312 Journal of Agricultural Research voi. xii, no. 6 



The existence of appreciable quantities of water-soluble phosphates 

 was the point longest in doubt, though the results of Heyden {25) and 

 Schulze {51) established this fact satisfactorily. 



The most notable of the early investigations and those which have 

 had the greatest influence on modern work were performed by Schloesing 

 (46, 47). His method consisted of treating 30 to 35 kgm. of soil with 

 an artificial rain and then collecting the first portions of clear solution 

 which ran through. This he believed to be identical with the actual 

 soil solution, and his was the first attempt to obtain it in an unaltered 

 condition. Schloesing showed the presence of all the principal elements 

 in this soil extract, and his procedure is still occasionally used in Euro- 

 pean work. Schloesing, jr. {4.8, 49), has continued this portion of his 

 father's work, devoting special attention to water-soluble phosphates, 

 and has concluded that there are differences in the phosphate content 

 of various soils, and also that there is almost enough soluble phosphate 

 present in most soils to supply an average crop. 



The first work performed in the United States on the water-soluble 

 material of soils was that undertaken by King (36- j8) , and his coworkers 

 at Wisconsin. This was largely devoted to studies of the nitrate con- 

 tent of cultivated field soils, and was later extended to studies of the 

 total salt content by the use of conductivity methods. The results 

 obtained in this work may be considered to have been merely prelim- 

 inary to King's more extensive investigations performed in the Bureau 

 of Soils of the United States Department of Agriculture {34, 35). 



Before this later work of King's appeared, Whitney and Cameron (61) 

 issued a publication from the United States Bureau of Soils which has 

 attracted more attention than any other single paper dealing with water 

 extracts. In it they gave the amounts of phosphoric acid, nitrates, cal- 

 cium, and potassium found in the water extracts of both good and poor 

 soils under varying conditions. The extracts employed for these analyses 

 were obtained by stirring 100 gm. of soil in 500 c. c. of distilled water 

 for three minutes. After standing for 20 minutes the liquid was decanted 

 into a cylinder of Brigg's (6) filtering apparatus and forced through 

 unglazed Pasteur-Chamberland filters under pressure. They concluded 

 that practically all soils gave essentially identical solutions and that 

 even where only a small quantity of one constituent was present, it was 

 sufficient for the growth of a fair crop if the mechanical condition of 

 the soil was good. 



It was concluded also that the soil moisture was practically a saturated 

 solution of the mineral substances present in the soil. Consequently, as 

 fast as salts were removed by the plant further quantities were quickly 

 dissolved, thus keeping the solution at nearly the same concentration 

 throughout the growth of the plant. One of the most significant facts 

 claimed to have been shown by the investigation was that the equilib- 



