In a recent GRDC Communities Crop Nutrition podcast, Dr Richard Bell from Murdoch University explained the importance of potassium (K) for optimal plant growth and reducing the effects of stressors such as drought, frost and salinity.
K is second only to nitrogen in importance for crop growth. Many soils in cropping regions are already low in K. Significant amounts of K are also exported when a crop is harvested and can be leached below the root zone in soils.
In the balance
It is therefore essential to look at the K balance in soil, and to see if you are removing more K as grain and hay than is being applied in fertiliser. A cereal crop typically removes 4-5 kg of K per tonne of grain harvested, while lupins remove 8-10kg/tonne. Canola is somewhere in between these two rates. Remember that additional K may also be lost by leaching.
To assess K levels in soil, it is important to test both the topsoil (0-10cm depth) and subsoil (10 – 30cm) levels. It is the subsoil K that plants will try to access in times of stress. Aim for topsoil K levels of 45-50ppm (at least 60 mg/kg using the Cowell K test) and increasing levels into the subsoil to minimise risk.
GRDC research trials in WA have shown K levels play a role in helping crops cope with many stressors, including sodic and saline soils, frost and drought conditions and disease.
If frost damage is very high (greater than 70% damage) or low (less than 15%) then above optimal levels of K are unlikely to make much difference to crop yield. But for frost damage between these levels, additional K applied to the soil at sowing and as an in-crop top up will help alleviate frost damage and protect yield.
For the full podcast, go to Richard Bell K podcast