EXPERIMENT STATION BULLETINS. 501 



(b) Collecting the water that percolates from the bottom of a can 

 that contains a large amount of soil and so arranged as to represent 

 natural conditions. 



Some pertinent objections may be brought against this method. The 

 gravitational or drainage water as it moves downward through the 

 soil, carries with it only a small part of the film water, but if it is con- 

 tinued long enough, this film water will be entirely replaced by a new 

 one which for the time being is not concentrated to the same degree as 

 the one before it. Schloesing^ claims that drainage water, collected 

 after it has passed through a meter of earth, does not faithfully repre- 

 sent the solution which the arable layers imbibe. Lysimeters gave a so- 

 lution for only a particular depth under certain conditions, but not at 

 any given moment. In all the drainage water, according to Hilgard,- 

 the chief nutritive ingredients of plants, except nitrogen, are present in 

 traces only; chlorides, nitrates, sulfates of sodium and magnesium form 

 the bulk of the permanently soluble matter, with usually a considerable 

 proportion of calcic (and magnesic) carbonate depending upon the 

 amount of the earth carbonates present in the soil as well as upon that 

 of oxidizable organic matter from which carbonic acid can be formed. 

 The drainage waters, therefore, contain too much of some ingredients 

 and too little of others to give any definite information as to the soil re- 

 quirements. 



2. Soil Extracts. 



The method of obtaining soil extracts has, no doubt, been used more 

 than any other for the study of the soil. It depends upon whether a 

 total analysis of the soil is desired, or whether just the ingredients that 

 may become easily available to the plant are desired, as to the nature 

 and strength of solvent used. Different investigators use different sol- 

 vents for this purpose. The solvents may be classified under three 

 heads : mineral acids, organic acids, and water. 



In all the above extractions, none deals with a solution which con- 

 tains the exact amount of substances available to the plant. While they 

 are certainly valuable for detei'mining the total amount of various in- 

 gredients present, it is seriously to be questioned, if they give data on 

 actual soil conditions. Consequently water, the least objectionable, has 

 been used as a solvent for determining the available plant nutrients in 

 the soil. 



The methods of obtaining extracts by the use of water vary somewhat. 

 Some use pure distilled water, others distilled water with some gas as 

 CO, dissolved in it, others with NH^Cl in solution. In these two latter 

 methods large amounts of soil and large amounts of water containing 

 COo are used. To one only a small amount of NH^Cl is added. 



The Briggs^ filter method gives a soil extract. In this method water 

 is mixed with soil in proportions of 5 to 1, and the turbid supernatant 

 liquid is clarified by being forced through a Pasteur-Chamberland filter. 



Another common method, the tumbler or beaker method, consists 

 in thoroughly mixing the soil with water and filtering through filter 

 paper. Either a definite amount of water is used, or the soil is washed 

 till the filtrate reaches the desired amount. 



^Schloesing, Th. Analyse des Eaux contenucs dans Ics terres arables,— Comptes Keudus 

 de I'Academie des Sciences, 1870. 70, p. 99. 



=Hilgard. E. W. "Soils" (1912) p. 271. 



■'Briggs, L. J.. Filtration of Suspended Clav from Soil Solutions. Bui. 19, Bureau of Soils, 

 U. S. Dept. of Agr. p. 31 (1901). 



