34 



The two soils used in the experiments described contained 

 practically equal amounts of total phosphoric acid, and phosphoric 

 acid soluble in hydrochloric acid 1.115 S P- r -> while the avail- 

 bility of that in the cultivated soil, as shown by the weak acid 

 solvents, was about five times as great as that in the uncultivated 

 soil. The chemical analyses of the two present no differences 

 that could serve as an explanation of the difference in availability. 

 This difference, therefore, is probably due, partly to the state of 

 cultivation, and partly to the physical characteristics of the soils. 



From the results obtained it is doubtful whether hydro- 



200 



chloric acid would prove a suitable solvent for the determination 

 of available plant food in all soils. Comparing the two soils, 

 however, the results with it indicate in a general way the same 

 relative availability of the phosphoric acid as is shown by the 

 solubility in i per cent citric acid. 



In regard to the action of lime, the solubility in citric acid 

 was increased slightly in both soils. The solubility in hydro- 

 chloric, acid was not increased in the case of the uncultivated 

 soil. This soil evidently held the phosphoric acid in a very 

 insoluble form. 



It is interesting to note the difference in action of lime on 

 natural and artificial soils. A comparison is made in the follow- 

 ing table. 



SOLUBILITY OF P 2 O 5 IN I PER CENT CITRIC ACID 



Without lime. With lime. 



Soil "A" 19.3% 21.3% 



Soil "A" 20.7% 22.0% 



Artificial soil i7-i% 43-7% 



Artificial soil ^7-9% 42>-o% 



The figures represent per cent of the total phosphoric acid. 

 It is evident that the natural soil was less susceptible to the 

 action of lime than the artificial product. 



(d) FIXATION OF PHOSPHORIC ACID IN THE PRESENCE OF 

 COMPOUNDS OF IRON AND CALCIUM 



While the tendency of soluble phosphates, applied to soils, 

 is to pass into the insoluble phosphate of iron, several investi- 



