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Soil Nitrogen, Sulfur, and Phosphorus in Relation to Growth Response of Western Hemlock to Nitrogen Fertilization

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Sixteen sites of western hemlock (Tsuga heterophylla) located in both the coastal and Cascade zones in western Washington were examined to determine the relationships of soil N, S, and P to growth response of the trees to N fertilization. The sites were chosen from among nineteen N fertilizer-test installations established in Washington in 1969 by the Regional Forest Nutrition Research Project of the University of Washington. The sites varied in productivity, but site index did not correlate with growth response. Concentrations of total N and extractable P were much higher in the forest floors than in mineral soils to a depth of 15 cm. Amounts of total N, mineralized N, and sulfate S were higher in mineral soils of the coastal sites than in those of the Cascades. Concentrations and amounts of extractable P of both the forest floors and mineral soils, however, were higher in the Cascades than on the coast. The nutrients studied and some of their ratios were significantly related to growth response to N fertilization. Overall, the strongest correlation was with extractable P in the forest floor (r = 0.77, P < 0.001) and the best correlations involving nutrients in mineral soil were with the ratios of extractable P/mineralized N (r = 0.67, P < 0.005) and extractable P/total N (r = 0.66, P < 0.01). The data strongly suggest that low levels of P alone or in combination with high soil N may be important factors in the reported lack of growth response of hemlock to N fertilizers. Forest Sci. 29:469-477.

Keywords: Tsuga heterophylla; extractable P; forest floor; mineral soil; mineralized N; mineralized S; nutrients; site index; sulfate S; total N

Document Type: Journal Article

Affiliations: Soil Scientist, Forest Land Management Center, Washington Department of Natural Resources, Olympia, Washington

Publication date: September 1, 1983

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