THE ORGANIC FRACTION - BULK-DENSITY RELATIONSHIP AND THE EXPRESSION OF NUTRIENT CONTENT IN FOREST SOILS

Bulk density of forest soils from nine locations in New England was closely and inversely related to the organic fraction of the soil. Measured data over the whole range of organic fractions followed the theoretical relationship D(b) = D(bm)D(bo)/[F(o)D(bm) + (1 - F(o))D(bo)] where D(b) is the bulk density (Mg/m3), F(o) is the organic fraction (kg(o)/kg), D(bo) is the bulk density when F(o) = 1, and D(bm) is the bulk density when F(o) = 0. The relation arises from assuming that (i) D(bo), the bulk density of ''pure'' organic matter, and D(bm), the bulk density of ''pure'' mineral matter, are constant and (ii) in a mixture, the volumes occupied by the organic mass and the mineral mass are additive. For forest soils on coarse-textured till in New England, D(bo) = 0.11 Mg/m3 and D(bm) varied from 1.45 Mg/M3 for sandy loams to 2.19 Mg/M3 for silt loams. When these parameters are known, D(b) can be estimated from F(o), which is more easily measured. When F(o) is greater than 0.1 kg(o)/kg, the organic mass per unit soil volume (F(o)D(b)), or organic density, is approximately constant at 0.1 Mg(o)/m3. For many nutrients, separate evaluation of the organic density and the amount of nutrient per unit organic mass may facilitate intersite comparisons for studies of nutrient availability and leaching.