The Good, the Bad and the Ugly of Wood Ash

In tight times it is common for growers to look to waste products as a source of nutrients. The products are also commonly marketed to growers.

Wood ash is often promoted primarily as a source of carbon and potash. It is often a good source of potassium, calcium, and carbon with a notable amount of phosphorus. The calcium is often calcium oxide which is powerful liming agent. The liming properties of wood ash are usually where problems can arise if not managed correctly.

While wood ash is a good source of carbon, it comes with a high C:N ratio. The more hardwood burnt to create the ash, the higher the carbon content. C:N ratios over 30:1 can become a nitrogen sink as microbes process the material. This can lead to nitrogen deficiencies in crops if not managed properly.

One ton of wood ash can have the same neutralizing capability of ½ ton of calcium or magnesium carbonate based ag lime. The concern arises when repeated applications of wood ash are made to meet a potassium nutrient recommendation without consideration for the liming potential of the material. Especially on high pH soils.

Excessive applications of calcium or magnesium carbonate based ag lime will result in a soil pH of 8.1 to 8.2. These soil pH levels can lead to a variety of challenges. The acid neutralization reactions of carbonate products stall at this pH. Unreacted carbonates will remain in the soil to neutralize any acid additions that maybe made to the soil. When soil reaches this point, it is very difficult to impossible to lower the soil pH. This is the main reason excessive lime applications are discouraged. Oxides, like those found in wood ash, can elevate soil pH into the low 9’s before stalling.

Whenever applying word ash, test the Calcium Carbonate Equivalent (CCE) of the material to compare to calcium or magnesium carbonate based ag lime application rates, avoid excessive or repeated applications, and monitor soil pH closely after application. If you have any questions about wood ash or other byproducts reach out to your regional ALGL agronomist.