Ash as a soil amendment

Fire ecology is the study of fire and specific ecosystems. Many plant species require fire in order to germinate or reproduce. Fire is an essential part of the ecosystem in coniferous forests, savannahs, and many other regions. There is another benefit to these regions other than regeneration. That is the impact of the ash on the soil. The ash benefits the soil with added nutrients, it buffers the pH of the soil, and it increases the population of soil microbes.

There have been numerous studies done on ash in soils. Specifically wood ash. Wood ash from home fireplaces, boiler ashes, and even from paper mills has been analyzed. All had about the same constituent makeup with varying percentages. All of these studies confirm that wood ash contains few noxious elements. As long as the source of the ash is from burning forests, untreated wood, and the like, the detectable limits are extremely low.

Macro and Micro Nutrients

The elemental analysis of wood ash is variable based upon the type of wood, and where the tree was grown. However, there are some general ranges that most of the wood ash falls into. According to the University of Wisconsin, wood ash is primarily composed of the following:

Symbol Chemical Percentage
Ca Calcium 22-45%
Mg Magnesium 1.2-2.2%
K2O Potassium 1.3-4.6%
P2O5 Phosphorous 1.1-2.3%
AL Aluminum 0.2-1.1%
Fe Iron 0.1-1.1%
Mn Manganese 0.06-0.3%
Z Zinc 0.01-0.5%
N Nitrogen 0.06%

These results are consistent with a study done with the University of Gent in Belgium(1).

From the table above, you can see that wood ash makes a good organic fertilizer with a low Nitrogen content. It has higher Phosphorous and Potassium levels than Worm Castings. It also has a high Calcium content which can lower soil pH.

Wood ash also contains large porous particles of carbon. Carbon is also very beneficial to the soil. It helps with water retention as well as sequestering nutrients. Carbon also helps the plants in regards to disease resistance.

Soil pH

Traditionally to raise the pH of acidic soils, lime is applied to the soil. Wood ash can be used in the same fashion. A study on acid soils in 2009(3) showed an equivalent increase in soil pH when compared to Lime. The study looked at introducing both lime and wood ash into Barley, Canola, Peas, and Timothy. They both increased the soil pH to between 6.71 -6.88 from 4.91.


MBC (Microbial Biomass Carbon) is the measure of the mass of living component of soil organic matter. The same study as above measured the quantity and diversity of soil bacteria in both the application of lime and of wood ash. The study found that with wood ash the soil microbial population increased 2.4 fold, and with lime it increased 3.2 fold. The increase in soil bacterial diversity was higher with wood ash.

Another study by the University of Lund(2) found that Spruce Seedlings grown with wood ash and ectomycorrhizal fungi significantly increased the plant growth.

In the first study, it is discussed that the increase in MBC may be influenced by a rise in the soil pH, whereas in the second study, the discussion is that the fungi had nutrients in a more available form to the plants.

Temperature of Combustion for Wood Ash and Type of Wood

The University of Wisconsin-Madison did a study(3) looking at different types of wood and combustion temperatures. The results of this study shows that there is variation in chemical analysis between different wood types, but they all stay with in some range. The combustion temperature should remain between 600-900°C. Above that temperature, some of the chemicals become a gas and are lost.


Wood Ash makes an excellent soil amendment. It contains many micro and macro-nutrients, raises soil pH, and encourages the diversity and quantity of soil microbes. It is easy to generate and apply yourself. It is also certified organic through OMRI  (Organic Materials Review Institute).





  1. Demeyer, A. at al., 2001, Characteristics of wood ash and influence on soil properties and nutrient uptake: an overview, Bioresource Technology 77, 287-295
  2. Mahmood, S. et al., 2003, Effects of hardened wood ash on microbial activity, plant growth and nutrient uptake by ectomycorrhizal spruce seedlings, FEMS Microbiology Ecology, 43, 121-131
  3. Lupwayi, N.Z., et. al., 2008, Soil microbial response to wood ash or lime applied to annual crops and perennial grass in an acid soil of northwestern Alberta, Agriculture and Agri-Food Canada
  4. Misra, M.K., et al., 1993, Wood ash composition as a function of furnace temperature, Biomass and Bioenergy, Vol 4 No. 2 pp 103-116

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