IQ4 SOILS AND FERTILIZERS 



221. Fixation of Ammonia. Ammonium compounds 

 readily undergo fixation, particularly in the presence of 

 clay. (See experiment No. 17.) The ammonium radi- 

 cal, NH 4 , like potassium, is capable of replacing soil 

 bases. After undergoing fixation, the ammonium com- 

 pounds readily yield to nitrification (see Section 156), 

 hence they serve as a temporary but important form of 

 insoluble nitrogen. The general tendency of the nitro- 

 gen compounds of the soil is to pass from insoluble to 

 soluble forms through processes of decay, and to resist 

 fixation changes. 



222. Fixation may make Plant Food Less Avail- 

 able. If a very heavy dressing of potash or phosphate 

 fertilizer be applied to a heavy clay soil, what is not 

 utilized the first few years may undergo fixation to such 

 an extent that part becomes unavailable as plant food. 

 It is not well to apply unnecessarily heavy dressings of 

 fertilizers at long intervals because of fixation. It is 

 always best to make light and frequent applications. 



223. Fixation, a Desirable Property for Soils. - - If it 

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

 heavy rains would soon become sterile. When the 

 plant food has become insoluble, it is retained in the 

 soil. That which undergoes fixation is, as a rule, in an 

 available condition or may readily become so by cultiva- 

 tion unless the soil be one of unusual composition. The 

 process of fixation regulates the supply of plant food in 



