274 THE PRINCIPLES OF SOIL MANAGEMENT 



portions of the soil to the depth represented by the 

 sample analysed. 



137. Advantages in showing manurial needs. The 

 action of each of these dilute acids upon the same soil 

 does not give equal amounts of the various constituents 

 in solution. Citric acid dissolves especially lime, mag- 

 nesia and phosphoric acid, and is the most satisfactory 

 solvent for purposes of analysis. The organic acids 

 naturally dissolve a much smaller amount of material 

 from the soil than does hydrochloric acid. The former 

 acids permit the detection of smaller amounts of easily 

 soluble phosphoric acid and potash than does the latter, 

 larger quantities of soil being used. For example, a 

 chemical analysis of the hydrochloric acid solution is 

 very likely not to show any increase in the phosphorus 

 or potassium in a soil that may have been abundantly 

 manured with these fertilizers, and its productiveness 

 increased greatly thereby. This is because the amount 

 of plant-food material added is so small in comparison 

 with the weight of the area of soil nine inches deep over 

 which it is spread that the increase in percentage may 

 well come within the limits of analytical error. An acre 

 of soil nine inches deep weighs about 2,500,000 pounds. 

 If to this be added dressings of 2,500 pounds phosphoric 

 acid fertilizer containing 400 pounds phosphoric acid, 

 it would increase the percentage of that constituent 

 in the soil only .016 per cent, which difference could not 

 be detected by the analysis of the hydrochloric acid 

 solution. 



138. Usefulness of citric acid. -As shown by Dyer, 

 the use of a 1 per cent solution of citric acid is well 



