• Bert Quin

A Deeper Look at Potassium Needs on Dairy Pastures

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Dairy farmers are coming under increasing pressure from different combinations of things in different areas of NZ; lack of rainfall, disappointing payouts, bank lending becoming more focused on cash-flow, and increasing environmental restrictions and requirements.

So why talk about potassium (potash) in this context? Well, a key fertiliser cost difference for dairy farming compared to sheep and beef has been the amount of potash dairy farmers are told they need. Around 80-100 kg K/ha (160-200 kg potash/ha) annually split into 2 applications (applied with superphosphate) is a typical recommendation.

This typically keeps potassium levels in grass and clover above 3.5%. Farmers are told they have to put this much on because clover needs it and grass responds to it, despite the complications this causes with milk fever. They are told they should have potash ‘Quicktest’ soil K levels of 8-10. But are pasture and soil levels this high actually needed, or is the problem somewhere else?

I have looked back over many of the field trial results conducted with potassium in the last 60 years. The vast majority just compared applying no K with large single applications of K. I could not find one that looked at applying K in much smaller amounts more frequently. Everyone, it seems, has missed the following vital point: just because the pasture K level goes from a deficient 1.5-2% K level to 3.5-4% and considerably better production with a 50 kg K/ha application, this does not mean this amount was necessary to get the full response!

I have not seen data from any single trial showing a pasture response to K when the level in the pasture was above 2.3-2.5%, and this can easily be maintained with Quicktest K levels of 5-6, not 8-10. But there is a proviso. The potash needs to be applied in small, regular intervals; about 10 kg K every 2 months, but skipping late autumn and winter, so say 4 applications per year. The current practice of 2 applications a year has come about largely to suit the spreading industry. Farmers have long been encouraged to reduce spreading costs by reducing the frequency. This wasteful practice is encouraged by most of the fertiliser industry as it means higher sales of potash.

Unfortunately, K is like N. When you put a lot of K on in one application, pasture - and particularly clover - take up far more than needed for growth, and vastly more than the 1.0-1.2% K the cow needs in its diet. This leads to well known metabolic problems. Farmers try to avoid these by not applying K too late in autumn, but if soil tests are in the industry ‘recommended’ range of 8-10 and above, metabolic problems can occur regardless.

Much of the excess K taken up by pasture is excreted in urine patches. Depending on the time of year, rainfall and/or irrigation and the type of clays in the soil (not just the CEC, but also how their K sorption and desorption behaviour is affected by drying and wetting cycles), much of this can be leached. This may not happen as predictably as excess nitrate does from the N in urine, but it is a big loss factor nevertheless. And leaching of K, a cation, takes with it nitrate and sulphate anions. The reverse is also true, of course.

So what would I do if I were a dairy farmer? I would apply the potash myself, at 10 kg K/ha four times per year from a tractor-mounted spreader, saving myself about 40 kg K/ha (about $70/ha delivered) in the process, and lots more in vet bills and the environmental plus of reduced nitrate leaching. The need for any additional labour and time spent spreading can be avoided by adding the K to planned N applications. And I would most definitely use urease-inhibitor-treated prilled urea, not granular, to considerably reduce how much N needs to be applied!

I think ‘Precision Farming’ has unfortunately become an over-hyped vague expression, but we certainly need to get a whole lot more thoughtful with fertiliser, both in terms of the form of nutrients we use, and their application.