Wisconsin industries produceapproximately 50,000–80,000 tons ofwood ash each year. Even though woodash has been used as a beneficial agri-cultural soil amendment for centuries,much of the ash generated today isstored in landfills at significant expenseto industry.This publication explainshow ash generated by woodburningindustries (such as paper mills and woodproducts processors) can be used as asafe, effective and economical soilamendment.

The potential ofwood ash as a soil amendment

Many of Wisconsin’s agriculturalsoils are naturally acidic and low inavailable potassium.This limits theirability to produce high yields of legumeforage crops such as alfalfa or redclover—the main species that supportWisconsin’s dairy industry.

To grow these crops,Wisconsinfarmers invest millions of dollars eachyear in lime and fertilizer.

Wood ash currently being put inlandfills has properties that make it avaluable soil amendment and a potentialsubstitute for much of the commerciallime and fertilizer now used onWisconsin farms. Since there are nolimestone deposits in northernWisconsin (see figure 1), farmers in thisregion pay high prices ($20–$30 perton) for agricultural lime.

Each ton of wood ash could substi-tute for one-half to one ton of agricul-tural limestone, and could supply 25–70pounds of potash (K2O equivalent) and30–32 pounds of phosphate (P2O5equivalent). Based on current lime andfertilizer prices, average ash is worth$24–$38 per ton. Much of the state’s ashproduction takes place in areas thatcould benefit greatly from an alternativelime source (figure 2).

Using industrial wood ash as a soil amendmentMark J. Kopecky, N. Larry Meyers and Wally Wasko

A3635

Figure 1. Limestone deposits

Light portions of the map show areas with limestone deposits. None

12 to 34 to 67 to 11

Figure 2. Commercial woodburning facilities in 1990 by county

Source: Wisconsin Energy Bureau

Chemical characteristicsBenefits

Wood ash from different sourcesvaries in the nutrients it contains and inits ability to neutralize soil acidity.Depending on the burning process andthe type of wood, the neutralizing index(NI) of industrial wood ash may varyfrom grade 40–49 to 80–89 on the scaleused to rate commercial aglime.Thismeans that ash must be applied at fromone to two times the recommendedrate of 80-89 ag lime to achieve thesame effect.Wood ash’s small particlesize may bring about a more rapid soilpH change than that produced by agri-cultural lime.

Table 1 shows the concentrationlevels of several chemical elements in 16ash samples.The samples came fromboth bottom ash and fly ash sourcesfrom the northern part of Wisconsin.

Lime and fertilizer valueof wood ash

When wood ash is applied at rateslikely to adjust soil pH, it also suppliessubstantial amounts of several plantnutrients including potassium, phospho-rus, calcium and magnesium.The mon-etary value of wood ash is greatest inareas where agricultural limestoneprices are high and where ash sourcesare located nearby.The potential dollarvalue of wood ash is shown in table 2.Depending on the source of the ash andsoil test recommendations, a wood ashapplication may satisfy a field’s fertilizer

needs in the year it is applied. In addi-tion to its value as a substitute for limeand macronutrients, ash can also supplysignificant amounts of sulfur, boron andother micro-nutrients.

Heavy metal contentThe concentration of heavy metals

in wood ash is very low.Table 1 showsthe range of concentrations of someheavy metals in ash samples taken fromnorthern Wisconsin.An application ofone ton of wood ash per acre wouldapply less than 1% of the cumulativeloading limit of heavy metal elements(Cd, Cr, Cu, Ni, Pb Zn) regulated bythe Wisconsin Department of NaturalResources (DNR).

Table 1. Elements found in 16 ash samples taken from northern Wisconsin.

Element Range of concentrations Pounds applied in 10–ton/acre ash application

Calcium (Ca) 22–45% 4,400–9,200

Magnesium (Mg) 1.2–2.2% 240–440

Phosphorus (P2O5 equivalent) 1.1–2.3% 230–460

Potassium (K2O equivalent) 1.3–4.6% 260–910

Aluminum (Al) 0.2–1.1% 42–230

Iron (Fe) 0.1–1.1% 26–210

Manganese (Mn) 0.06–0.3% 12–56

Zinc (Zn) 0.01–0.5% 3–100

Boron (B) 0–210 ppm1 0–4

Chromium (Cr) 0–14 ppm 0–0.3

Copper (Cu) 0–54 ppm 0–1.1

Nickel (Ni) 0–7 ppm 0–0.1

Cadmium (Cd) 0–22 ppm 0–0.4

Cobalt (Co) 0–6 ppm 0–0.1

Lead (Pb) –17 ppm 0–0.3

Sulfur (S) See footnote 2 See footnote 21 ppm = parts per million2 Sulfur concentration was not determined in the Wisconsin research. Other research reports sulfur concentrations of

0.38%–2.0% in wood ash, which would yield 76–400 lbs in a 10–T/A application.

Research resultsResearch conducted recently in

Minnesota and Wisconsin demonstratesthat wood ash can be a safe and effec-tive soil amendment. Greenhouse stud-ies at the University of Wisconsin–River Falls show alfalfa and barley yieldsfrom wood ash applications of 5–20tons per acre to be significantly greaterthan those from commercial lime andfertilizer applied at rates recommendedby soil test.

Crop responses under field condi-tions show favorable trends similar tothose observed in greenhouse studieswith no statistically significant differ-ences.Alfalfa plots in Price County,treated with ash at rates from 2.5–20tons per acre exhibited yields compara-ble to plots treated with commerciallime and fertilizer applied at rates rec-ommended by soil test. No harmfulcrop effects were noted when ash wasapplied at rates of up to 20 tons peracre.

