Weather permitting, the coming weeks will provide Irish farmers with the perfect opportunity to spread lime. According to Teagasc, 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 and K) applied in organic and chemical fertilisers.
Lime is continually being lost from the soil and needs to be replaced as part of a nutrient management programme, e.g. drainage water can remove approximately 250–625kg/ha of lime equivalent each year, depending on the soil type.
Soil type and lime loss
Light free-draining soils will lose lime more quickly than heavier soils. Therefore, light land may need extra attention, particularly in areas where limestone is not present in soil parent material or bedrock.
Crops and livestock also remove lime e.g. a crop of first cut grass silage (5t/ha DM) removes approximately 75kg/ha of lime equivalent.
A finishing bullock will remove approximately 25kg while 1,000L of milk removes approximately 3kg of lime.
Nitrogen (N) fertilisers also have an acidifying effect on soils. Each kg of N applied as calcified ammonium nitrate (CAN) or urea, will generate acidity that will require approximately 2kg of lime to neutralise.
Urea tends to require more lime per kg of N compared to CAN and continuous application of urea will increase soil acidity.
Lime requirements
During the period 1970 to 1985, lime usage in Ireland averaged 1.5 million tonnes annually. For the specific years 1979 and 1985, the actual amount of limestone spread was in the region of 1.7 million tonnes.
According to Teagasc, these figures are 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 in Ireland.
In contrast, annual lime usage has only exceeded one million tonnes on four occasions during the last 30 years.
As a consequence, 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 and fertilisers).
Lime application
Traditionally, the perceived practice was to treat 20% of available farmland with lime based on soil test results.
Teagasc confirms that such an approach remains good advice, when it comes to managing soil pH.
Significantly, the last decade has seen lime applications exceeding the previous 10-year average, except for 2017 and 2019.
In each of these two years, weather conditions did not favour the application of ground limestone.
Weather is one of the largest uncontrollable factors influencing the application of lime in Ireland annually, e.g. 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.
When to spread
Advisors stress the need for every opportunity to be taken during the year to apply ground limestone to correct soil acidity. These include: after silage making; grazing and reseeding.
Lime application on soils alone will bring major benefits in terms of soil nutrient availability (especially N and P) and the more efficient use of applied nutrients (N, P and K) as either organic or bag fertilisers.
This will result in extra grass production of one to 1.5t DM/ha from a small annual investment (€25/ha) in ground limestone.
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 requiring more lime to achieve the same increase in pH.
Soils that are heavier textured (clay soils) or soils with 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.
Crop types
Lime can be applied at any convenient time of the year. For lime-sensitive crops such as beet, cereals or maize, lime can be applied two 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, lime can be applied 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.