ACTIVE PLANT FOOD, ETC. iSl 



cent, citric acid thus corresponds with the treatment of the plot 

 and the yield of wheat. The phosphoric acid soluble in strong 

 hydrochloric acid was 0.114 to 0.219 per cent., potash 0.197 to 

 0.285 ; thus while there was some difference, it is not a clear 

 indication as to the fertility of the soil. 



The application of the citric acid method to soils of varying 

 character has not always given good results. This may be due to 

 several reasons : ( i ) The plant may have greater difficulty in 

 obtaining citric acid soluble plant food in some soils than in 

 others. In other words, it may be necessary that the standard 

 vary according to the character of the soil, as Hilgard's standards 

 do. (2) The soil may not have been deficient in phosphoric acid 

 or potash when it was supposed to be. (3) Different rates of 

 change of potash and phosphoric acid into more soluble com- 

 pounds in different soils may interfere. 



According to Dyer, less than o.oi per cent, phosphoric acid or 

 potash soluble in phosphoric acid indicates a deficiency. 



Other Weak Solvents. 1 These include N/5 nitric or hydro- 

 chloric acid, N/2OO hydrochloric acid, 2 and N/5 oxalic acid which 

 have been suggested for the same purpose as I per cent, citric acid. 

 Some of these solvents have the advantage of greater ease of 

 manipulation than citric acid. The results vary according to the 

 solvent employed. 



The most promising solvent is fifth-normal nitric acid. Fifth- 

 normal nitric or hydrochloric acid gave the same results on cer- 

 tain of the Rothamsted soils as citric acid, and upon some other 

 soils tested by the Association of Official Agricultural Chemists 

 the results were more nearly in accordance with the needs of the 

 plant. 



Factors of Availability of Plant Food. 3 The amount of any 

 given plant food which is withdrawn from the soil by the plant 

 does not depend upon one condition only, but is dependent upon 



1 See Proceedings Association Official Agr. Chem., Bulletins 47, 49, 

 51, 56, 67, 73, Division Chem., U. S. Dept. Agr. 



2 Moore, Jour. Am. Chem. Soc., 1912, p. 791. 



3 Fraps, Am. Chem. Jour., 1904, p. i. 



