Potassium plan for pasture growth


Potassium is an important nutrient for summer pasture growth, and if farmers didn’t apply it in spring, there is still an opportunity to apply K with their summer nitrogen application.

June 24, 2016

Potassium plan for pasture growth 

Potassium (K) is an essential nutrient to deliver good pasture growth, particularly during summer, and a three year trial by Ballance Agri-Nutrients shows one size does not fit all for application rates.

New Zealand’s Central Plateau and Bay of Plenty regions may be famous with tourists, but they are also renowned for coarse pumice free-draining soils which leach K.

Ballance Science Extension Officer, Jeff Morton, said farmers on these pumice soils should consider small K fertiliser applications, often, while their farming counterparts on ash or sedimentary soils could apply K in larger, less frequent applications.  Mr Morton was presenting the research findings at the recent Grasslands Association Conference in Alexandra.

“The results showed that farmers on pumice soils are better off applying K through split fertiliser applications, rather than attempting to raise soil Quick Test K levels to the target range of 7-10 for near maximum pasture production.

“Potassium is an important nutrient for summer pasture growth, and if farmers didn’t apply it in spring, there is still an opportunity to apply K with their summer nitrogen application.”

“While a soil test is always advised, farmers can check for pasture K deficiency by taking white clover samples from three to four paddocks in late spring when they are growing really well. Potassium content should be 2.0 - 2.5% to ensure adequate K uptake,” said Morton.

Dairy farms lose between 60 and 120 kg/ha/ year of K, some of this in dairy shed effluent. If this is spread back on the land, the loss is mitigated, and the need for fertiliser K is reduced. Where silage or hay is produced, the K loss can be 200-300 kg/ha/year. If the silage is fed out on the farm, some of the K will be returned to the soil in dung and urine. So, part of the equation determining your K requirements is driven by how much K you remove from the land (in milk and crops), and how much of this you return (in effluent and animal waste).

Ballance undertook trials at two pumice soil sites, at Pouakani, near Whakamaru and at Mamaku, Bay of Plenty. Initial soil Quick Test K (2-4) were low at both sites. K was applied at 0, 75, 150, 300 and 600 kg/ha/year, equally split between September, November and February for three years, with pasture growth measured throughout the trial.

Results showed that at least 300kg/ha/year was needed to raise K levels to the normal target of 7-10. However, this increase was not maintained between May and September, when Quick Test levels dropped back to near their starting point.

“The trial showed that during winter months, when drainage is high and plant K uptake is low, K leaches at a higher rate than other times of the year. Therefore, it’s recommended that three applications of K at 40-50 kg/ha are used.”

“While this work was done on pumice soil, the results are applicable to other soils with high soil K loss such as sands and podzols,” said Mr Morton.

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