Greenhouse experiments with ryegrass were conducted to evaluate and characterize plant availability of native and added sulfur in samples of Iowa soils. Fourteen surface soil (0-6 inches) and five subsoil (18-24 inches) samples from different sites in Iowa and two surface soil samples from S-deficient out-of-state sites were studied. Laboratory analyses were made to characterize the soil samples and to evaluate the results of different extractants as indexes of the S-supplying abilities of the soils. Plant uptake of S from the soil samples with no added S during a 202-day cropping period (five harvests) varied from 1 to 39 mg S/1500 g of soil (39 mg S/1500 g of soil is approximately equivalent to 50 lb S/acre six inches of soil in the field). Sulfur uptake was greatest during the first 70 days of cropping (two harvests), but continued at a slower, essentially constant daily rate throughout the rest of the cropping period. Although relatively slow for all soil samples, the rate of S uptake during this later cropping varied markedly among the different soil samples, with the rates for the surface soil samples being directly related to the amounts of S taken up by the plants in the earlier cropping period. The rates of uptake were very slow from most of the subsoil samples, and many plants on these subsoils died. Air-drying the soil samples before cropping resulted in increased plant yields and increased S uptake by the plants
