850 ANNUAL REPORT OF THE Off. Doc. 



elements as different ones are attacked, thus making some phos- 

 phates available that Nveie previously unavailable, or even the reverse 

 may take place. 



It is probable that of the many methods proposed that none 

 of them will be equally well adapted to all classes of soils owing 

 to tiie selective power of certain acids for different combinations of 

 pliosphoric acid, and they will attack different types of soils with 

 more or less vigor, but in the main the relative action of all acids 

 on all soils will be alike. 



Hall and Plvmen have recentlv made an extensive research and 

 review of the methods proposed for available phosphoric acid, and 

 have reached the conclijsion that one per cent, solution of citric 

 acid gave results which are most in accord wath the known history 

 of soils. On soils well provided with carbonate of lime, there was 

 little difference in the results obtained with the different acids 

 tried. 



With the present state of the perfection of chemical analysis 

 of soils it will still be necessary to put much reliance upon the re- 

 sults obtained by the practical use of the various phosphates in 

 connection with the growing of different crops upon a variety of 

 soils. Upon the following pages will be given brief summaries of 

 prominent experiments of this character. 



EXPERBIEXTS WITH DIFFEEEXT FORMS AXD SOURCES OF 



PHOSPHORIC ACID. 



Almost as soon as the value of phosphoric acid began to be recog- 

 nized as an essential plant food, various experiments were conducted 

 as to the value of the phosphoric acid from different materials 

 and sources. These experiments have been repeated from time 

 to time under varying conditions and circumstances. The experi- 

 ments upon points which seem to have particular value to the 

 farmers in the United States are those conducted at the Pennsyl- 

 vania Experiment Station and at the Maryland Agricultural Ex- 

 periment Station. These two series of tests have been performed 

 upon two distinct classes of soils, which are representative of a 

 large percentage of those commonly cultivated, and the conditions 

 under which the tests were carried on are fairly representative of 

 ordinary farm practice. So the results would seem to have much 

 value for practical application. 



Article on the determination of available Plant Food, Jour. Cham. Soc. , Jjond.. Jan.. 1902, pp. 

 117-144. 



