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Potash fertiliser guide


All you need to know about potash, and how to use it.

Potash is the term commonly used for potassium. It’s one of the three major nutrients that plants require for healthy growth and is represented by the chemical symbol ‘K’. The other two major nutrients are Nitrogen (N) and Phosphorus (P). Potassium helps flowers and fruit to form and also toughens growth in order to resist pests and diseases. It also helps increase resistance to drought or extreme cold. Potassium deficiency is more common on light, sandy soils and signs include brown scorching and curling of leaf tips. The term ‘potash’ comes from an early production method that used wood ash in large pots, hence ‘pot-ash’.


About fertilizers

Back of a fertiliser bottle, showing its NPK ratio
Back of a fertiliser bottle, showing its NPK ratio

Fertilizers are food for plants that can be bought in concentrated and compact form such as pellets, granules, powder, or liquid. Fertilizers can be artificially produced (inorganic) or based on plant or animal products (organic). They’re used to improve plant performance and higher crop yields, although plants that are growing on healthy soil shouldn’t need fertilizer. Most fertilizers are what is known as ‘compound’, which is a blend of the three major nutrients (N, P and K), and product labels carry a detailed breakdown of these as a percentage. In addition, there are ‘straight’ fertilizers which are composed of a single nutrient. In the case of potash, the ‘straight’ form is sulphate of potash and garden potash.

Which plants benefit from potash?

Adding sulphate of potash to feed apples
Adding sulphate of potash to feed apples

Plants that bear flowers or fruit are likely to perform better or produce higher yields when given fertilizer that is high in potash. Fertilizers that are produced for specific plants, such as rose fertilizer and tomato fertilizer, are rich in potash and also have other minerals that these plants need.

When to apply potash

As with most nutrients, plants can only take up potash when in active growth, and also when the soil is moist (or the fertilizer is applied as a liquid) so it can be drawn up through the roots or absorbed through the leaves if applied as a foliar feed. Apply potash from early spring to late summer.

How to make your own potash-rich comfrey fertilizer

Making a potash-rich comfrey solution
Making a potash-rich comfrey solution

Comfrey (Symphytum officinale) is a plant that is naturally high in potash with deep roots that draw up nutrients into its leaves. Grow your own comfrey plants and you can make your own organic fertilizer, by harvesting the leaves, keeping in a large container for a few weeks, and drawing off the resulting liquid that can be used to feed flowering and fruiting plants.


Advice on buying potash

  • Fertilizers are widely available from many retail outlets as well as online
  • Potash-rich fertilizers include tomato fertilizer (either in liquid concentrate or granular form) such as Vitax liquid tomato feed or fertilizer for flowering plants such as Vitafeed Flower and Fruit Soluble Feed (Thompson & Morgan)
  • ‘Straight’ or potash-only fertilizer comes in powder form.
  • Wool potting compost is also rich in potash, hence good for growing flowering and fruiting plants in containers (from Suttons).

Where to buy potash