Potassium (Potash) deficiency

Potassium (potash) deficiency is a common plant disorder that mainly results in discoloured leaves, including brown edges, as well as poor flowering and fruiting.

1. Yellow or purple colouring in the leaves
2. Leaves with brown edges and curled tips
3. Poor flowering
4. Poor fruiting
5. Plants sensitive to cold weather
6. Reduced resistance to plant diseases

Potassium or potash (K) is one of the macronutrients, needed by plants in relatively large amounts. It is important in plant growth and many of the plant’s internal metabolic processes, including water uptake. Potassium also helps promote flowering and fruiting, as well as ensuring good cold weather and disease resistance and hardiness.

Potassium deficiency normally starts on the older, lower leaves.

Potassium deficiency is caused when there are insufficient suitable potassium-containing materials in the growing medium (soil or compost). It can also be caused when plants are growing in dry soil, which prevents the roots taking up the potassium from the soil.

As potassium is very water soluble, it is easily washed out of the soil or compost by excessive rainfall or overwatering, leading to a deficiency. Shortages are worse on light, sandy and chalky soils that drain quickly and contain little organic matter to hold on to the potassium.

To check if your soil is short of potassium, you can use a simple soil-testing kit.

• All plants can be affected
• Especially flowering and fruiting plants

Keep soil potassium levels topped up by feeding with a balanced fertiliser especially in spring after winter rains may leach potassium from the soil. Where soil levels are very low, or for flowering and fruiting plants, feeding with a specific high potassium fertiliser – such as sulphate of potash or a liquid tomato fertiliser – may be a better choice.

The soil or compost should be kept consistently moist, never allowing it to dry out, without cycles of drying out followed by overwatering.

Make sure soils contain lots of bulky organic matter to hold soil moisture and nutrients. This is particularly important on light, sandy and chalky soils, which drain quickly.

Covering the soil with a 5-7.5cm (2-3in) thick mulch will help conserve moisture. Using well-rotted manure or garden compost may also provide a small supply of potassium.

Foliar feeding with a suitable liquid fertiliser is a good way to get potassium into the plant quickly.

Sowing green manure crops in summer or early autumn is particularly useful in vegetable gardens and allotments, or other areas of bare soil. These cover the soil over winter and help prevent nutrients, including potassium, leaching out and are dug into the soil to improve fertility overall.

Borderline hardy garden plants can be fed with sulphate of potash in late summer or early autumn to help improve their hardiness and cold weather resistance. Similarly, lawns should be fed in autumn with an autumn lawn fertiliser to improve their resistance to cold weather, frost and waterlogging.

• Garden fertilisers
• Tomato fertilisers
• Mulches
• Soil-testing kits