Bulletin #2279 Using Wood Ash on Your Farm 2004 reprint. This fact sheet is based on research done between 1990 and 1993 (see references). Much of the information is still valid and valuable; however, please be aware that the content of Maine ash sources may have undergone change in the intervening years. ulp and paper companies in Maine the effect of wood ash on plant growth. often burn wood, bark or paper-mill Much of the research summarized in this sludge to make electricity. Every fact sheet was conducted through the Maine Pyear, more than 300,000 tons of Agricultural and Forest Experiment Station. wood ash is produced in this way. One way to dispose of this wood ash is to put it Nutrients in Wood Ash in a landfill. However, landfill space is becoming scarce and costly. That’s why You know what wood ash looks like. It’s other methods of ash disposal have been a fine black material, like the ash left in a developed. One method that has become wood-burning stove or after a campfire. But common in Maine is to spread the ash on what is wood ash made of? fields. People have used ash as a low-grade Calcium is the most abundant nutrient, fertilizer and liming agent for a long time. averaging almost 20 percent of the ash. (As Up to 70,000 tons of wood ash are we will see later, this means that ash can be spread on farmland every year in Maine. used to raise soil pH, just as agricultural This figure will probably continue to lime is used.) If you applied five tons of ash increase. It is important that farmers and per acre, you would be applying about one communities know the benefits and the ton of calcium. Compared to calcium, other problems of using wood ash. This fact sheet covers research on nutrients in wood ash, the effect of wood ash on soil nutrient levels, and A Member of the University of Maine System nutrients are present in much smaller amounts. Wood ash is about four percent If you applied five tons of ash per acre, potassium, and less than two percent you would be applying about one phosphorus, magnesium, aluminum and ton of calcium. sodium. The small amounts of these nutrients is the reason that ash is considered a “low grade”fertilizer. In terms of amounts by plants. Wood ash may also commercial fertilizer, average wood ash contain heavy metals. These metals are of would be about 0-1-3 (N-P-K). concern because they may cause health problems for humans, livestock or wildlife. Figure 1 shows the average amount of Both micronutrient and heavy metal calcium, potassium, phosphorus, concentration in wood ash are usually magnesium, aluminum and sodium in measured in parts per million (ppm). In wood ash. These averages amounts are from other words, the number of pounds 12 different wood ashes—six produced in contained in every million pounds (or 500 Maine and six produced in other states. tons) of ash. Other nutrients are present in wood ash Figure 2 on page three provides an in much smaller amounts. Some of these example of the heavy metal content of wood nutrients, like boron, copper, molybdenum, ash. As you can see by looking at this sulfur, and zinc, are needed in trace Figure 1. Pounds of nutrient per acre in five tons of wood ash 2,000 1,500 1,000 Pounds 500 0 Sodium Phosphrus Magnesium Aluminum Potassium Calcium Nutrient Nutrient content calculated as average of 12 different wood ashes. (See appendix tables for more detailed information.) 2UNIVERSITY OF MAINE COOPERATIVE EXTENSION Figure 2. Pounds of heavy metal per acre in five tons of wood ash 1.5 1.2 0.9 0.6 Pounds Wood ash is commonly used as a 0.3 liming agent of its high calcium, because 0 content. Cadmium Chromium Copper Lead Metal (See appendix tables for more detailed information.) figure, applying five tons of ash adds less There are several ways to estimate the than two pounds of these elements. If liming of wood ash. Lab measurements can concentrations of cadmium, chromium or be taken (by boiling the ash in acid) to find lead are too high, you may not be able to out the calcium carbonate equivalent (CCE). use wood ash. The CCE tells you how well the wood ash would raise soil pH compared to lime Wood Ash as a Liming Agent (calcium carbonate). Like the wood ash nutrients discussed above, there are big Wood ash is commonly used as a liming differences in wood ash CCEs. When CCE agent, because of its high calcium content. was determined in the