Wood ash is a wonderful resource in the vegetable garden and everyone who has easy access to it should use it rather than disposing of it in some other way. It can be nutrient rich and used to improve nutrient access for your garden plants, and it is a very rich source of potash - a substance that sweetens soils and is rich in soluble potassium - an essential nutrient for healthy plants.
But not all burnt wood produces the same type of ash containing the same amount of valuable nutrients. I will get to the different types of wood ash in a minute, but all wood ash is useful to sweeten soils.
All types of wood ash are alkaline and therefore are good soil sweeteners. Sweet soils are soils that are not too acidic, and soil sweeteners help to raise the pH of a soil; in other words, make them less acidic. It is important that soil pH is correct for the plant - incorrect soil pH impacts on a plant’s ability to uptake nutrients. Therefore, if the pH is incorrect, even if the nutrients are present, the plant will not be able to uptake them - you could add lots of fertiliser, it would still have no affect, because the plant is unable to uptake them.
The section of plant stem or tree trunk that contains the bulk of nutrients is the bark and the outer extremities. It is in this section that most “living” activity takes place - sap containing many nutrients are transported around the plant. But most wood used in domestic fireplaces is “heart” wood from a tree trunk. It burns the hottest and most efficiently, but it contains few, if any nutrients. Therefore the ash from this wood is mostly useful as a soil sweetener, not to add nutrients to the soil.
However, small branches, twigs, weeds and the type of plant material that results from prunings usually contains the bark and outer extremities of the plant. The ash produced from burning this material is nutrient rich and contains a valuable material called potash.
In years gone by, wood ash was soaked in pots in order to separate the potassium from the other material contained in ash. The potassium is highly soluble, therefore enabling it to be collected in the water within which the ash was soaking. Potassium is critical to plant health - second only to nitrogen, and is crucial to the plants ability to both uptake and regulate water and also is important in the process of photosynthesis.
To summarise …… All wood ash has use in the garden, either as a soil sweetener, or as a potassium rich fertiliser. However, the ash that results from the “heart” wood that is burnt in most domestic fireplaces has little nutrient value and is mainly useful as a soil sweetener. Ash that results from the burning of smaller twigs, branches and even weeds is also valuable for its high levels of nutrients; particularly potassium.
So, ensure that you save the twiggy garden material that will not compost and when the conditions are right in your area, burn it and collect the ash for use in your garden - neither you, nor your plants will regret it.