Wood Ash as a Source of Phosphorus and Potassium

Ash is composed of many major and minor elements needed by the tree for plant growth (Table 1). Since most of these elements are extracted from the soil and atmosphere during the tree's growth cycle, they are elements that are common in our environment and are also essential elements in the production of crops and forages. Calcium is the most abundant element in wood ash and gives the ash properties that are similar to agricultural lime. Ash is also a good source of potassium, phosphorus, magnesium, and aluminum. In terms of commercial fertilizer, average wood ash would probably be about 0-1-3 (N-P-K). In addition to these macronutrients, wood ash is also a good source of many micronutrients that are needed in trace amounts for adequate plant growth. Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low and not in a highly extractable or available form.

Field and greenhouse research have confirmed the safety and practicality of recycling wood ash on agricultural lands. It has shown that wood ash has a liming effect of between 8 and 90% of the total neutralizing power of lime and can increase plant growth up to 45% over traditional limestone. The major constraints to land application of wood ash are transportation costs, low fertilizer analysis, and handling constraints. With ever increasing disposal costs, land application of wood ash will probably be the disposal method of choice in the coming century resulting in savings for the industry, an opportunity for agriculture, and conservation of our resources.

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