Wood ash can substitute for lime and other fertilizer applications

Staff report

MADISON, Wisconsin: Wood ash from wood-fired boilers can substitute for agriculture lime on a one-to-one ratio in some cases, according to Wisconsin research.

In forested northern Wisconsin, there are no limestone deposits and farmers are forced to pay high prices for agricultural lime. The soils are naturally acidic and low in potassium which limits their usefulness for legume-based pasture systems.

Wood ash from industrial boilers in the region is currently being put in landfills but could be a valuable soil amendment and a potential substitute for commercial lime and fertilizer. This would reduce disposal costs to industry and fertilizer costs to the farmer.

Each ton of wood ash could substitute for one-half to one ton of agricultural limestone and could supply 25-70 pounds of potash (K20 equivalent) and 30 to 32 pounds of phosphate (P205 equivalent). Based on recent lime and fertilizer prices, the average wood ash is worth $24 to $30 a ton.

The fertilizer benefit of wood ash differs as to its source. Depending upon the burning process and type of wood, the neutralizing index of industrial wood ash may vary from grade 40 - 49 to 80 - 89 on the scale used to rate commercial ag lime.

This means the ash must be applied at from one to two times the recommended rate of 80 - 89 ag lime to achieve the same effect.

However, wood ash’s small particle size may bring about a more rapid soil pH change than that produced by agricultural lime.

When wood ash is applied at rates likely to adjust soil pH, it also supplies substantial amounts of several plant nutrients including potassium, phosphorous, calcium and magnesium.

In addition to its value as a substitute for lime and macronutrients, ash can also supply significant amounts of sulfur, boron and other micro-nutrients.

Research in Minnesota and Wisconsin have found that barley and alfalfa yields from fields that received 5 to 20 tons of wood ash to be significantly higher than those from commercial lime and fertilizer applied at rates recommended by soil test.

No harmful crop effects were noted when ash was applied at rates of up to 20 tons per acre. However, wood ash is highly alkaline and care must be taken to prevent inhalation or contact with the skin and eyes.

Fresh ash is very difficult to spread because of its dustiness; therefore water should be added to it before it is spread.

Wood ash that has been made damp can be spread with manure spreaders, lime trucks or fertilizer wagons. Ash can be loaded from a stockpile with a skid steer or tractor with a front end loader.

Wood ash landspreading is exempt from Wisconsin’s landspreading code as long as the ash has been tested, found to have a soil amendment value and applied according to standard agricultural practices.

General procedures for obtaining a “grant of exemption” for wood ash landspreading can be obtained from the Wisconsin Department of Natural Resources.

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