E. J. Russell and J. A. Prescott 



97 



are convenient quantities; this makes three sticks, which for better 

 protection are put into a muslin bag; this is then suspended from a 

 glass rod into a 250 c.c. bottle containing the acid. Twice in each 

 24 hours the liquid is changed, and the whole is evaporated to dryness. 



The results given in Table VI show how the P2O5 comes out from 

 the soil ; the experiment was continued for 21 periods each of 24 hours. 

 For comparison the "Total P2^5" (*-^- ^^^ amount soluble in boiling 

 concentrated HCl) and the " available P2O5" {i.e. the amount soluble 

 in 1 % citric acid) are also given. 



In practice it is unnecessary to pour of? more than 10 times for 

 citric acid, or 20 times for hydrochloric or nitric acid. There is, of 

 course, no need to make separate determinations : it is easier and more 

 accurate to continue the evaporations in the same basins till the process 

 has gone sufficiently far. 



When the total extractable P2O5 has been determined the value 

 is used as above for determining y, and the equation is then easily 

 completed. 



The results obtained with five difierent soils are given in Table VII, 

 and plotted in Figs. 4 and 5. The logarithms of y are also plotted 

 against those of c, giving straight lines, showing that the equation holds 

 both for rich and for poor soils. In the rich Hoos 2C soil (Fig. 5) the 

 experimental numbers all lie on the same side of the bend so that the 

 curves cannot be verified as completely as we would like, but in the 

 poorer soils there is no difficulty on this score (Fig. 4). 



Table VII. Values for y and C for different soils treated with NJ 10 

 or iV/5 acid containing varying quantities of Na-^HPO^. 



Agdell soil. Time = 1 hour. N/IO HNO^. Temperature 23° C. 



100 gms. of soil to 1000 c.c. of solution. 

 P2O5 brought out by diffusion = 0-025 % = 25 mgms. per 100 gms. 



of soil. 



PoOg added (mgms. per 



"lOOOc.c.) 



P2O5 found in solution (G) 



(mgms. per 1000 cc.) 



P2O5 left in soil (y) (mgms. 



68-6 

 44-5 

 491 



Equation obtained : y = ir3C2'6 

 Journ. of Agric. Sci, vni 



