Potassium is an essential plant nutrient that is commonly in short supply in Western Australian soils. This nutrient is critical for plant growth as it is involved in most of the biological processes of plants. It’s classified as a macronutrient because plants take up large quantities of potassium throughout their life cycle. As a result, limited supply of potassium can limit crop yields.
As demonstrated in this nutrient removal table, only small amounts of potassium are removed in grain. The majority of potassium is held in the leaves and stems.
Example of grain removal
A wheat crop which yields 3.0t/ha would have a mature weight of around 10.0t/ha (roots, leaves, stem and grain). At 2.0% potassium, the crop has to access 200kg/ha of potassium (equivalent to 400kg/ha of muriate of potash). Out of this potassium, only 12 to 15kg/ha would be removed in the grain. The rest is available to be returned to the soil, removed by livestock or lost through stubble burning or erosion. Greater losses will occur if the ash is removed by wind, rain or soil erosion. When soil sampling, much of the potassium may be still in the plant at the time of sampling as it will be leached into the soil with rainfall. Marginal levels in November may be adequate in May.
If removing large amounts of plant material, as in hay and silage production, some grazing systems and chaff carts behind headers, removal of potassium can be large. Replacement of potassium is essential, using muriate or sulphate of potash.
Role of potassium in plants
Potassium is a mobile nutrient within the plant. When deficiencies occur, the plant will remove potassium from the older leaves and pump it into the younger leaves, therefore the symptoms will appear on older leaves first. Plants lacking potassium will have speckling along the leaves, spreading quickly to the tip and the margin. Complete senescence (death) of the older leaf may occur.
Generalised guide to soil test levels and potassium requirements
P.T. = Plant Test. Levels are marginal, but may not respond to applied K.
*Canola does not appear to respond to low potassium levels. However, data is very scarce. To help the following years crop, and to cover all eventualities, it is suggested that up to 50kg/ha muriate of potash be applied pre-sowing.
# Balansa: Balansa clover is a vigorous growing clover often used for hay making. Observations from the Great Southern and South Coast regions indicates that Balansa is more sensitive to potash deficiency than other pasture legumes. If soil tests indicate levels around the 120 to 150ppm then use of potash would be a wise precaution.
One of the myths about potassium is that it leaches rapidly. In fact, potassium is a cation and binds quite strongly to the soil particles. Leaching may be a problem in high rainfall, deep sands. However, in lower rainfall areas where the soil has a reasonable clay or gravel content, or in duplex sand on top of clay/gravel, leaching is not a major concern. In waterlogged clays, or soils where surface erosion is likely to be a problem, then delayed or split applications may be beneficial.
Lime and Potash
Applying lime to soils with lower potassium levels (up to 140ppm) can have a detrimental effect on the availability of potassium to the plant. This is particularly prevalent in lupins. The application of lime applies large amounts of calcium. Calcium and potassium appear to be absorbed through the same part of the root, so the plant will absorb the nutrient in abundance. The other possibility is that the calcium (also a cation) will replace the potassium on the soil colloid, releasing the potassium for ready leaching.
To find out more about preventing or correcting a potassium deficiency, speak to your local Area Manager.