Industrial wood ash on crop land
When available and cheap, wood ash can be a great deal for acidic, low fertility soils.
In heavily wooded areas of northern Michigan, wood ash is a fairly common byproduct of manufacturing companies. Paper mills, sawmills, log home manufacturers, flooring mills, pole plants and smaller wood products companies produce a stream of wood ash. Where soil conditions include low pH, low potassium and generally low fertility, application of wood ash on crop land makes good sense.
Depending on wood ash characteristics, each ton of wood ash could substitute for one-half to one ton of agricultural lime, and could supply 25 to 70 pounds of potash (K2O equivalent) and 30 to 32 pounds of phosphate (P2O5 equivalent). Much of the ash production in Michigan takes place in areas with limited access to agricultural lime. The qualities of wood ash from different sources vary quite widely. This variation is due to the type of wood burned and the burning process. In general, hardwoods result in a higher quality wood ash than softwoods.
Ash samples collected in northern Wisconsin in the 1990s showed nutrient contents as follows:
- Calcium 22 – 24%
- Magnesium 1.2 – 2.2%
- P2O5 equivalent 1.1 – 2.3%
- K2O equivalent 1.3 – 4.6%
In addition, the samples included small amounts of iron, manganese, zinc, boron and sulfur. Heavy metal concentrations were very low.
Research from Minnesota and Wisconsin demonstrates that wood ash can be a safe and effective soil amendment. On-farm wood ash demonstrations and replicated trials on several crops in the Upper Peninsula during 1997 – 1999 were complicated by drought conditions, but resulted in data suggesting positive yield and soil fertility outcomes. Barley yields were increased significantly by applying 2 tons per acre wood ash on sandy loam soils in Alger County. Oat and alfalfa yields also tended to increase with wood ash application, but not significantly. On acidic clay soil in Chippewa County, 0.6 tons of wood ash per acre was top-dressed on mixed grass or legume hay on an organic farm. Yield checks suggested a positive yield response, although not statistically significant.
Transportation, storage and spreading wood ash pose some unique challenges. It is very dusty when first taken from a combustion facility and should be covered during transport. It can also be very hot when fresh and create a fire hazard. Once stockpiled on the farm, it quickly forms a crust and blowing is not usually a problem. Ash should be analyzed for lime equivalency and nutrient content before application. For most farmers, spreading can be accomplished most practically with a manure spreader. Because of its dustiness, water should be added to wood ash before it is spread.
In many cases, there are more people requesting access to wood ash from combustion facilities than the facilities can satisfy. One of the challenges with using wood ash on crop land is simply getting access to it. An interested farmer should pencil out the potential benefit of wood ash use, including the cost of wood ash, lime and fertilizer value of the ash, trucking and spreading costs, before negotiating with a potential wood ash generator.
For more information, check out University of Wisconsin Extension publication A3635 Using industrial wood ash as a soil amendment and University of Maine Cooperative Extension bulletin #2279 Using Wood Ash on Your Farm.