AGRONOMIC EFFECTIVENESS OF WOOD ASH AS A SOURCE OF PHOSPHORUS AND POTASSIUM

Wood ash is a residual material produced when wood is burned for energy production. It is a useful soil amendment because it raises soil pH and has the potential to supply plant nutrients. The two studies described in this paper were designed to assess the plant availability of P and K in wood ash. Both studies compared nutrient uptake by corn (Zea mays L.) grown in the greenhouse in wood ash-amended soils to nutrient uptake from fertilizer-amended soils. The wood ashes used were chemically characterized for available P and soluble K content by ammonium citrate extraction, as well as for total P and K. In addition wood ash- and fertilizer-amended soils from the P availability study were extracted with Bray-1, Olsen, and NH4OAc solutions after harvest. Wood ashes were found to be more similar to conventional P fertilizer materials when the amendments were compared on a citrate-extractable P basis than when they were compared on a total P basis. For K there was little difference between ash and fertilizer sources, whether they were compared on a total K or a citrate-extractable K basis. The buffered NH4OAc (pH 3.0) extract was adequate for determining K availability in wood ash-amended soils, but inadequate with regard to P availability in these soils. The buffered solution may have dissolved residual wood ash and released P that was not actually plant-available. Either the Bray-1 or the Olsen extract provided a relative measure of P availability that was consistent for both fertilizer- and wood ash-amended soils.