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Long-Term Availability of Applied Phosphorus to Loblolly Pine on a Piedmont Soil¹

ABSTRACT

A loblolly pine (Pinus taeda L.) phosphorus (P) fertilization trial was evaluated 20 years after establishment on a Tatum silt loam (clayey, mixed, thermic, Typic Hapludults), in the Virginia Piedmont. Triple superphoshate (TSP) was applied at 160 kg P/ha and ground rock phosphate (GRP) at both 160 kg P/ha and 670 kg P/ha. Lime was applied (4.48 Mg/ha) with and without TSP. Tree growth was not significantly affected by treatment and foliar P levels were above 0.10% on control plots indicating that a deficiency was not the immediate growth-limiting factor. Soil and foliar P levels were highest where GRP was applied at 670 kg P/ha. Foliar P levels and double acid (0.05M HCl + 0.0125M H₂SO₄) extractable soil P were highest when 160 kg P/ha was applied as GRP vs. TSP. Liming caused a pH increase which persisted even after 20 years, but had no direct effect on P availability.

¹ Contribution from the Dep. of Forestry, Virginia Polytechnic Institute and State University, Blacksburg.

² Former Graduate Research Assistant and Assistant Professor of Forest Soils, respectively, Dep. of Forestry, Virginia Polytechnic Institute and State University, Blacksburg.