AVAILABILITY OF PLANT NUTBIJBNTS 335 



a strong hydrochloric acid analysis has led to the use of 

 weak acids for obtaining the solution. These weak acids 

 dissolve much less of the soil constituents than do the 

 strong acids, and the portion so dissolved is supposed to 

 represent more nearly the amount that the plant can make 

 use of. Both dilute organic acids and dilute mineral 

 acids have been used. Among the former are citric, 

 acetic, oxalic, and tartaric acids. The assumption on 

 which the use of the organic acids is based is that they 

 correspond to the solvent agents in the soil combined with 

 the solvent action that the plant is supposed to possess, 

 and thus dissolve from the soil the quantities of nutrients 

 that the plant could take up if it came in contact with 

 all the soil particles to a depth represented by the sample 

 analyzed, 



238. Advantages in the use of dilute acids. — The ac- 

 tion of each of these dilute acids on the same soil does 

 not give equal quantities of the various constituents in 

 solution. The dilute acids naturally dissolve a much 

 smaller amount of material from the soil than does strong 

 hydrochloric acid. The dilute acids permit the detection 

 of smaller quantities of easily soluble phosphoric acid and 

 potash than does the latter, larger quantities of soil being 

 used. For example, a chemical analysis of the strong 

 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 greatly increased 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 



