Like all soil nutrients, potassium (K) requires both maintenance and build-up if below the target level. From a maintenance perspective, the target index is 2+ for soil K in NI and 3 in ROI.

For grazed grass, 30 units per year is required to maintain index 2 in NI and for build-up from index 0 to index 2+, 80 units/acre is required.

The most K that should be spread in any one application is 90 units/acre. So while the K requirement for a one-cut silage system is 96 units/acre and for a two-cut silage system 124 units/acre, these rates include what was spread before closing, what was spread at closing and what was spread after cutting. The value of cattle slurry should be taken into account also.

Some Dairylink programme farms were showing signs of K deficiency, with soil analysis highlighting only 30% optimal K in 2015 across the group.

Some possible reasons for this include the increased use of cutting surplus grass paddocks for baled silage, zero-grazing and continued silage cutting on farms without replacing K.

Research from Teagasc suggests that for every 1t of silage dry matter, 20kg K/ha is taken up. Therefore, a paddock of 2,000kg DM/ha cut for silage will require 40 units of K to replace what was taken off.

Maintenance levels of K should be spread during the main growing season, with build-up rates spread in autumn to prevent issues with luxurious K uptake in animals.

Slurry is a valuable source of K on your farm. Typically, every 1,000 gallons of cattle slurry contains five units of nitrogen, five units of P and 30 units of K.

Care should be taken when spreading slurry not to apply more than 3,000 gallons per acre. The risk with this is luxury uptake of K in the grass and when grazed or eaten as silage it can cause metabolic disorders.

A high-K diet reduces cows’ ability to mobilise magnesium, which increases the risk of milk fever or grass tetany.

Where a soil test shows soil K levels are high (index 4 ROI, index 3 NI), which is the case on a small proportion of many farms, then K should be omitted for one year and revert back to advice for K at soil index 3 (index 2 NI) until the soil is tested again.

Nigel Corbett

“Soil K was flagged as a concern in 2015 when we got the farm soil-sampled. Back then, approximately 80% of the farm was below optimal for soil K.

“The target is index 2+ and we had 47% of the farm in index 2- and another 32% index 1. The index 1 paddocks got 60 units of K in the autumn of 2016 and three applications of 25:5:5 used during the growing season of 2016.

“The analysis clearly highlighted that low-K paddocks were more likely to be outlying blocks of land not receiving much slurry and some of the heavier grazing paddocks which would be occasionally taken out for baled silage and rarely get slurry.

“My current policy is to get 60 units of muriate of potash (MOP) on low-index paddocks (index 1/0) in the autumn, then target these and index 2- areas with an 18:6:12 compound during the growing season.

“Getting on with slurry can be difficult, especially with heavy grass cover in the spring. However, if we get slurry on, less compound will be required.

“Slurry will be used on the silage area after first and second cuts. Based on 2017 analysis, only 30% of the farm is below optimal for K. By the end of 2017, we should be down to around 5% below optimal for K.

Charles Clarke

“Over 90% of the farm was optimal for soil K when we sampled in 2015. Generally speaking, Cavan soil would be high in K. So maintenance is the key issue on this farm.

“The 2015 samples highlighted 40% of the farm to be above optimal soil K at index 4.

“These paddocks have received no chemical K during 2016.

“The soil samples taken in 2017 highlighted the same trend. However, only 20% of the farm is now index 4.

“On the grazing block, I am using 30 units to replace K take-off throughout the season, using compound fertiliser 18:6:12 or 24:2.5:10 as part of the rotation.

“On silage area, some compound will be used. However, most of the K will come from slurry at 2,500 gallons/acre. I aim to get 60 units/acre of K on after silage cuts.”

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