Given all the benefits associated with lime usage and having soils at the optimum pH, why are farmers reluctant to apply it? Generally, the return far outweighs the cost.

I get asked a lot of questions about liming, by farmers looking to address any soil fertility issues on their farms, usually after soil-sampling underperforming fields.

Liming has to be carried out at times of the year when ground conditions are suitable, and it often just doesn’t get done if left too late into the autumn, when trafficability may be the stumbling block.

These are the main areas I get asked about:

Residue

Ideally lime should be applied to recently grazed fields with low grass covers.

Rainfall will typically wash most of the lime from the grass onto the soil.

Where a small amount of lime remains on the leaf, it is not harmful to grazing livestock.

Lime on silage ground

The same can’t be said for residues on the grass destined for silage-making. Lime is a neutralising agent so it is counter- productive to the acidic conditions required for good silage preservation.

I always err on the side of safety: I recommend at least three months between lime application and closing for silage. This should avoid under-dry conditions, as any of the lime entering the pit or bale will affect grass preservation.

Interaction with slurry

Spreading cattle slurry on freshly limed land where the lime has not had sufficient time or rainfall to be washed into the soil can result in a loss of up to 50pc of the available nitrogen in the slurry.

To minimise these losses, the advice is to apply the slurry first and then the lime 7-10 days later.

Interaction with urea fertiliser

Loss due to the volatilisation of nitrogen in the form of ammonia can occur where straight urea is applied to recently limed land.

Again, apply the urea first followed by the lime 7-10 days later.

Where protected urea is being applied, Teagasc research indicates that it is safe to apply protected urea to fields that have been recently limed, as the urease inhibiter protects the urea granule from losses due to volatilisation.

Other obstacles

In the Sligo/Leitrim area where I work, lime use leaves the soil surface softer and more prone to poaching.

Also most of the soils are derived from carboniferous shales and are high in molybdenum.

Soil types with a thick organic layer above the topsoil are more prone to poaching during the wetter periods of the year.

This layer holds a large store of acidity, and when limed this is neutralised. The pH increase creates favourable conditions for biological activity and the release of nutrients stored in the organic matter.

As the nutrients are released, the top layer of the soil becomes temporarily more prone to poaching by livestock.

“Liming will cause the ground to run away” was a saying I often hear.

To minimise these effects, the advice is to apply lime on a little-and-often basis, improving the pH over time.

No more than 5 ton per hectare should be spread on these soil types in any single application.

Soils with high molybdenum status may increase the risk of inducing copper deficiency in grazing animals. My advice is to apply the lime and supplement the animals with copper.

How fast does lime work?

Once lime is applied and washed in, it starts to adjust the soil pH.

At least 35pc of ground limestone (350kg/tonne) has a particle size < 0.15mm. This fine component is fast-acting and will start increasing the soil pH immediately.

The remaining 65pc will be broken down in the soil over an 18-month period and will help to maintain soil pH levels in the long term.

Is liming cost-effective

Research shows that liming acidic soils increases grass production by 1tDM/Ha. On a drystock farm this is valued at €105/ton, and €180/ton on a dairy farm.

An application of 5t/Ha of ground limestone to correct soil pH represents a cost of €25/Ha/year over five years.

The return on investment gives €4-7 worth of extra grass for every €1 invested.

Correcting and maintaining soil pH at optimum levels will result in up to 70kg/Ha of nitrogen being released from the soil and made available for plant growth on an annual basis.

This is equivalent to 56 units of free nitrogen per acre.

Liming will also substantially increase the uptake of chemical and organic phosphorus and potassium applied and will also release P that has been locked up in the soil due to acidic conditions.

Now is the time to lime.

Summary

Long-term improvement in soil pH and structure;

Immediate impact, with improved grass yields;

Maximised efficiency of use and availability of chemical and organic N, P and K applied;

Environmentally friendly reduced need to apply chemical nitrogen.

Tom Coll is a Teagasc business & technology advisor, based at Drumboy, Co Leitrim; tom.coll@teagasc.ie