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Friday 9 December 2016

Mind your 'special K'

Potassium reserves are key in push for profit

Mark Plunkett

Published 16/02/2010 | 05:00

Potassium (K) is a major plant and soil nutrient and is vital for optimum grass growth and livestock nutrition. It is an essential nutrient for the growth of grass, clover and especially for silage. Where insufficient levels of K are applied, soil K fertility will fall rapidly, resulting in a loss of yield and loss of productive perennial rye grasses and clover in the sward.

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It also plays an important role in the response of grass growth to nitrogen and survival of clover.

However, over the past few years, with pressure on farm profitability, there has been a drive to cut costs of production, resulting in a reduction of potassium application.

If savings have to be made, it is essential to check the soil K index to ensure soil K reserves are not low, as a shortage of K will have serious yield and quality effects.

Regular soil analysis is a key part of modern agriculture and is an essential tool, allowing the farmer to adopt a suitable fertiliser programme to maintain the soil K nutrient status for economic sustainability.

Over the last decade K usage in Ireland has declined by 58pc. The consumption of K has decreased by 71,000t/yr, from 123,000t/yr in 1999/2000 to 52,000t/yr in 2008/09.

Soil analysis results from Johnstown Castle show that around 50pc of grassland samples are in index 1 or 2 and are very responsive to the application of K. A further 26pc are in index 3 and require a maintenance application of K every year.

As K application falls, the proportion of soils requiring annual K fertiliser is increasing. Low K levels can result in a costly yield reduction and poor use of valuable fertiliser and manure plant nutrients.

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A major problem is that this is not evident with deficiency symptoms or identifiable poor growth. There is very little information on crop performance on a field-by-field basis as grassland forage equip- ment generally provides no information on grass yield.

Therefore, regular soil analysis is the starting point in the decision-making process to give a sound basis to the application of K as fertiliser and manure. This will tailor fertiliser and manure application to satisfy crop nutrient requirements, improve manure recycling and reduce the cost of expensive fertilisers.



  • Grazing ground -- Potassium has a very important role in ensuring early and vigorous grass growth. The K requirement of grazed grass is small because most of the K is recycled back to the sward in the dung and particularly in the urine. The potassium advice for grazing ground will be determined by the soils nutrient status and the stocking rate.
  • Silage -- Intensively cut grassland has a high requirement for K, and soil K can become exhausted if sufficient levels of K are not applied to meet crop demand.


A typical two-cut grass silage system, yielding around 50t/ha of fresh grass at 25pc DM (10t/ha DM), will remove a total of 250kg K/ha.

The K advice for silage ground will depend on the soil K index, the silage yield and the number of grass cuts.



  • Slurry -- Cattle slurry is an excellent source of K and will contain around 4.3kg/m³ or about 40 units per 1,000ga of slurry depending on the soils K nutrient status. In soils of low K status (Index 1) slurry will supply 3.9kg/m³ of K or 35 units per 1,000ga of slurry. For higher index soils (Index 2 and 3) slurry will supply 4.3kg/m³ of K or 40 units per 1,000ga.


An essential part of nutrient management planning is that slurry should be recycled on silage ground to return the K removed in cut grass.

Slurry quality is important, as slurry that has been diluted will be of lower nutrient value. This should be taken into account when selecting a suitable fertiliser type and rate to balance crop K requirement for optimum yield.

Research has shown that silage fields that are furthest away from the farmyard usually receive the least amount of slurry and these fields tend to have lower soil K levels. This results in a large yield penalty where crop requirement for K is not satisfied with a suitable fertiliser programme.



  • Timing of K application -- No more than 90kg/ha K (72 units/ac) should be applied for any one cut during the growing season; this will help reduce luxury uptake of K which may upset the potassium-magnesium-sodium balance in grazed grass.


The luxury uptake of K can be reduced if the rate of applied K fertiliser takes account of the rate of nitrogen applied, the soil K index, the use of organic manures and the soil type. Where more than 90kg/ha K is recommended, the balance should be applied to the aftermath or in late autumn.



  • Cross compliance -- Under the new nutrient regulations (SI 101, 2009), K application is not an issue as K has no environmental problems associated with it.


General rules for fertiliser and manure use covering field boundaries and margins must be complied with. Records of K application in fertiliser and manures should be kept as these will be useful for future decisions on K use.

New regulations have placed restrictions on phosphorus (P) use and have resulted in a reduction on whole farm P application. This varies with stocking rate and soil P index. It is essential to remember that silage and hay crops remove 3 to 3.5 times as much K as P. Therefore it is good practice for both environmental and economic reasons to ensure nutrient applications match crop requirement as close as possible and select fertilisers on the basis of soil P and K index.



  • Monitor soil K fertility -- In order to maximise grass dry matter yield it is essential to maintain adequate soil K (target soil K index 3).


Frequent soil analysis is an essential tool to monitor soil potassium levels (soil sample every 3-5 years). K reserves in the soil are more effective at supplying the plant than fresh fertiliser applications. Therefore it is vital to apply fertilisers and manures to replace K removed through cutting and grazing in order to maintain adequate levels of soil K to feed the crop.



  • Summary -- Potassium is an essential nutrient for the growth of grass, clover and especially for silage. Where insufficient levels of K are applied, soil K fertility will fall rapidly, resulting in a loss of yield and loss of productive perennial rye grasses and clover in the sward. Potassium also plays an important role in the response of grass growth to nitrogen and survival of clover. With increasing pressure on nitrogen use and efficiency, the supply of K is now of even greater importance to ensure maximum uptake and usage of N by the plant. When deciding on a suitable fertiliser type, take account of crop requirement, the nutrient value of organic manures applied and the soil fertility status.


Irish Independent



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