CHEMICAL CHANGES 235 



withdrawing a portion of the solution for analysis. By keeping 

 all conditions constant except the one to be studied, we may 

 determine the effect of (a) the nature of the soil, (b) the ratio of 

 soil to solution, (c) the concentration of the solution, (d) the 

 time, (e) the temperature, and (/) the nature of the salt used. 

 These are the principal factors which affect fixation. 



Another way 1 of studying fixation is to allow a solution of de- 

 finite composition to percolate through a column of soil, but this 

 method is open to the objection that the solution afforded to dif- 

 ferent layers of the soil is of different composition, since it be- 

 comes progressively weaker as it percolates into the soil. This 

 method is in principle the same as mechanical washing devices, 

 in which fresh water flows where the clean material comes out, 

 and the dirty material enters where the dirty water flows out.- 



Factors of Fixation. The important factors governing fixation 

 are as follows : 



(i) The Character of the Salt. Potash, phosphoric acid, am- 

 monia, lime, magnesia, and soda are fixed by the soil. Chlorine, 

 nitric acid, and sulphates are not fixed from strong solutions ; but 

 from very weak solutions it appears that some fixation may take 

 place. 



That is to say, if potassium chloride, nitrate, or sulphate in 

 solution are brought in contact with a soil, a portion of the potash 

 disappears, but the amount of chlorine, nitrate, or sulphate in the 

 solution remains nearly as it was before. The potash is replaced 

 by an equivalent amount of lime, soda, and magnesia. 



Different percentages are absorbed when potash, phosphoric 

 acid, etc., are brought in contact with the same soil in equivalent 

 proportions, that is, in the proportion of their combining weights, 

 such as ninety-five parts of potash to 62 parts soda and so on : 



K 2 : Na 2 : CaO : MgO : 2 NH 3 : i/3P 2 O 5 



95 62 56 39 34 47+ 



The percentages absorbed are not always in the same order for 



1 Schreiner and Failyer, Bulletin No. 32, Bureau of Soils. 



