Potassium is generally linked to seed, flowers and fruit production. It is highly soluble and easily leached out of the soil. For this reason is uis more common in sandy soils. Though it is present in manure in small quantities, it is much higher in ash. Primitive cultures discovered that burning off the vegetation somehow created better soil for their crops. What they were doing was raising the levels of potassium in the soil through the resulting ash.
Typical symptoms of Potassium deficiency in plants include brown scorching and curling of leaf tips. The symptoms generally first appear on older leaves. The mature leaves will begin to show yellowing at the edges, which will eventually turn brown at the extremities. In tomatoes they develop a yellow shoulder on the fruit and the leaves turn brown and wither. There is some debate whether this is directly due to a deficiency in potassium or the fact that it causes the vines leaves to wither and exposes the fruit to too much sun and heat – either way, there is a relationship.
Most tomato fertilisers are higher in potassium. If you have been growing tomatoes and using a commercial tomato fertiliser, other vegetables planted in the same location may show magnesium deficiency symptoms. This is not due to a deficiency of magnesium, it can be due to plants taking up the more reactive potassium instead of magnesium. Use chemical fertilisers sparingly, especially when they are plant specific.
Ash from treated wood, coal, plastics and general household rubbish can also contain high levels of toxins, like heavy metals and should not be used on food gardens. Ash is also quite caustic and will harm most plants if applied directly. It’s best to steep ash in water and use the water to water the plants. It can be added directly to the soil, if it is left to age a few weeks before planting. This allows the soil bacteria time to break it down and reduce the caustic levels in the soil.
All plants require a mix of all three nutrients but depending on their growth, will require slightly more of one or the other nutrient.