Using wood ash

Transportation and storage

Wood ash is a caustic substance(strongly alkaline) that is very dustywhen it is first taken from a combustionfacility. Its disposal is regulated by theDNR.Ash that is stockpiled in theopen tends to hydrate (absorb moisture)after several months, becoming lessdusty. Some combustion facilities addwater directly to ash to reduce dusti-ness.

Dry ash must always be coveredduring transport. It can be stockpilednext to fields until it is spread. Once ashis stockpiled it quickly forms a crust, sohaving it blow away in the wind is notusually a problem. If the ash is veryfluid, storage areas can be banked withlow earthen berms to contain the pile.

(Note: Fresh ash can be very hot—keep all flammable objects away from it.)

Spreading wood ashFresh ash is very difficult to spread

because of its dustiness; therefore, watershould be added to it before it is spread.Wood ash that has been hydrated can bespread with manure spreaders, limetrucks, or fertilizer wagons, dependingon the equipment available. For manyfarmers, manure spreaders will be themost practical.Ash can be loaded fromthe stockpile with a skid steer or a trac-tor with a front end loader.

Regardless of the spreading equip-ment used, it’s important to calibrate theapplication to obtain the desired rate.Ash must be analyzed for chemicalcharacteristics before it is applied.Ashshould usually be applied at the rate thatwill satisfy soil test recommendationsfor lime, but recommendations forpotash or phosphate can also be used incertain situations.This is especiallyimportant when using ash with veryhigh levels of these nutrients. Fordetailed information on calibratingmanure spreaders, see the University ofWisconsin-Extension publicationNitrogen Credits for Manure Applications(A3537).

Table 2. Economic value of wood ash at various nutrient levels and limestone costs.

Constituent Range in content Economic value (dollars per ton)1at different limestone costs2

Lime Value (NI) 40–49 to 80–89 $10–30

Potash (K2O) 26–92 lb/ton $2.86–10.12

Phosphate (P2O5) 22–46 lb/ton $5.06–10.58

Total range in value: $17.92–50.701Assumes cost of nitrogen (from urea) equals $0.22/lb N; cost of potash (from 0–0–60) equals $0.11/lb of K2O; cost of phosphate (from diammonium phosphate ) equals $0.23/lb P2O5.

2Based on NI 80–89 limestone, ranging from $20–30/ton.

Safety considerationsWhen working around ash, wear

personal protection equipment (goggles,dust mask and protective clothing).Ashcan be harmful if it is inhaled or comesinto contact with eyes or skin.

Landspreading regulations

Wood ash landspreading is regulat-ed under Wisconsin’s solid waste man-agement rules.The state landspreadingcode (NR 518.04(5)) exempts ash fromspreading prohibitions if certainrequirements are met.The requirementsstate that the wood ash must be testedaccording to the code; it must be shownto have value as a soil amendment, andit must be applied according to standardagricultural practices with a writtenapproval from the DNR.

To receive a “grant of exemption,”follow the general procedures outlinedbelow.1. The firm generating the ash should

submit a request to the DNR askingfor an exemption to landspreadingprohibitions under NR 518.04(5).The request should include informa-tion on:

a) the source of the wood ash;

b) the volume of ash produced;

c) the process used to burn thewood;

d) any chemicals added during processing;

e) any other relevant information.

2. Representative ash samples must betested according to NR 518.06 todetermine liming value and metalcontent.

3. Fields proposed for ash spreadingmust be identified and soils must betested to determine lime needs.

4. The request for exemption shouldalso include information on how theash will be stored, transported tofarms, and spread.

Exemption permits will specifyhow ash can be stored and spread andwill indicate setbacks to wells, water-ways, homes, wetlands, and other sensi-tive areas.

More detailed information onrequirements for landspreading exemp-tions is available from your DNR areasolid waste specialist. For the phonenumber of the specialist for your part ofthe state, contact your local DNRoffice.

SummaryIndustrial wood ash can be used as

a safe, effective, and economical soilamendment. It is especially useful wherelime and potash needs are high orwhere forage legumes are grown.

Ash from woodburning industriescan potentially be spread on agriculturallands instead of being landfilled.Thispractice can substantially reduce ash dis-posal costs and provide increased cropyield at low cost to farmers.

For more informationTo obtain more information about

using wood ash as a soil amendment,contact your Wisconsin countyExtension office or local DNR districtoffice.

Acknowledgments:This publication was made possible by a grant from the Sustainable Agriculture Program,Agricultural ResourceManagement Divison,Wisconsin Department of Agriculture,Trade, and Consumer Protection with funding from the Wisconsin EnergyBureau, Department of Administration. Research support came from the University of Wisconsin–River Falls, Solid Waste ExperimentCenter,Wisconsin Department of Natural Resources, and Pri-Ru-Ta Resource Conservation and Development Council.

Authors: Mark J. Kopecky is the Price County agriculture agent for the University of Wisconsin–Extension, Cooperative Extension.N. Larry Meyers is a professor of soil science in the Department of Plant and Earth Science, University of Wisconsin–River Falls.Wally Wasko is an area waste management specialist with the Wisconsin Department of Natural Resources.

Issued in furtherance of Cooperative Extension work,Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department ofAgriculture, University of Wisconsin–Extension, Cooperative Extension. University of Wisconsin–Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need this information in an alternativeformat, contact the UWEX Affirmative Action Office or call Extension Publications at (608)262-2655.

This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications,Rm. 245, 30 N. Murray St., Madison,WI 53715. Phone: (608)262-3346.

A3635 Using industrial wood ash as a soil amendment I-5-95-2M-30-E