Not always prioritised, potassium (K) is an important nutrient of grassland. In this article, Climate and Sustainability Advisor based in Ballinasloe, Colm O’Leary reminds us that it’s time to maintain or build soil K levels.

While each farm has a nitrogen and phosphorus allowance based on regulated limits, K is not a regulated nutrient. Potassium is a vital nutrient for building grass yield potential and improving nutrient (especially N) uptake and utilisation.

It is a key nutrient in building plant structure, helping keep grass upright, while maintaining water balance in the plant which is a help during dry conditions. A high percentage of K is recycled by grazing livestock (90%+ in urine) onto grazing land with the most significant offtakes associated with silage crops (typically 3-4 times the K requirements versus grazing).

Paddocks that have had surplus grass removed as baled silage will also need to return K with each bale of grass silage removing approximately 10 units of K/bale.

Potassium – where to start

Most farmers have a nutrient management plan or soil sample results to establish lime, phosphorus (P) and K rates. Check in with your advisor or consult your results to identify fields low in K which will appear as Index 1 (<50mg/l very low K supply) or 2 (<100mg/l low K supply). Any soils at Index 3 (101-150 mg/l optimum soil supply) can be targeted to replace offtakes to maintain their optimum levels. If soils are at index 4 (>150 mg/l K), there is no K requirement for the year after the soil sample was taken and then revert to a K Index 3 maintenance application rate until the next soil sample is taken.

Peaty or high organic matter soils (>20% SOM) don’t have the ability to retain K like mineral soils. In the case, where a farm is low in K, prioritise fields that are most relied upon to deliver optimum growth (grazing platform or silage fields) and build these to the optimum level (Index 3).

Requirements

On silage ground, the offtakes can be significant. A good rule of thumb when closing for silage would be 1 unit of K per every unit of N applied.

The predicted offtakes for a first-cut silage crop yielding 5t DM per ha (10 bales/ac) is 125kg K/ha (100 units/ac). Second and third cuts have a predicted offtake of 75kg/ha (60 units/ac) assuming 3t DM/ha yield.

Offtakes from grazing are stocking rate dependant and significantly lower. For a dairy farm grazing at 2.5-3 LU/ha, the predicted maintenance requirement is 40kg K/ha (32 units of K/ac). A beef farm stocked at 2 LU/ha is predicted to require 15kg/ha (12 units/ac) for maintenance. Actual growth and farm offtakes will impact the true maintenance figure required. K application is often referenced as being particularly important for red clover silage.

Sources of potassium

The best source of K on a livestock farm is cattle slurry or farmyard manure. The typical K content of slurry is 32 units of K/1,000 gals at 6% DM, although slurry testing can show variances at farm level due to differences in livestock type and feeding regime.

Where compound fertilisers are used in line with the farms P allowance, 18:6:12+S is balanced fertiliser for grazing areas. On silage areas, 0-7-30 is suitable due to its P:K ratio. Where there is no P allowance on farm or it has been distributed elsewhere, 29-0-14+S can be used in line with normal N application to build on the grazing platform. MOP 50% (Muriate of Potash) is a suitable product for build-up in particular and with 50 units of K it is ideal for meeting the demands of silage areas after the final yearly cut has been taken.

Timing of K application

K is susceptible to ‘luxury uptake’, which means grass is likely to take up more than it needs during rapid growth periods. This can cause issues with grass tetany in lactating animals. High K levels when closing for silage can lead to excessive K in silage which is associated with milk fever susceptibility.

For this reason, a single application is recommended to not exceed 90kg K per ha (73 units/ac). The typical route to build K is to apply a little and often on grazing land, use slurry to apply maintenance on silage areas and apply build up at the end of the growing season.

Key messages:

• Start identifying if there are K deficits on the farm with your advisor
• For silage ground, start planning MOP 50% where needed for maintenance or build up (1 bag/ac K Index 1, 0.5 bag/ac K Index 2)
• Have paddocks where surplus bales have been harvested received slurry or chemical K?
• Payback in increased grass yield and N use efficiency
• Optimum soil fertility allows reduced chemical N use on productive land key to water quality and GHG emission targets.