FIXATION l6l 



204. Humus May Cause Fixation. Other com- 

 pounds of the soil as humus and calcium carbonate 

 also take an important part in fixation. In the case 



of humus, a union takes place betweenthe minerals 

 in the fertilizers and the organic acids formed from 

 the decay of the humus in the soil, resulting in the 

 production of humates. (See Section 104.) 



205. Soils Possess Different Powers of Fixation. 



All soils do not possess the power of fixation to the 

 same extent. Heavy clays have the greatest fixative 

 power while sandy soils have the least. Experi- 

 ments have shown that in the first nine inches of 

 soil, from 2,000 to 8,000 pounds per acre of potash 

 and phosphoric acid may undergo fixation. 54 Hence 

 it is that a fertilizer, after being applied to a soil, may 

 be entirely changed in composition, so that the plant 

 feeds on the chemical products formed, rather than 

 en the original fertilizer. 



206. Nitrates Cannot Undergo Fixation. Nitro- 

 gen in the form of nitrates or nitrites cannot undergo 

 fixation. This is because all of the ordinary forms of 

 nitrates are soluble. If potassium nitrate be added to 

 a soil, calcium or sodium nitrate will be obtained as 

 the soluble compound. The potassium undergoes fix- 

 ation, but the nitrate radical does not. Chlorides also 

 are incapable of undergoing fixation because all of the 

 chlorides found in soils are soluble. 



207. Fixation of Ammonia. Ammonium com- 

 pounds readily undergo fixation, particularly in the 

 presence of clay., (See experiment No. 15.) Theam- 



