Keeping your K okay

Keeping your K okay

24 July 2018

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Don’t let potassium-deficient soil limit your pastoral farm’s potential

Potassium (K) is generally a very affordable nutrient, but soil test data from Ballance customers has shown it is in short supply on almost 35 percent of New Zealand pastoral farms. Ballance gathered the data between 2009 and 2015 from nearly 17,000 dairy farm samples and more than 9,000 sheep and beef farm samples.

Potassium deficiency can compromise pasture growth and quality and consequently affects mating, production and growth of stock further down the track.

A potassium deficiency will show up in clover first. Ballance Science Extension Officer Josh Verhoek explains: "Clover is a quality feed and supports the growth of other pasture species too. However, it is worse at extracting potassium from the soil than perennial grasses. If K levels are low, clover will suffer first, triggering a decline in your pasture’s feed value." Speckling around leaf edges is a classic sign that clover is lacking potassium.

Regular soil testing and an appropriate fertiliser programme will help you keep on top of your potassium levels. The Quick Test K (QTK) result is the one to watch. The range for near maximum pasture production is QTK five to eight on sedimentary soils or between QTK seven and 10 on ash or pumice soils.

On free-draining soils which leach easily in wet conditions, it may not be practicable to keep K levels in the optimum range. "In this case, it’s recommended to replace the K used by pasture to avoid mining soil reserves, rather than aiming for the optimum," advises Josh. "Elsewhere, increasing levels - even by a couple of points – offers advantages. Shifting from QTK three to QTK five could increase pasture production by five to 10 percent. With a payout of $6 per kilogram (kg), this could generate an additional $480 per hectare," says Josh.

K and hay

Every tonne of dry matter removes 15kg of potassium for hay and 20kg for silage. If hay or silage is fed out on the paddock from which it was cut, some potassium returns to the soil in dung or urine. However, if fed out elsewhere on the farm, its nutrient value goes with it. Either way, if the nutrients are not replaced, pasture quality will eventually suffer.

"Apply replacement fertiliser separately after harvest or combine it with maintenance fertiliser, ensuring a heavier rate goes on your hay and silage paddocks," says Josh. Split your applications if you need to apply more than 50kg K/ha, have low cation exchange capacity (CEC) soils or get more than 1,500 millimetres (mm) of rain per year. If applying in spring, it is recommended to keep two months clear of calving or lambing. Excess potassium in pasture or feed can interfere with calcium and magnesium uptake, increasing the risk of milk fever and grass staggers.

What’s in the bag?

Not all fertilisers are created equal.

It’s important to select a quality fertiliser, tailored to New Zealand soils. Michael Leaf, Procurement Category Manager for Ballance Agri-Nutrients, explains: "Raw materials for fertilisers are natural in origin and can vary in composition. To be certain you’re going to get results from a product on farm, you have to be confident in the manufacturer’s commitment to quality sourcing and production."

A good manufacturer will focus on exactly what nutrients New Zealand farmers need and when. "Agronomic benefit is number one – delivering a product that is going to be effective in a New Zealand context. Timing is also important: you don’t want product stockpiled for too long as it can deteriorate," explains Michael. "To maintain a regular supply of key nutrients, we develop long-term relationships with suppliers that are a match for the NZ market. Potassium is a good example. Ballance has an exclusive arrangement with a Canadian supplier which allows us to secure the best product for local agriculture."

For more advice, talk to your Ballance Nutrient Specialist or your local Farm Source team.