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Foliar and Soil Tests for the Prediction of Phosphorus Response in Loblolly Pine¹

ABSTRACT

Loblolly pine (Pinus taeda L.) fertilization studies in the southeast USA were used to relate tree growth response to soil tests and foliar analyses data. In the Lower Coastal Plain, a needle P concentration of 0.80 g kg^–1 and less indicated a severe P deficiency, and 5-yr height growth increased by 50 to 140% after fertilization of young stands with 50 kg P ha^–1. Stands with foliar P from 0.80 to 1.0 g kg^–1 generally responded 15 to 50% in height growth to fertilization. Height and volume response to P fertilization in the Upper Coastal Plain and Piedmont were not correlated to P concentration of needles. Soil drainage class of the Lower Coastal Plain was useful in separation of soils responsive and unresponsive to P. Loblolly pine growth on 12 of 13 very poorly and poorly drained soils was stimulated 15% or more by P fertilization, whereas this level of response was found on only two of 11 better drained soils. Soils influenced by phosphatic deposits and previous P application should be excluded when using drainage alone as an indicator of P responsiveness. Total soil acidity separated P responsive and unresponsive soils in the Lower Coastal Plain. Where total soil acidity was 5 cmol (+) L^–1 and less, height response to P fertilization varied from 15 to 138%. Extractable P as measured was not useful. A maximum volume response of 30% was obtained from P fertilization in the Upper Coastal Plain. When Mehlich 1 extractable P of the 0- to 20-cm depth was <3 mg L^–1 the 5-yr volume response generally was from 14 to 30%. Improved separation of P-responsive stands appeared possible by using soil acid saturation and pH. As acid saturation increases and pH decreases, the response to P fertilization tends to increase. Phosphorus response in the Piedmont ranged from a negative 19 to positive 21% and no relationship was found for prediction of response within the range of 2 to 6 mg L^–1 of Mehlich 1 extractable P.

¹ Contribution from USDA Forest Service, Southeastern Forest Experiment Station, Asheville NC.

² Soil Scientists, USDA Forest Service, Southeastern Forest Experiment Station, Research Triangle Park, NC 27709; formerly Associate Director, North Carolina State Forest Nutrition Cooperative (NCSFNC), North Carolina State Univ., Raleigh, NC; Director NCSFNC, NCSU, Raleigh NC, respectively.

Received for publication September 27, 1985.