Do wood ashes affect your garden soil pH and nutrients?

Wood ashes can be a valuable resource for amending garden soil.

Anyone using a fireplace or wood stove knows that a large amount of wood ashes can accumulate over a heating season. When applied under the right conditions, ashes can be a good source of needed plant minerals for your garden.

There are three major minerals that plants require, they are: nitrogen (N), phosphorus (P) and potassium (K). Plants also need secondary nutrients that include minerals such as calcium and sulfur. Various concentrations of micro-nutrients are also used by the plants, including include boron, manganese, zinc, iron, copper and others. These are used by plants in very small amounts, hence the name micro-nutrients.

Since they are derived from our largest kinds of plants, wood ashes can be considered a type of plant residue. With their massive, extensive root systems trees can generally absorb from the soil all of the different plant minerals and deposit them into its wood and other parts. Then when the wood is burned, the carbon-based compounds are burned off releasing heat. After burning, the minerals in the wood remain behind in the ashes in proportion to what was in the tree.

As a fertilizer, wood ash potash (K) runs about 7% to 10%. Phosphorus is about 1 1/2 %. Nitrogen, another major nutrient, is released as a gas during combustion so it is no longer present in ashes.

The amount of K in ashes is a relatively high percentage for a no-cost, naturally occurring material that is readily available. Plus, it doesn't need to be mined from the ground, packed and shipped long distances to a garden center.

Optimum amounts of potassium increase disease resistance in plants. It also helps plants deal with cold weather and helps them endure droughts by preventing loss of water from the plants.

Many natural sources of potassium, such as greensand, release their nutrients very slowly over a period of months or years. Wood ash potassium on the other hand, is in a form that is very water soluble making it immediately available for plants to use. Because of this high solubility, potassium is quickly lost if the ashes are stored where it can be rained on. The rainwater essentially washes away the potassium, so it's important to store ashes in a dry area.

Calcium, classified a secondary nutrient, is another major ingredient of ashes. It comes in at about 50% to 60% by weight. It is in the form of calcium carbonate, the same compound that makes up the active ingredient in agricultural limestone.

Application of calcium carbonate as a soil amendment helps correct soil pH when it is too low. With low pH, the soil chemistry changes to the point where some minerals are no longer available to the plants. So farmers and gardeners add limestone to raise the pH to the optimum level for plant growth, generally around 6.5 to 7.

Depending on soil conditions about 30 to 100 pounds of limestone is commonly added to 1,000 square feet of garden. Because wood ash particles are much smaller than limestone particles, only about 3 to 10 pounds of wood ashes are required to get the same result, about 1/10th the amount.

The biggest disadvantage to using ashes is that the nutrients are released quickly and do not persist in the soil. Conversely, calcium carbonate in ground limestone stays active over a period of years.

While calcium is a critical element for plants, it is the carbonate part of the molecule that does the work of increasing pH.

Wood ashes work so quickly to raise soil pH that there is a danger that the soil pH can quickly get too high and cause other problems for your plants.

Ashes contain trace amounts of micro-nutrients, too. Those are essential nutrients needed for plant growth but only in tiny amounts compared to the three major nutrients of nitrogen, phosphorus and potassium.

While you can simply spread wood ashes over the ground, mixing it with other materials such as compost is much better.

The other thing to consider is that some plants require soil with low pH, potatoes and blueberries come to mind. Never apply wood ashes to low pH soil loving plants. So it's always a good idea to check your garden's soil pH to get an idea of how much wood ash to apply.

Now is a good time to check your garden’s soil pH. Soil pH meters and test kits make it easy to check your soil pH. They are inexpensive and readily available at garden centers and online.