Autumn is the time to tackle potassium deficiency in silage fields

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Joe Kelleher

Joe Kelleher

Spring is usually the time of year when soil sample results appear on advisors' desks across the country and farmers start looking for advice on the appropriate fertiliser to spread.

Approximately 60pc of our soils nationally are at index 1 and 2 for potassium.

This means that six out of 10 silage fields are deficient in potassium, and this poses a challenge. Autumn time is the ideal time of year to rectify potassium deficiencies.

So why is potassium such an important fertiliser? Potassium is the nutrient taken up in the greatest quantity by grassland swards and has a wide-ranging role in the plant, affecting nutrient uptake, photosynthesis, rate of growth and feed value.

It is particularly important for increasing stem strength, improving drought resistance and cold tolerance, and importantly for increasing yield.

Potassium fertilisation is vital, especially in autumn and on older grass. If adequate amounts of potassium are not available, the rate of growth and yield will be restricted.

There is also a relationship between nitrogen and potassium, as the response of grass to nitrogen is dependent on an available supply of potassium to allow N uptake as nitrate and conversion into proteins.

In silage fields in particular, stem strength is of huge importance. Silage crops low in potassium are more prone to lodging, as the stem cannot hold up the seed head and grass plant.

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Padraig O'Kiely's work in Teagasc, Grange on the factors that impact most on silage quality show that lodging is the factor that can lead to the greatest deterioration in the quality of a silage crop and estimates that up to nine units of DMD percentage could be lost by your crop lodging. This means your 70pc DMD silage pit might now be only 61pc DMD.


The table below shows the amount of potassium (K) required for first-cut silage crops. Fields that are index 1 and 2 for potassium require 120-140 units of potassium/acre.

However, as the recommended amount of K to be applied in a single application is 90 units (three bags 0-7-30/acre), the advice is generally to spread the 90 units and wait until the autumn to spread the balance. In practice, this balance is rarely spread.

The reason why it is advised to only spread 90 units in a single application is because luxury uptake of potassium occurs in rapidly growing spring grass, which has the ability to take up a lot more potassium than it needs for normal plant functioning.

High levels of potassium in the plant interfere with the uptake of magnesium, and low magnesium in the animal's diet is one of the factors causing grass tetany.

Many of us depend on cattle slurry to rectify our potassium deficiencies; as 1,000 gallons of thick cattle slurry will contain approximately 30 units of K, it will go some distance to replacing some of the K off-take from cutting or grazing.

However, in a silage situation, 3,000 gallons/acre provides only 90 units of K at best, meaning we are still short 30-50 units of K on index 1 and 2 soils. Now is the best time to apply this amount.

This autumn there is increased risk of stripping the land of potassium because of the volume of late cuts of silage that are going to be taken in September/October.

It is vital to replenish this K to ensure next year's silage crops will grow to their potential and help fill the empty silage pits around the country. A bag of Muriate of Potash (50 units K) should be adequate in most situations.

As there is no legal deadline or maximum rates in relation to potassium, every farmer should look at getting a pallet or two of potassium spread over the next month.

Joe Kelleher is a Teagasc consultant based in Newcastle West, Co Limerick

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