Graziers in many parts of the country often deal with acidic soils that are low in several nutrients. Potassium is one of the soil nutrients that’s often present in amounts that limit pasture yield. In many parts of the United States and Canada, farmers could improve their pastures by occasionally applying wood ash.
Wood ash is a byproduct of several industries that burn wood for energy to make electricity, steam, or just heat. Wood ash is a good substitute for agricultural lime, and at application rates similar to lime it provides substantial amounts of potassium and calcium. The phosphorus content of wood ash is generally much lower, but some of the phosphorus may be available fairly quickly after the ash is applied. Ash contains lower amounts of magnesium and several micronutrients. It’s a very good source of zinc and manganese, and has trace amounts of boron and copper. It usually contains low amounts of nitrogen or sulfur, because these nutrients usually are lost during combustion. There are trace amounts of non-nutrient heavy metals in ash, but companies that provide ash generally need to show that these are present in such low amounts that it’s safe to use the product. Ash from clean wood and bark with no painted wood, adhesives, solvents, etc. is an approved soil amendment for organic farmers in the United States. Some industries even have their product listed by the Organic Materials Review Institute (OMRI), which makes it very easy to document with certifying agencies.
The lime value and nutrient content of ash vary from source to source, but it’s common for ash to be about half as effective as ordinary aglime at neutralizing soil acidity. Ash commonly provides somewhere around 50 to 70 lb/t of potash (K2O). Ash may contain 30-40 lb/t of total phosphorus, with around 5 to 8 lb/t in a form that is readily available to the plants. Most industries that provide ash for farm uses have the complete analysis available, so you can customize the application accordingly.
The nutrient content and liming ability of wood ash vary with the type of wood being burned (hardwood vs. softwood, straight wood vs. wood and bark, etc.), the type of combustion process used to burn it, the collection and storage method, and whether it’s fresh or aged. In general, wood ash is very dusty and is usually very caustic because many of the compounds in it are oxides. As ash is exposed to air and moisture, these oxides convert to bicarbonates and carbonates, and the material mellows. Different types of ash respond differently to wetting. Some become cakey, which usually makes them a lot nicer to work with, and some form a sort of concrete-like material that can be really hard to handle. The businesses that provide ash usually know how to handle it safely and effectively.
Where industries have a lot of wood ash to deal with, the cost for farmers to receive it is usually very reasonable compared to the price of the amount of lime and fertilizer it can replace. Nutrient and lime values of $60-80/t are quite typical. Depending on the soil pH and potassium level and the characteristics of the ash, spreading rates are usually between one and five tons per acre. Spreading ash is a lot easier with the type of spreader that you would usually use for poultry litter or gypsum. Using an ordinary lime truck usually doesn’t work as well with dusty ash, and some lime spreaders don’t work well at low application rates. Ordinary fertilizer spreaders don’t work well because of the physical characteristic of the ash. Manure spreaders can work, but it’s usually a lot easier to spread the ash if you load it on top of a layer of manure to carry the ash as you spread. It also takes a lot longer to spread this way than with a gypsum spreader.
It’s usually better to spread ash in the summer when the first few grazing cycles are done, or in the fall. For fall applications, it’s a good idea to apply the ash when most grazing is finished but the plants are still active and there’s time for some rains to work the ash into the ground. Because the nutrients and lime in ash are quite soluble, it’s easy to over-apply it. On pastures, it’s better to be conservative with the rate you use, because too much ash will cause high pH and potassium levels that can be a problem for forage quality and soil biology. On the other hand, correcting acidity and providing enough potassium and the other nutrients in ash can help improve the quality, yield, and persistence of pasture swards. Legumes in particular often show a dramatic response to ash applications, and these in turn can feed the grasses and forbs they’re growing with.
If the soils on your farm are acidic and need extra potassium, wood ash could be an excellent amendment. Before you apply ash, make sure you have a recent soil test for the area you’re considering. If either the pH or the potassium levels in the soil are high, or even near the target range, it’s probably better not to apply it, or to only apply it at very low rates. If you decide to apply ash, you should follow up with more soil testing two or three years after you start to see if you need to adjust the rate you use. Your university extension agent or consulting agronomist can help you to determine the best way to use ash on your farm.