EVALUATION OF SULFUR MINERALIZATION POTENTIAL OF MEADOW SOILS AND AVAILABILITY TO ALFALFA

With the reduction in S inputs through atmospheric deposition and high analyses fertilizers, soil organic S will become a more important sou rce of S to crops. Sulfur mineralization in 20 meadow soils was invest igated using a long-term aerobic incubation and a greenhouse bioassay with alfalfa (Medicago sativa L.). The cumulative S mineralization (S( m)) was linear with time for 13 soils whereas 7 soils showed an expone ntial phase over the first 10 wk followed by a linear release phase. P odzolic soils dominated the latter group. The data were described by a n incremental zero-order model where an initial flush was accounted fo r. The total amount of mineralizable S (S(m)), the amount of S mineral ized in the first 1.4 wk (S(e)), the S mineralization potential (S(o)) and the rate constant (k) were all very closely correlated to the N m ineralization potential and to the amount of N mineralized in the firs t 1.4 wk of incubation (r = 0.64-0.85; P < 0.01). The S mineralization parameters were not significantly correlated to soil pH, total C, N, and S, C:S and N:S ratios and extractable P content. The S(e) and S(o) mineralization parameters were significantly correlated to the soil s and content (r = 0.51 and 0.53; P < 0.05) suggesting the possible invo lvement of particulate organic matter as a pool of mineralizable S. Al falfa yield response to S addition was observed only on seven soils wi th lowest cumulative S. Total S uptake of alfalfa represented, on aver age, 10% of S(m) and was strongly correlated to S(m) (r = 0.81*) and k (r = 0.78*). The effect of S fertilizer on plant S uptake was obser ved only for the third cut (P < 0.01), this being probably due to mine ralizable S depletion in soils. The results of this study suggest that the initial potential of N mineralization is a good indicator of S(m) and that a short term incubation could measure the S supplying power of Quebec soils.