FIXATION 139 



A1 L A ] A1,0, 1 



£*°^ | x .(Si0. 2 ) x .H 2 0+2HCl =p^o, [x(SiO,) x .H. 2 + CaCl 2 . 

 etc. J etc. J 



166. Humus May Cause Fixation. - - Other com- 

 pounds of the soil as humus and calcium carbonate 

 may also take an important part in fixation. In the 

 case of humus a union takes place between the basic 

 material and the organic matter of the soil. 



167. 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. Experiments 

 have shown that in the first nine inches of soil from 

 2,000 to 8,000 pounds per acre of potash and phos- 

 phoric acid may undergo fixation. 53 Hence it is that 

 a fertilizer, after being applied to a soil, may be en- 

 tirely changed in composition, so that the plant feeds 

 on the chemical compounds formed, rather than on the 

 original fertilizer. 



168. 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 nitrate will doubtless be obtained as the 

 soluble compound. The potassium undergoes fixa- 

 tion, but the nitrate radical does not. Chlorides are 

 likewise incapable of undergoing fixation. 



169. Fixation May Make Plant Food Less Availa- 

 ble. — If a liberal dressing of phosphate fertilizer be 



