No. 6. DEPARTMENT OF AGRICULTURE. 849 



From these figures it will be seen that the commercial or trade 

 vaiue of 100 lbs. of soluble phosphoric acid in a dissolved bone would 

 be §5.00, while 100 lbs. in a dissolved rock would be but §3.00. From 

 an agricultural standpoint, in view of all the experiments which 

 have been conducted, the value of phosphoric acid from the two 

 sources would be exactly the same. 



Again, the above valuations place a higher valuation upon the 

 total phosphoric acid in cotton-seed meal than that from bone, 

 whereas, when applied, probably there would be no difference so far 

 as phosphoric acid is concerned. 



From what has been said it would be plain to every farmer that 

 he should buy a phosphate or, indeed, any fertilizer with reference 

 to the value it has to him in increasing the productive capacity 

 of his soil and not purchase solely upon the basis of commercial 

 valuations as represented by agents or tabulated analysis of fer- 

 tilizers. 



THE DETERMINATION OF AVAILABLE PHOSPHORIC ACID IN SOILS. 



The purpose of the agricultural chemical examination of soils, 

 from the earliest time when the science was employed in this way, 

 vva« to throw some light upon the relations of the various constitu- 

 enr.>s to plant growth and especially to determine the amounts 

 of the essential constituents which are in a condition to be used 

 by crops or. in other words, "available." There have been numer- 

 ous methods proposed for distinguishing between available and 

 unavailable plant foods in various kinds of soils, and this problem 

 has ocr-upied the attention of the best minds in agricultural chem- 

 istry for many years. No one element in this study has received 

 as much attention as the phosphoric acid. 



At the present day there seems to be a general agreement that 

 the use of weak solutions or solvents give results that more nearly 

 correspond to the results obtained by cropping, yet there is much 

 di^Verence of opinion as to the proper acid or solution to use. 

 This condition, no doubt, is due to the variations in the eKeimGal 

 characteristics of the soils experimented upon. 



The present status of the results obtained in this research would 

 seem to indicate that it is very improbable that a marked distinction 

 of any kind can be drawn between "available" and "unavailable" 

 compounds of phosphoric acid in the soil, for the reason 

 that is is not probable that any soil contains a compound or 

 group of compounds which can be wholly removed by plants or dis- 

 solved by an acid that is "available," before the remaining com- 

 pounds are attacked. From the very nature of the changes which 

 are taking place in soils, produced either by crops or natural agen- 

 cies, there mu^t be more or less change and re-arrangement of the 

 54—6—1902 



