AVAILABILITY OF PLANT NUTRIENTS 337 



acid is well adapted to show the amount of easily- 

 soluble phosphoric acid and potash in certain soils, but 

 for other soils it has failed to give satisfaction in 

 the hands of a number of analysts. It is doubtless 

 best suited to soils rich in calcium and low in iron and 

 aluminium. 



The reason urged by Dyer for the superiority of the 

 citric acid method over the hydrochloric acid extraction 

 is that soils, shown by experience, to need phosphoric 

 manures, yielded a relatively much greater quantity of 

 phosphorus to citric acid than to hydrochloric acid when 

 compared with soils not needing this element. 



The application of both the hydrochloric and citric acid 

 methods to a soil, when used to supplement each other, 

 may add greatly to a knowledge of the potential and 

 present productiveness of the soil. 



According to Dyer, 1 for cereals and for most other 

 crops there should be present in a soil at least .01 per cent 

 of phosphoric acid, soluble in one-per-cent citric acid. 

 A soil containing less than this quantity is deficient in 

 phosphoric acid, unless this acid exists largely in the form 

 of ferric or aluminium phosphate, which is not readily 

 soluble in citric acid but is fairly available to the plant. 

 Sod land contains organic compounds of phosphorus that 

 are readily available to the plant; hence such soil, to 

 indicate sufficiency, should show by analysis more than 

 0.01 per cent of phosphoric acid. The quantity of potash 

 soluble in the same solvent should also be not less than 

 0.01 per cent in arable land. 



1 Dyer, Bernard. A Chemical Study of the Phosphoric 

 Acid and Potash Contents of the Wheat Soils of Broadbalk 

 Field, Rothamsted. Philosoph. Trans. Royal Soc. London, 

 Series B, Vol. 194, pp. 235-290. 1901. 



