Liming is key to unlocking your soil's real potential

Liming is a fundamental step in improving and maintaining soil fertility
Liming is a fundamental step in improving and maintaining soil fertility

Mark PlunkettDavid Wall

LIMING is a fundamental step in improving and maintaining soil fertility. Over the last four decades, lime usage has declined from an average of 1.7m tonnes in the 1980s to an average of 725,000 tonnes in the 2000s.

Approximately 50pc of the total lime applied at present is applied in the south of the country - 29pc in counties Wexford, Carlow, Kilkenny, Waterford and the remaining 21pc in Cork.

Soil test results indicate that 60pc of Irish grassland soils are below the target pH 6.3 for grassland.

Given the reduced lime usage and our high annual rainfall, it's not surprising that soils are becoming more acidic. This is possibly the largest limiting factor to grass production in Ireland.

Benefits of liming

Lime has many effects in the soil. These include increasing soil biological activity resulting in nitrogen recycling, improving nutrient availability, promoting better soil structure and increasing the survival of productive rye grasses and clover.

Nitrogen (N) and phosphorus (P) are expensive when purchased as chemical fertilisers, plus they are tightly regulated under current Nitrates legislation.

Our soils contain large quantities of both N and P, but lime is the key to unlocking these valuable nutrients from the soil on an annual basis.

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Maintaining a soil pH 6.3 to 6.5 in grassland soils can result in the release of up to 80kg N/ha/year.

This will help reduce overall N that needs to be purchased, which is worth approximately €80/ha/year. Recent research from Johnstown Castle has shown that where lime alone is applied soil test P levels have the potential to increase by a similar degree as where P fertiliser alone is applied.

This research showed that where lime and P fertiliser are both applied maximum soil P availability is reached.

Currently many farms are aiming to build soil P fertility levels in order to increase grass production. This work clearly shows the important role that lime has to play in improving the P status of Irish soils.

At a time when fertiliser prices continue on a gradual upward trend all farms need to check soil pH levels and lime accordingly part of their soil fertility improvement strategy

Analysis and liming

Lime should only be applied based on recent soil test results as soils will differ in the quantity of lime required to bring them up to the target pH.

Now is a good time to apply lime as it will be starting to work over the autumn period. Ground limestone is most active over a two-year period with the very fine lime particles (<0.15mm) working instantly.

The larger lime particles take longer to breakdown and help to maintain soil pH at or close to the target soil pH in the preceding years.

Check soil test results and apply lime firstly to fields that have the lowest soil pH values. It is good practice to apply lime to 20pc of your farm on an annual basis.

Lime Loss

Due to our high annual rainfall lime is mainly removed from soil through leaching and water drainage.

The next pathway for lime loss is in neutralising the acidity stemming from N fertiliser applications. For example, each kilo of N applied requires close to 2 kgs of lime with urea consuming more lime than CAN.

There are also small amounts of lime removed by livestock and crops at harvest time. For example a crop of first cut grass silage will remove approximately 75kg lime.

In the region of 0.5-1.0t/ha/year of lime is lost annually. Therefore, to maintain the productivity of our soils we need to apply lime on a planned approach to replace lime loss and keep soil pHs at the target pH of 6.3.

Application Rate

The soil test result will show the rate of lime recommended based on the soil type.

On very acidic soils it is best to spilt the lime recommendation applying half in the first year and the remainder in year three. The maximum application rate at any one time should not exceed 7.5t/ha (3t/ac). Where soils are low in magnesium, Mg lime should be used as the most effective way to improve soil Mg levels.

In general Irish soils are not low in Mg but there are pockets of land in specific counties low in Mg.

Lime and High Mo soils

In grassland soils that are high in molybdenum (Mo), it is important not to raise the soil pH above 6.2.

As soil pH levels increase above this more Mo becomes available and may induce copper deficiency in animals. High Mo occurs on specific soil types and care should be taken where previous experiences of fields or farms with high Mo levels have caused problems.

Liming and slurry applications

Applying slurry on recently limed soils should be avoided as it may result in increased losses of the N component through accelerated ammonia volatilisation. Where lime is applied, slurry should not be applied until the lime is well washed into the soil.

Liming checklist

Aim to maintain a 
target soil pH 6.3 to 6.5 in mineral soils

Well limed soils can release up to 80kgN/ha/year

Liming will increase the availability of soil P and K

Don't exceed 7.5t/ha lime in a single application

Don't exceed pH 6.2 
on high Mo soils.

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