6. FERTILIZATION IN YOUNG STANDS

The benefits of early P fertilization on the poorly drained P-deficient flats of the lower coastal plain have long been recognized. Volume growth gains averaging 40 to 50 ft3/acre/year (3 to 4 m3/ha/year) are typical on severely P-deficient sites. Because the duration of response to a single application of 50 lbs/acre of P may last for 20 or more years (Pritchett and Comerford 1982), P fertilization on deficient sites may yield volume gains of over 100% and consequently is viewed as an improvement in site quality. Site index gains (25-year base) of 6 to 10 feet or more are typical when P is applied at or near time of planting (Gent et al. 1986). More recently, results from several Forest Nutrition Cooperative trials have shown that large areas of well-drained sites on the upper Gulf Coastal plain are also P-deficient (Figure 5; Allen 1990, Allen and Lein 1998). Unlike the well-drained sites on the Atlantic coastal plain, many Gulf coastal plain sites have never been in row crops and, consequently, have not been fertilized.

Identification of stands in need of early fertilization is been based on landscape/soil type, geology, soil and foliar tests, and experience. The critical value for soil P below which a fertilizer response is expected is 6 ppm (Mehlich-3 extraction procedure). Critical values for foliar P concentrations vary by species and range from 0.09% for slash pine and 0.11% for loblolly pine (Allen 1987, Jokela et al. 1991). The sources of fertilizer P that are typically used include diammonium phosphate (DAP), triple superphosphate (TSP), and rock phosphate. DAP is now the most widely used source for fertilization at time of planting. Rates of application vary from 25 to 50 lbs/acre of elemental P (125 to 250 lbs/acre of DAP). In 2002, almost 250,000 acres received fertilizer at or near time of planting.

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