Effects of Different Urea Fertilizers on Soil and Trees in a Young Thinned Stand of Western Hemlock
- M. A. Radwan and
- D. S. DeBell
Effects of three different urea fertilizers on selected chemical characteristics of soils and foliage, and on growth per tree of a 24-yr-old thinned stand of western hemlock [Tsuga heterophylla (Raf.) Sarg.] were compared at a site in coastal Oregon. Treatments were the following: untreated control (C), urea (U), urea treated with N-Serve®1 [nitrapyrin, 2-chloro-6-(trichloromethyl) pyridine] (U + NS), and sulfur-coated urea (SCU). Fertilizers were uniformly applied by hand to 0.03-ha plots in March at 224 kg N ha−1. Soil, to a depth of 20 cm, and current-year foliage were sampled periodically for 2 yr. Height and diameter of 10 dominant or codominant trees per plot were measured annually for 6 yr. Treatment effects on soil pH varied by fertilizer and sampling date; throughout, pH was lower with U + NS than with U, and the smallest change in pH was associated with SCU. Effects of fertilizer on inorganic N in the soil and on foliar nutrients varied by fertilizer and sampling date. Initially, N-Serve inhibited nitrification and produced the highest NH4-N concentrations. In general, all fertilizers increased inorganic N in the soil and total N in the foliage. Fertilizers significantly reduced foliar concentrations of some other macronutrients, especially in the first year after fertilization; some depressions were significantly less with SCU than with the other two fertilizers. Height growth per tree was not significantly affected by any of the fertilizers. Basal-area and volume growth per tree were significantly greater with the SCU treatment than with the control, U, or U + NS treatments. Beneficial effects of SCU seemed to be mostly due to the slow release of N from the fertilizer, although some positive effect of S cannot be ruled out.
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