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Increase Yields, Reduce Emissions and Improve Profitability with Potash

Increase Yields, Reduce Emissions and Improve Profitability with Potash

John McCabe, Dairy Adviser, Teagasc/Aurivo Joint Programme, talks us through the benefits of using potash.

Moderate to highly stocked farms need between 25 and 35 units of K (Potassium) to maintain soil K levels at where they were at the start of the year. Simply put, if you are using 18-6-12 to get K onto grazing ground then you need to spread 2 to 3 bags of 18-6-12 to stand still. (Phosphorus is restricted and allowances are on a farm by farm basis so I will not discuss Phosphorus here.)

In 2021, half of all soil samples taken on dairy farms and analysed by Teagasc were index 1 or 2 for K. It was the same in 2020. It is worth building index 1 and 2 soils up to index 3 because we can expect to grow an extra tonne of dry matter per hectare or the equivalent of 2 bales per acre more grass grown in the year after spreading, and in subsequent years too.

To build an index, it takes roughly 50 units of K as well as the 25-35 maintenance units mentioned above. Some soils will take more, some will take less. Slurry is a great way of lifting K indexes however most farms don’t have enough slurry to satisfy our silage crops plus lift fertility on the low index soils. If you can - spread slurry on paddocks that are low K or paddocks harvested as surplus bales. Cows return most of the K they consume in the grass they graze but bales take it all. 

Compound fertilisers with 5 or 10 units of K tend to be expensive and insufficient ways of lifting indexes unless being spread in huge quantities.  Some fertilisers have 19 units in a bag along with Nitrogen. This mean you would need to spread 4.5 bags of it to lift an index.

Muriate of Potash (MOP or 0:0:50) is a straight K fertiliser with 50 units of K per bag. There are 20 x 50kg bags in a tonne. Spreading MOP at a rate of one bag per acre means that a tonne of it will cover 20 acres and will go a long way towards building an index.
What I like about Muriate of Potash is that it is simple – take out the soil samples, pick out the low index soils, order the MOP, spread one bag per acre and the job is done.
Department of Ag figures published 2 weeks ago show sales of K were down by 30% in the period 1st Oct 2021 – 30th June 2022. The price (roughly €900 a tonne), along with the uncertainty of how the year would pan out, probably put a lot of people off ordering K in the spring. However now that milk price has been at such high levels, it would be very good business to spend on high K fertiliser. If we take a 100 acre farm with national average soil fertility status as an example, 48 acres of the 100 would need a bag of MOP meaning the investment would be 2.5 tonnes X €900 = €2,250. It will reduce tax this year and insulate the business against outside shocks in years to come. Our predecessors spread 2 bags of 0-7-30 in the autumn. It was much harder work back then – lifting hundredweight bags into small fertiliser spreaders. On most farms nowadays, the small bag is gone.

Spreading in the high rainfall winter months may not be giving the best results on some farms. September and October is a very good time to spread it as rainfall is not too high, growth is slowing and the risk of grass tetany is reducing. It also means that on silage ground, it isn’t at risk of being taken up into the silage crop (as is the case when spreading high levels closer to harvest in the apring). High K silage being fed to dry cows can drive milk fever as they calve.

In terms of the environment, having correct K indexes mean that we make much better use of our Nitrogen. It means we have to spread less to get the same growth which is good for the pocket and good for the planet. It also means that the grass will perform better in dry weather and even though we live in the West of Ireland, we seem to be experiencing more frequent periods of dry weather during the mid-season over the last number of years.

In summary, to build indexes you must put out more potassium/K than the soil needs for maintenance. So pick out the low K fields and put out slurry or 50 units on top of maintenance to achieve the benefits stated in the title of this article.


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