54 

 the problem of turbid extracts. The Ca ions suppress organic matter 

 extraction by promoting f locculation, and the phosphate anions dis- 

 place adsorbed sulfate. Hoeft et al . (1973) also obtained significant 

 correlations with Ca(H ? PO,) 9 extracts and S uptake and yield of alfalfa. 

 A Ca(H ? P0,) 9 solution of 500 ppm ? in 2 N acetic acid solution ex- 

 tracted an average of 4 ppm more S per sample and gave slightly higher 

 correlations than Ca(H ? P0,)„ solutions alone; this was probably due to 

 the extraction of some plant-available, organic sulfates. Other 



researchers have used Ca(H„?0,)„ extractions as effective indices of 



2 4 2 



available S (Barrow, 1967; Rehm and Caldwell, 1968). Sodium acetate 

 and ammonium acetate solutions have been effectively used as sulfate 

 extractants (Ensminger, 1954; Bardsley and Lancaster, 1960; Bartlett 

 and Neller, 1960; Bardsley and Kilmer, 1963; Jordan, 1964; Nelson, 

 1964a, 1964b; Rehm and Caldwell, 1968). However, acetate is not a 

 strong replacer of sulfate in neutral solutions (Beaton et al. , 1968). 



Acidic solutions would not be expected to increase sulfate extrac- 

 tion because the adsorption of sulfate by soils increases with decreas- 

 ing pH (Ensminger and Freney, 1966). Spencer and Freney (1960) com- 

 pared a number of extraction methods and found that the amount of 

 sulfate extracted from soils increased in the following order: 



acetate < cold-water < phosphate < hot-water . 



Bardsley and Lancaster (1965) reported that the alkaline extrac- 

 tion of soils with NaHC0_ removed more S than is obtained with acetate 

 extractants. Sulfur extracted from 30 soils with NaHCO-, at pH 8.5 

 correlated well (r = 0.89) with S "A" values. Plants with less than 

 10 ppm extractable S responded to applications of S. This extractant 

 was effective in solubilizing and replacing anions as well as some 



