Potassium, or potash as it’s known in its fertiliser form, represents the ‘K’ in the NPK trio of most essential nutrients for plants. Although it doesn’t currently get the attention that nitrogen and phosphorus does, potassium is equally vital to grow the amount of high quality food we need for ourselves and our overseas markets.
Potassium and potash are essential elements for pasture and crop quality.
What is potash (potassium)?
The word potash comes from “pot ash”, a potassium product once recovered from the wood ashes of coastal plants. Prior to the 1790s, this was a thriving cottage industry in the seaside communities of developing nations like America. Sadly for these communities, this source of potassium was superseded by the discovery of huge mineral deposits in places like Chile and later, Canada, Russia, and Belarus.
Today, the term potash is used to talk about potassium fertilisers in general, but particularly potassium sulphate (K₂SO₄) and potassium chloride (KCl), also commonly referred to as muriate of potash.
Potassium in New Zealand
Potassium makes up around 2% of the earth’s crust, so it’s an abundant element around the globe. However, there are no large mineral deposits in New Zealand. And many of our soils are deficient in potassium or have been depleted of their natural reserves. So our volcanic ash soils such as allophanic soil, and our organic soils like peat, are likely to need potash.
Why plants need potassium
While nitrogen and phosphorus are key components in building proteins and the structural parts of plants, potassium functions quite differently.
Many studies made in the last few decades tell us that potassium controls the transport of water around the plant. In particular, it controls the movement of water in and out of cells so that plants remain turgid and upright with strong, sturdy stems.
Potassium also regulates the opening and closing of the stomata or pores found on plant leaves. One of the key roles of the stomata is to prevent water being lost from the plant. But potassium-deficient stomata are slow to close. This leaves the plant vulnerable to drought stress.
Apart from water movement, potassium is involved in photosynthesis. The stomata which open and close for water, also allow gases like CO₂ and oxygen to be exchanged with the atmosphere. When there is adequate potassium, the gases can move freely in and out and photosynthesis can proceed normally.
Additionally, potassium is involved in activating enzymes to produce ATP (adenosine triphosphate), the plant’s primary source of energy. And ATP is essential for transporting nutrients like nitrates, phosphates, calcium, and magnesium around the plant. So potassium and ATP are important for maintaining optimum utilisation of any applied nitrogen and phosphate fertilisers.
Potassium and crop quality
Apart from regulating water movement, photosynthesis, and nutrient utilisation, potassium is vital for crop quality. For example, the starch content of grains, and the sugar content of fruit are dependent on the availability of potassium. Root and tuber development in crops like potatoes also depend on adequate potassium supplies.
Potash deficiency
So, how do you know you need to apply potash? Symptoms typically appear in late spring/early summer on older leaves. Some plants will show necrosis (leaf spots, blight, scab, rots, etc), and chlorosis (leaf yellowing). Clover may show white spots on the leaves. In all cases there may be a reduced growth rate and a decrease in turgidity.
Pasture will exhibit a patchwork pattern not unlike that seen with nitrogen deficiency where urine patches show more growth. However, there will be a contrast in what grows in the urine patches and surrounding pasture. With a potassium deficiency, both ryegrass and clover will be found in the urine patches, but no clover present elsewhere. With a nitrogen deficiency, the urine patches will be grass dominant, and the surrounding pasture will be a mix of ryegrass and clover.
As always, if a shortage is suspected, the best solution is to start with a soil test.
This article was published in the Coast & Country News.
