53 

 procedure has been consistently superior in predicting response to 

 applied S. 



Most surface soils contain so little water-soluble S that fre- 

 quently poor correlations with S uptake are obtained with water 

 extracts, dilute, neutral-salt extracts, and weak-acid extracts which 

 contain no replacing anion. Water, especially hot water, may extract 

 a portion of the soil organic matter. This imparts a color co the 

 solution and interferes with the precipitation of sulfate. The 

 organic matter must be digested before sulfates can be determined tur- 

 bidimetrically, and digestion could increase the measured sulfate con- 

 centration of the extract. If only reducible S is determined (Johnson 

 and Nishita, 1952), then color and organic matter are no problem in 

 the extract. Neutral salts such as CaCl„, MgCl„, and LiCl extract 

 less organic matter than water but are not effective in removing 

 adsorbed sulfate (Ensminger and Freney, 1966: Harward and Reisenauer, 

 1966; Roberts and Koehler, 1968). 



Extractants which have gained confidence among researchers as the 

 most satisfactory indices of plant S uptake and yield have been those 

 extractants which remove readily-soluble sulfates, portions of the 

 adsorbed sulfates, and possibly some organic S. Beaton et al. (1968) 

 pointed out that adsorbed sulfate is in kinetic equilibrium with the 

 sulfate in solution, and it may be replaced by other anions of greater 

 coordinating ability according to the 13/otropic series: 



hydroxyl > phosphate > sulfate = acetate > nitrate = chloride . 



Ensminger (1954) showed that adsorbed sulfate could be extracted 

 with a KH PO, solution containing 500 ppm P. Fox et al . (1964) used 

 Ca(H.- > P0,)„ because it gave similar values to KH 9 P0, and also eliminated 



