Soil pH & Liming

Soil pH plays a key role in soil fertility. Maintaining the soil pH at the optimum level will increase the microbiological activity of the soil, and result in better soil nutrient recycling and release. Soil pH is also critical for maximising the availability of nutrients (N, P & K) applied in organic and chemical fertilizers.

Lime is continually being lost from the soil and needs to be replaced as part of a nutrient management programme. For example drainage water can remove approximately 250 – 625kg/ha, depending on the soil type, of lime equivalent each year. Light free draining soils will lose lime more quickly than heavier soils. Therefore, light land may need extra attention; particularly in areas limestone is not present in soil parent material or bedrock. Crops and livestock remove lime, for example, a crop of first cut grass silage 5t/ha DM) removes approximately 75 kg/ha of lime equivalent. A finished bullock removed approximately 25kg while 1,000 litres of milk removes approximately 3kg of lime. Nitrogen fertilizers also have an acidifying effect. Each 1 kg of N applied as CAN or Urea will generate acidity that will require approximately 2 kg of lime to neutralise. Urea tends to require more lime per kg N compared to CAN (27%N) and continuous application of urea will increase soil acidity.

Lime Usage 1970 to 2019

In the 1970’s to the mid 1980’s lime use exceeded 1.5 tonnes every year except for 1979 & 1980 as shown below. During these years ground limestone applications averaged 1.7 million tonnes. This is indicative of the national lime requirements that are required to maintain soil pH in the optimum zone of pH 6.3 to 6.5 on mineral soils for Ireland. Since the mid 1980’s lime use has only exceeded 1 million tonnes on 4 occasions in the last 30 years. During this period soil acidity has increased on Irish farms resulting in a large requirement for lime. Correcting soil pH needs be carried out on an annual basis with ground limestone applications to maintain soil pH in the optimum zone for efficient use of both soil and applied nutrients (manures & fertilisers). Traditionally, lime advice was to treat 20% of your annually for lime based on soil test results, this was good advice and holds true today for managing soil pH and liming.

Over the last 10 years lime applications have exceeded the previous year 10 year average except for 2017 & 2019. In each of these 2 years weather conditions did not favour the application of ground limestone. Weather is one for the largest uncontrollable factors influencing the application of lime here in Ireland annually. For example in 2016 the favourable weather conditions from August to December resulted in large quantities of lime being applied. This was quite the opposite for 2019 where rainfall levels well above the average for October to December had a dramatic impact on lime applications.

It is important to take every opportunity that arises during the year to apply ground limestone to correct soil acidity (after silage harvest / grazing/ reseeding). Liming soils alone will bring major benefits in terms of soil nutrient availability (esp. N & P) and the more efficient use of applied nutrients (N, P & K) as either organic or bag fertilisers. This will result in extra grass production of 1 to 1.5t DM/ha (+ 5 to 10 bales/ha of grass silage per year) from a small annual investment (€25/ha) in ground limestone.

Target soil pH for a range of crops

The target soil pH for a range of crops is shown in Table 1. Aim to maintain soil pH close to the target level and apply lime as recommended on the soil test report.

Table 1. Optimum soil pH for a range of crops

Determining Lime Requirement (LR)

The lime requirement is calculated in the laboratory based on a test that measures the buffering capacity of the soil. Buffering capacity is a measure of how much lime it takes to change the soil pH. Therefore, soils that are returned with the same soil pH may be shown to have different lime requirements. This is because the soils have different buffering capacities require more lime to achieve the same increase in pH. Soils that are heavier textured(clay soils) or higher organic matter levels tend to have higher buffering capacities and higher lime requirements as a result. However, while these soils may require more lime following the soil test, the higher buffering capacity should result in the soil retaining lime better in the future once it has been applied.

Timing of lime application

Lime can be applied at any convenient time of the year. For lime sensitive crops such as beet, cereals, maize, apply lime 2 years before sowing. If lime has not been applied it should be spread after spring ploughing so that it can react with the soil and be thoroughly mixed with soils during spring cultivations.

For grassland, it is preferable to apply to fields with very little grass cover, and to avoid grazing or cutting until sufficient rainfall has occurred to wash the lime off the herbage. For silage swards apply lime before mid-March for first cut or within one week after cutting on land being closed for a second cut. Applying lime to heavy covers of grass intended for silage can reduce the silage quality if the lime is not washed off the grass by rain.

Over Liming

When a soil is over limed, some crops may suffer from lack of plant foods such as boron, iron and manganese. Crown Rot, a disease of beet, and Brown Heart, a disease of turnips are caused by boron deficiency.These diseases generally occur on alkaline or over limed soils. Grey speck in cereals is caused by manganese deficiency and this is brought on by too much lime. In practice, over liming is more likely to be caused by uneven spreading than by applying too much per hectare.

Freshly applied lime may increase the amount of Common Scab on potatoes and this is a serious blemish in potatoes. It is a good practice to lime the soil after harvesting the potatoes. Where a good rotation is followed, at least five years will have elapsed before the next potato crop.

Testing Equipment Information

Soil pH Meter 151

This piece of equipment will give an indication of the soils pH and can be very useful to diagnose lime shortage or crop suitability.

Remember it is important to maintain the soil pH meter, through regular callibration with pH 4 and 7 buffer solutions for good results. Please refer to guidelines below for set up and maintainence of soil pH meter.

Lime advice should only be based on a laboratory pH reading as it takes into account the soils buffering capacity. The pH meter is only to give an indication of pH and is not to be used for lime advice.

For more information please see the details below.

Articles and Publications

The following are some articles and publications

• Liming FAQ, answers many frequently asked questions about lime and queries about liming.