Agricultural Use of
WOOD ASH
in California
UNIVERSITY OF CALIFORNIA
Agriculture and Natural Resources
Publication 21573
B
AGRICULTURAL USE OF WOOD ASH IN CALIFORNIA
ROLAND D. M EYER
Extension Soils Specialist , Department of Land , Air and Water Resources, University of California , Davis
D ANIEL B. MARCUM
University of California Cooperative Extension Farm Advisor, Shasta-Lassen Countie s
H OLLY A. G EORGE
University of California Cooperative Extension Farm Advisor, Plumas-Sierra Counti es
GARY G . M ARKEGARD
University of California Cooperative Extension Farm Advisor, Humboldt Count y
G ARY M . NAKAMURA
Extension Forestry Specialist , Department of Ecosystem Science, Policy, and Management , University of California, Berkeley
According to the California Integrated Waste Management
Boards Phase 1 Report, Ash Quantification and Characterization
Study (Khan et al. 1992), approximately one-half million tons
of wood ash were produced in California in 1989. Although
most ash was initially taken to landfills, it is now being used
in a variety of ways. Many of the current uses are in response
to a California law (Public Resources Code §40000 et seq.
[AB 939]) requiring the development of alternative disposal
measures to reduce the amount of material sent to landfills.
Since the mid- l 980s , University of California Cooperative
Extension personnel and many cooperating landowners have
conducted field research studies to examine how wood ash
can be used in agriculture. This publication discusses the
benefits and potential risks of using wood ash as a low-value
liming material and plant nutrient source in order to assist
potential users of wood ash in distinguishing beneficial from
nonbeneficial agricultural uses.
Origin and Composition
of Wood Ash
Wood-fueled power plant ash (bottom ash and fly ash) is a
byproduct of steam generation that is produced by burning
wood chips in a furnace at 1,400° to l ,900°F (766° to
l ,046°C). The wood fuel comes from in-forest operations ,
orchards, sawmills (bark, sawdust, shavings , or wood
chips) , and occasionally urban trees and shrubs (tree stems,
limbs, stumps , leaves, or needles). One bone dry ton (BDT)
(100 percent dry matter) of wood fuel produces about 1
megawatt-hour of electricity, sufficient to power 1,000
homes for 1 hour. Depending on the combustion character-
istics of the furnace , 1 BDT also produces 3 to 7 cubic feet
(0.08 to 0.20 m3) of ash, which may weigh from 10 to 100
pounds per cubic foot (160 to 1,600 kg!m3) .
It is very important that ash produced from wood fuel
( wood ash) be distinguished from ash produced from other
fuel sources, particularly coal, sewage sludge, or urban
waste. These other ash materials may contain lead and other
heavy metals or organic compounds from burning plastics
or petroleum products . They may not be suitable for agri-
cultural applications . If the concentrations of these elements
or compounds exceed prescribed limits, the ash is classified
as a hazardous waste according to Title 22 of the California
Code of Regulations.
The composition of wood ash is determined largely by
the type of power plant , whether bottom and fly ash are
mixed together, and how the ash is collected and removed
from the facility. Wood ash typically contains carbon
(unburned wood), calcium, magnesium , sodium, potassi-
um , and phosphorus , and it may also contain beneficial
amounts of sulfur, zinc, copper , and other micronutrients
(fig. 1). Boron, molybdenum , and selenium may also be
present at low concentrations , depending on the wood used
in the power plant.
Some power plants produc e a "high-carbon " ash as a
result of incomplete combustion of fuel, while others pro-
duce a "low-carbon" ash. High-carbon ash can consist of
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Plate 1. Chipped wood, a fuel used by many power plants. Plate 2. Large piles of wood chips are mixed together before
being burned in the power plant in the background.
Plate 4. Large blocks of ash, rocks, and other debris
should be removed from ash before spreading in the
field.
Plate 3. Two most common types of ash: high-carbon black ash, left,
and low-carbon gray ash, right.
Plate 5. A spreader equipped with a grate or screen on the top can
be used to remove large particles from the ash.
Plate 6. A wide opening in the spreader—24 to 28 inches
(61 to 71 cm) wide, as shown here—allows ash to feed
properly onto the spinners for uniform distribution in the
field.
24 – 28”
Plate 7. Ash application rates (100 percent dry matter basis)
on rangeland: 20 tons per acre (45 Mg/ha), right; 40 tons per
acre (90 Mg/ha), left and in back; and 80 tons per acre (180
Mg/ha), center foreground.
Plate 8. Ash applied on irrigated pasture
at 16 dry tons per acre (36 Mg/ha). The
inset shows a close up of no ash, left, and
ash applied at 16 dry tons per acre, right.
Plate 9. A truck with a moving floor unloads ash in strips in
a field.
Plate 10. Driving stakes in the field at planned spacing and inter-
vals provides for more accurate rates of application.
Plate 12. Small, uniform windrows that are more closely spaced
allow for uniform spreading in the field.
Plate 11. A large dump truck unloading ash in piles in a
field.
Plate 13. A grader, scraper (shown here), blade, or landplane
traveling perpendicular to the windrows can be used to
spread the ash uniformly in the field.
Plate 14. Ash is incorporated effectively using a large tandem or
double disk with disks 18 to 24 inches (45.5 to 61 cm) in diameter.
Plate 15. A second pass over the field with the tandem disk at a 45° to 90° angle
to the first disking assures more complete incorporation of the ash.
Plate 16. An aerial photograph showing very uneven spreading of ash in a field. Uneven spreading decreases the benefit of ash to
crops.
Plate 17. Areas of high ash application rates
may reduce germination and early seedling
growth, resulting in very low crop yields.
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Plate 20. Avoid storing or applying ash near streams or
intermittent watercourses.
Plate 22. Avoid excessive movement by wind during spreading and incorporation of ash.
Plate 21. Ash should be stored in areas surrounded by trees where it
can be protected from movement by wind. Some movement is
unavoidable during loading and spreading of ash.
Plate 18. The quarter gives a perspective as to how great-
ly plant density has been reduced by heavy ash applica-
tion compared to the upper right corner of photo, where
a higher plant density exists.
Plate 19. A large legume-wheat plant growth response of greater than
2.5 tons per acre (5.6 Mg/ha) from ash application of 100 dry tons
per acre (225 Mg/ha), left, compared to no ash, right. The soil was
slightly acidic (pH 5.7) and low in phosphorus and potassium.
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© 1999 The Regents of the University of California
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
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Publication 21573
ISBN-13: 978-1-62711-069-3
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