1 62 SOILS AND FERTILIZERS 



moniuin radical, NH 4 , like potassium is capable of re- 

 placing soil bases. After undergoing fixation, the am- 

 monium compounds readily yield to nitrification (See 

 Section 145), hence they serve as a temporary but 

 important form of insoluble nitrogen. The gen- 

 eral tendency of the nitrogen compounds of the soil is 

 to pass from insoluble to soluble forms through pro- 

 cesses of decay, and to resist fixation changes. 



1 08. Fixation May Make Plant Food Less Avail- 

 able. If a liberal dressing of phosphate fertilizer be 

 applied to a heavy clay soil, the phosphoric acid which 

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

 ation to such an extent that part becomes unavailable 

 as plant food. It is not desirable to apply heavy 

 dressings of fertilizers at long intervals because of fix- 

 ation. It is always best to make lighter applications 

 and more frequently. 



109. 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 ac- 

 count of the plant food being rendered insoluble, it is 

 retained in the soil. The plant food which undergoes 

 fixation is, as a rule, in an available condition or may 

 readily become so by cultivation unless the soil be one 

 of unusual composition. The process of fixation in 

 the soil regulates the supply of plant food. Many 

 fertilizers, if they did not undergo this process, would 

 be injurious to crops for 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 injurious 



