable to apply heavy dress- 

 ings of fertilizers at long intervals because of fixation. 

 It is always best to make lighter applications and more 

 frequently. 



170. Fixation, a Desirable Property of Soils. — If it 

 were not for the process of fixation, soils in regions of 

 heavy rains would soon become sterile. On account 

 of the plant food being rendered insoluble, it is re- 

 tained in the soil. The plant food which undergoes 

 fixation is, as a rule, in an available condition, unless 

 the soil be one of unusual composition. The fixation 

 of fertilizers regulates the supply of crop food. Many 

 fertilizers, if they did not undergo this process, would 

 be injurious to crops. There would be an abnormal 

 amount of soluble alkaline or acid compounds which 

 would be destructive. The process of fixation first 

 taking place removes, to a great extent, the water-solu- 

 ble salts, particularly when the reaction is one of 

 union rather than replacement. Then the plant is 

 free to render soluble its own plant food in quantities 

 and at times desired. 



Farm manures and commercial fertilizers alike un- 

 dergo the process of fixation and, in studying fertilizers, 

 their action upon the soil and the products of fixation 

 are matters of prime importance. 



