Fertiliser nutrients are not applied to make plants grow; it is rather a lack of an adequate supply of one or more nutrients which prevents a plant, such as perennial ryegrass, from achieving its yield potential.

And so, fertilisers or organic manures are applied to remove any nutrient constraints, not to make plants grow.

This is according to Chris Dawson, a private consultant in the UK, who spoke at the Fertiliser Association of Ireland’s (FAI’s) spring 2020 scientific meeting last week.

During his presentation, Chris focused on the nutrient potassium (K) – which he said can have major effects on yield and quality as well as the general health and vigour of a plant, such as grass.

He also pointed out that a crop deficient in K uses nitrogen (N) less efficiently and is less able to handle stress caused by frost, heat, water-logging and wind.

Based on 2018 figures, 50% of Irish soils are inadequate for K – that is, are in Index 1 or Index 2 for K (see table below).

Image-source-Fertiliser-Association-of-Ireland

Image source: FAI

Effect of K on N response

As aforementioned, crops require an adequate availability of all essential nutrients, for maximum production, particularly N and K, which are both required in the greatest quantities.

N and K are linked, as stated by Chris – an imbalance or shortfall in availability of one will alter the need for the other.

For instance, if an inadequate amount of N is applied to a soil then the requirement of K will also be lower and less K will be taken up from the soil reserves.

Even though this will limit production, K tends to stay in the soil, so will be retained in the soil for later use.

However, Chris explained that if a rate of N is applied to achieve a full potential yield, but there is an insufficient supply of available K, then the full yield cannot be achieved.

Also, not all of the N will be usable by the crop – leading to a lower N use efficiency (NUE); but unlike K, N is prone to leaching so this surplus N will then be lost to the environment – which can pose a major risk to water quality in the area.

Image-source-Fertiliser-Association-of-Ireland-P-and-N

Image source: FAI

Response to K

Chris added that some farmers are reluctant to apply much K because they do not see a response; but this he said is usually just an indication that the soil is not deficient in K.

Image source: Fertiliser Association of Ireland

Image source: FAI

Visual deficiencies are usually only seen in Index 1 soils; however, yield depression and other symptoms of a K deficiency can often be seen in Index 2 soils, he noted.

This, he said, is referred to as ‘hidden hunger’ because symptoms are rarely seen.

Finally, Chris advised farmers to apply build-up rates of K until soils reach an optimum K status (Index 3).

Once Index 3 is achieved, it must be maintained by applications of nutrients from manures and/or fertilisers to replace the nutrients removed by silage.

Where high application rates are needed, the FAI advises farmers not to exceed 90kg of K/ha in a single application on silage fields in the springtime and the balance after silage is harvested or in late autumn.

Only maintenance rates of K should be applied on grazing ground in the spring with the balance in August/September, it says.