336 SOILS: PBOPMETIBS AND MANAGEMENT 



2,500,000 pounds. If to this there is added a dressing 

 of 2500 pounds of phosphoric acid fertiHzer containing 

 400 pounds of phosphoric acid, it would increase the per- 

 centage of that constituent in the soil only 0.016 per cent 

 — a difference that could not be detected by the analysis 

 of the hydrochloric acid solution. 



239. The one-per-cent citric acid method. — This 

 method was proposed by Dyer ^ and w^as shown by him 

 to give results with Rothamsted soils that permitted of 

 an accurate estimation of their relative productivity. 

 Dyer adopted the one-per-cent strength as the result of 

 an investigation in which he determined the acidity of 

 the juices in the roots of over one hundred species or 

 varieties of plants representing twenty different natural 

 orders. The average acidity of the juices of the twenty 

 orders, calculated to crystallized citric acid, was 0.91 

 per cent, which led Dyer to adopt a strength of 1 per cent. 

 It must be said, however, that the different varieties 

 varied greatly in this respect, some having ten times as 

 much acidity as others. The implication is that plants pro- 

 duce a solvent action on a soil in proportion to the acidity 

 of their juices, but an examination of Dyer's figures does 

 not show that the size of the crop ordinarily produced by 

 the plants tested would in many cases correspond to the 

 acidity of these juices. Thus, of the Cruciferse the horse- 

 radish has several times the acidity of the Swedish turnip 

 or of the field cabbage, although the crop produced by the 

 former is much less than that of the latter. 



240. Useftilness of the citric acid method. — As shown 

 by Dyer, the use of a one-per-cent solution of citric 



1 Dyer, Bernard. On the Analytical Determination of Prob- 

 ably Available *' Mineral" Plant Food in Soils. Jour. Chem. 

 Soe., Vol. LXV, pp. 115-167. 1894. 



