Wood Ash: Free Potash — Used Carefully
Wood ash from a clean fire is a free source of potash and calcium, a pest dust, and a grain-storage protectant - but it raises soil pH.
Vaibhav Dhama2 min readOrganicK ~5-10%, Ca highUpdated 3 August 2026
Wood Ash
Free potash and lime from a clean fire — a gift to acid soils, a mistake on alkaline ones. Everything about using it well comes down to that.
- Composition
- K ~5-10%, Ca high, P trace
- Form
- Fine grey ash
- Price
- Free (from clean fuel)
- Best for
- Potash + lime on acid soil
Overview
Wood ash from a clean cooking or crop-residue fire carries 5–10% potash and a high level of calcium, with only trace phosphorus and micronutrients. It also carries lime value, which is exactly why applying it raises soil pH.
That single fact drives every rule for using it: a genuine benefit on acidic soil, worked lightly into the topsoil or dusted on leaves against soft-bodied pests, but a mistake on soil that is already alkaline or saline, where it pushes the pH higher still and can lock up other nutrients.
What it is and when to reach for it
- A free potash-and-lime source for acidic soil — reach for it where soil pH needs nudging up and potassium is short.
- It also works as a light leaf dust against soft-bodied pests and slugs, and as a traditional grain-storage protectant.
- It carries only trace phosphorus, so it does not replace a phosphorus source.
How to use it well
- Apply a light dressing worked into acidic soil — it is concentrated, so a heap is a mistake, not a bonus.
- Dust it lightly and dry on leaves against soft-bodied pests; it stops working once it gets wet.
- Layer or mix it through stored grain and pulses as a traditional protectant against storage insects.
- Use only ash from clean, untreated wood or crop residue — never painted or treated timber, coal or rubbish.
Common mistakes
- Applying it to already-alkaline or saline soil, where it pushes pH higher still and can lock up nutrients.
- Heaping it on rather than spreading it thin — over-application skews soil chemistry.
- Expecting it to supply nitrogen or phosphorus — its value is almost entirely potash and lime.
Crops it is used on
Open a crop for its full nutrient schedule.
Guides
Long-form guides on this fertiliser, with the rates and timings worked out.
Common questions
Is wood ash good for the soil?
On acidic soils, yes — it supplies potash and calcium and gently raises pH toward neutral. On already-alkaline or saline soils it does more harm than good, because it pushes the pH higher still. Always match it to your soil, and apply modestly.
How much wood ash can I apply?
Sparingly — a light dressing worked into the soil, not heaped on. Because it is alkaline and concentrated in potassium, over-application skews soil chemistry and can lock up other nutrients. Spread it thin and observe, rather than treating it like a bulk fertiliser.
Does wood ash control pests?
Dusted lightly on leaves it deters some soft-bodied pests and slugs, and it is a traditional protectant dusted through stored grain and pulses against storage insects. It is a mild, supporting measure, best kept dry to keep working.
Composition and doses follow ICAR / Soil Health Card guidance; prices are the notified subsidised rates for the season shown and are guidance, not a quote. A soil test for your field always beats a general rate. See our editorial policy.