Sulfur Status of Some Central Oregon Pumice Soils

In a fertilizer experiment established in a 45‐year‐old stand of ponderosa pine ( Pinus ponderosa Laws) on a pumice soil, 112 kg S/ha added as gypsum resulted in a small increase in tree growth in the first 5‐year period. The combination of S with 224 kg N/ha gave a greater response. Over a 10‐year period, basal area growth was 50% greater than in untreated plots. The soil layers in the tree rooting zone at this and four other sites were collected for a pot trial bioassay of their S status. In all cases the growth of Monterey pine ( Pinus radiata D. Don) seedlings was improved by the addition of S. At all sites the soil layer least deficient in S was the A1 horizon which varied from 5 to 15 cm in thickness. Although the results indicated that the field trial site is more S deficient than the others, they also show that many of the pumice soils in central Oregon are marginally deficient in S. Forest management operations, such as site preparation, which remove the surface soil, will seriously lower the S as well as the N and P levels of the soil and if productivity is to be maintained, the use of fertilizers will be essential.