lab, it ranged from 25 Liming agents are used to increase soil pH to 59 percent. This is shown on page four in (pH is a measure of how acid a soil is). Soils Table 1. This means that if you added the in the Northeast are naturally acid. Soil pH same amount of lime and wood ash, the of 4.5 to 6.0 is common. A soil pH of 7.0 is wood ash should be 25 to 59 percent as neutral. Most plants prefer soil pH between effective in raising soil pH. 6 and 7 (although potatoes and blueberries Wood ash CCE can also be determined prefer lower soil pH). in the lab by mixing ash with soil and USING WOOD ASH ON YOUR FARM, BULLETIN #2279 3 measuring the change in soil pH over time. depending on the CCE. The amount of Then this is compared to soil pH after moisture in the ash also affects the adding lime. These figures are also shown application rate. Water is commonly added below in Table 1, in the column “CCE to the ash at the power plant to cut down on (incubation).” dust. The more water in the ash, the higher You will notice that, in most cases, the the application rate on an “as is” basis. CCE determined with this method is lower than the other lab method. Using this method, wood ash CCE ranged from 14 to In Maine, top-dressed applications of 56 percent. Again, remember that there are wood ash are usually limited to two tons big differences in how well wood ashes of lime equivalent per acre. increase soil pH. Let’s look at what happens to soil pH when wood ash is applied. Figure 3 shows Potassium and Phosphorus in Wood Ash how soil pH increases as wood ash rate Plants remove large amounts of increases. This relationship between soil pH phosphorus (P) and potassium (K) from the and ash rate is linear. This means that twice soil. One ton of alfalfa hay contains about as much ash raises the soil pH twice as six pounds of P (14 pounds of P O ) and 48 much. 2 5 pounds of K (56 pounds of K2O). Corn In Maine, top-dressed applications of silage yielding 15 tons per acre removes wood ash are usually limited to two tons of about 50 pounds of P2O5 and 120 pounds of lime equivalent per acre. If the ash is tilled K2O. Over time, P and K must be added to in, the limit is set at three tons per acre. The the soil to maintain crop yields. How good actual ash applied may range from about is wood ash in meeting crop needs? three to eight tons of dry ash per acre, There is a difference between the total Table 1. Calcium carbonate equivalents and available P and K in wood ash. Three (CCEs) for six Maine wood ashes different methods are used to estimate how much of these nutrients are available: Ash Source CCE (in acid) CCE (incubation) 1. A standard fertilizer test procedure, which extracts nutrients from the ash in % an ammonium citrate solution, treating Ultrapower 56 33 the ash the same as a commercially available fertilizer Greenville Steam 26 24 2. The Maine Soil Test extraction, which Beaverwood Chester 53 56 measures changes in the amount of Sludge-Ash mixture 50 40 available P and K in the soil after ash application, using ammonium acetate Bioash 25 14 Pinetree 59 34 3. Measurement of differences in plant Average CCE from incubation in three different uptake of P and K after ash application Maine soils. Data from: Ohno and Erich. 1990 4UNIVERSITY OF MAINE COOPERATIVE EXTENSION We use methods 1 and 2 in research differences between the ash samples. because we want to know quickly what Availability of these nutrients, especially P, nutrients are available from the ash. is affected by the ash itself, the pH of the However, it is important to compare these soil, and the nutrient level in the soil before laboratory methods with method 3. you apply ash. The percentage of P and K that is Plant uptake of P and K following wood available using methods 1 and 2 is shown in ash application has been measured in Table 2 on page six. From this table, you can greenhouse studies. The relationship of soil see that wood ash P and K are not test procedure and plant uptake is usually completely available to the crop. If wood linear or straight-line. This relationship is ash containing 100 pounds of P and 100 important. It is the first step in being able to pounds of K were applied, soil tests results predict crop response to nutrients in wood would increase only about five pounds for P ash. and 40 pounds for K. Again, there are There is a difference Figure 3.