Soil Boron in Loblolly Pine Plantations of the Southeastern United States

Abstract

Boron (B) is not well understood in intensively managed pine plantations across the Southeastern United States. Forest fertilizer research has focused primarily on N and P applications, with essentially no research being conducted on micronutrients such as B. The objectives of this study were to quantify soil B fractions and to identify relationships that may exist between soil B fractions and soil and site properties. Soil samples from 50 sites scattered across 11 states were used to provide a wide range of soil and site properties. Soil properties including total carbon, pH, exchangeable acidity, exchangeable base cations, effective cation exchange capacity, and particle size were determined. Site properties such as 30-year average annual precipitation, drainage class, soil taxonomic information, and parent material were used for the analysis. Three B fractions were chosen for analysis due to their relative quantities and overall contribution to the B pool. The readily soluble B fraction was determined by boiling a 2:1 solution of 0.02 M CaCl2 and soil. This fraction ranged from 0.00 to 1.35 mg B kg-1, with a mean value of 0.28 mg B kg-1. The specifically adsorbed plus oxide bound fraction was determined using a sequential extraction procedure with 0.2 M acidic ammonium oxalate. The oxide bound B fraction ranged from 3.8 to 30.5 mg B kg-1 soil and the mean was 11.7 mg B kg-1 soil. The organically bound fraction was extracted with 0.02 M nitric acid (HNO3) and 5-mL of 30% hydrogen peroxide (H2O2). The organically bound B fraction ranged from 5.1 to 650.6 mg B kg-1 soil, with a mean value of 75.5 mg B kg-1 soil.

The readily soluble fraction was significantly and positively (r = 0.57) correlated with 30-year average annual precipitation. The specifically adsorbed plus oxide bound fraction was significantly and positively correlated with the effective cation exchange capacity (r = 0.62) and exchangeable base cations (r = 0.75). The organically bound fraction was significantly and positively correlated with total carbon (r = 0.60) and exchangeable acidity (r = 0.60).

Organically bound B contained the highest overall levels of B. Organically bound B from poorly drained soils were approximately triple that of well-drained soils. Poorly drained soils contained nearly twice as much total carbon as well drained soils. The readily soluble fraction contained the lowest levels of B followed by the oxide bound fraction. Soils derived from marine sediments contained the highest levels of B across all fractions.

Results from this work should provide the groundwork for understanding B in forest soils through the characterizations of B outlined in this study.