Lots to gain in keeping potassium replenished

from The Farmlander - November 2021 North

Crew helps Monowai chart stormy waters

Potassium (K) is critical for pasture quality and productivity but large amounts of it are removed when hay or silage is harvested.

“Repeatedly cropping a paddock without replacing K and other nutrients removed at harvest can lead to declining pasture quality and productivity and leave pasture increasingly vulnerable to undesirable species,” Ballance Science Extension Manager Ian Tarbotton says. “Herbage contains a lot of K as plants take up more than they need, which is known as ‘luxury uptake’. Potassium is the second most concentrated nutrient in herbage, after nitrogen, so large amounts are removed at harvesting.” Potassium is especially important for clovers, which are poorer foragers of the nutrient than grasses so are more prone to K deficiency. Lack of K can limit clover growth, affecting longerterm nitrogen fixation and pasture production and quality. Post-grazing or silage-making clovers in particular need potassium as they recover from being shaded out by grasses.

What about K in the soil?

“Most K in the soil isn’t plant available so the soil can’t provide new pasture with the K it needs,” Ian says. Unlike other nutrients, K is not stored in soil organic matter and 90 to 98 percent of K in the soil is in mineral form. This slowly becomes available to plants as soil minerals weather and break down. “Potassium leaching is also an issue in some soils. Pumice soils and sands, for example, have a low cation exchange capacity so can’t hold onto mineral K and if high rainfall occurs, K can easily be lost via leaching.”

Replacing K

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. “Applying K at the right rate and time can minimise luxury uptake of K, as well as leaching losses,” Ian says. Potassium requirements are largely driven by the amount removed in product, so more intensive farming activities will require more K to be applied. Soil testing to determine existing reserves of K will help to refine the rate. Use either a Quick Test to measure K readily available for plant uptake or the TBK test to measure both readily and slowly available K. On soils prone to K leaching, 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 replenish the phosphorus, sulphur and K removed by pastures and crops during their growth cycle. To boost growth with nitrogen and replenish K levels at the same time, SustaiN also comes in 15K, 20K and 25K options. Post-harvest fertiliser takes care of the nutrients removed by the harvest but regular maintenance fertiliser is still needed.