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Special K
High concentrations and removal rates of potassium (K) in plants combined with low amounts of plant available K in the soil mean this nutrient often needs to be applied.
Like other macronutrients potassium (K) is essential for plants, but a couple of features set it apart.
“As the concentration of K in plants is second only to nitrogen, large amounts of it are removed in farm produce, cash crops and silage,” says Ballance Science Extension Manager Ian Tarbotton.
“Also unlike other macronutrients, K is not stored in soil organic matter.”
While many soils contain large amounts of K, most of it (around 90 to 98 percent) is in mineral form and not available to plants until released very slowly as soil minerals weather and break down.
In addition, some soils with low cation exchange capacity (especially pumice soils and sands) are not able to store much mineral K, and this combined with high rainfall results in K losses via leaching.
“So K’s high concentration and removal rate in plants combined with it not being readily available in the soil means that K fertiliser is often needed, especially in intensive farm systems.”
Lack of K can easily limit clover growth, which in turn can affect longer term pasture production and quality and nitrogen supply. Clovers need greater amounts of K than grasses do, and it is especially important for clovers post-grazing, which can take some time to recover from being shaded out by grasses. If any nutrient is in short supply clovers suffer first as their root system is shallower than ryegrasses’, making them a poorer competitor for nutrients.
MOP to the rescue
Muriate of potash (MOP), a concentrated source of K that dissolves readily when applied, is the most widely-used, affordable K fertiliser on pastoral farms.
Historically, MOP was made by soaking wood or leaf ash in a pot and boiling off the water, hence the term potash. MOP is potassium chloride, so not suitable for use on chloride-sensitive horticultural crops, which generally require more expensive sulphate of potash.
“Plants take up K in luxury amounts (more than needed), but applying it at the right rate and time can minimise this, as well as leaching losses,” says Ian.
K requirements are largely driven by the amount removed in product, so more intensive farming activities will require more K applied. Soil testing to determine existing reserves of K, using either a Quick Test to measure K readily available for plant uptake, or the TBK test to measure both readily and slowly available K, will help to refine rate.
On soils prone to K leaching (low cation exchange capacity and high rainfall), frequent applications will minimise losses and maximise plant use.
MOP is inert, and may be blended with virtually all other fertiliser products. It is contained in the Superten range (5K, 7K, 10K, 15K and 25K options) which provide the phosphorus, sulphur and K removed by pastures and crops during their growth cycle. If you need to boost growth with nitrogen and replenish K levels at the same time, SustaiN also comes in 15K, 20K and 25K options.
For more support, talk to your Ballance Nutrient Specialist or Ruralco Representative.