Physical and Chemical Properties of Pinus ponderosa Charcoal: Implications for Soil Modification

Charcoal is the result of incomplete combustion of plant material. It is common as residue from wild and prescribed fires in many soil surface horizons, including those of the extensive Ponderosa pine forests in the western United States, but its effects on soil properties are incompletely understood. The objectives of this study were (i) to determine the soil-relevant chemical and physical properties of Ponderosa pine (Pinus ponderosa, P. Lawson and C. Lawson) wood charcoal produced under controlled conditions, (ii) to compare the properties of that charcoal with those of charcoal produced by a prescribed fire and aged in the soil environment for 7 years, and (iii) to determine how charcoal additions to soil affect water-holding capacity and soil color. Laboratory-produced charcoal had a C/N ratio (644) about three times larger than that of the field-collected charcoal (195-261). Low specific surface area values for all samples (1.6-4.5 m(2) g(-1)) were apparently the result of relatively low combustion temperatures. All charcoals were highly water repellent (water drop penetration time 92 h), except samples collected immediately below the litter layer (water drop penetration time <10 sec). Water repellency was eliminated when the laboratory-produced charcoal was leached with various solutions (distilled water, ethanol, pine litter extract, dilute acetic acid) and air-dried. Laboratory-produced and field-collected charcoals had cation exchange capacity values of 19 to 29 cmol kg(-11). Addition of finely divided charcoal to the mineral soil material in this study significantly increased the available water-holding capacity and darkened the soil. Charcoal may play an important role in determining the properties, behavior, and classification of some surface soil horizons.