Sulfur Requirements of Nitrogen Fertilized Douglas-fir
In a series of Pacific Northwest Douglas-fir nitrogen fertilizer trials, foliage SO4-S was found to be a correct predictive index of response in 17 of the 19 stands studied, whereas using foliage nitrogen concentrations the correct prediction was obtained in only 12 stands. To obtain a tree growth response to an application of 220 kg/ha N, resulting in an increase of 0.4 percent foliage N, approximately 0.04 percent foliage SO4-S is required. As SO4-S concentrations are not routinely determined in the fertilizer project laboratories, other parameters providing potential indirect estimates of SO4-S status have been assessed. Using a combination of soil parent material and the separately measured soil nitrogen content (kg/ha), predictions of fertilizer response comparable to those using foliage SO4-S concentrations have been obtained. Trees located on soils derived from sedimentary and glacial parent materials responded to N fertilization when the soils contained less than 8,000 kg/ha N in the rooting zone whereas no trees were found to respond on soils derived from pumice (less than 2,400 kg/ha N) and basalt parent materials. In these latter cases foliar SO4-S levels were less than 0.005 percent. Forest Sci. 25:461-467.
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