Lime holds the key to unlocking soil fertility

Over the last five decades the application of lime a major soil nutrient has collapsed more than two-fold, resulting in Irish soils that are now massively deficient in lime.

In the 1970s and early 1980s there was an average of 1.7m tonnes of lime applied annually. Over the last 30 years we have applied an average of 800,000t lime annually - less than half of the previous average.

In Ireland our soils naturally have a requirement for lime to control soil acidity generated by our high annual rainfall. For example, from the east to the west coast we get in the region of 1-1.5m (40-60 inches) of rain each year. Rainfall is the biggest driver of soil acidity and reducing the production power of our soils.

The accumulated reduction in lime applications over the last 30 years has resulted in a large percentage of soils with low pH levels.

Irish soils are some of the most productive soils in the world in terms of either grass and grain production due to our temperate climate. But if we continue to ignore lime as key component of soil fertility we are slowly eroding away our competitive advantage.

Soil test results analysed by Teagasc show that on average 65pc of all soils have a lime deficiency. In some counties this figure is as high as 80pc. The data present in table 1 shows the percentage of soils county by county in different soil pH ranges.

For example soils in the red bar (pH<5.5) have a high lime requirement while samples in the green bar are in the optimum soil pH range (pH 6.2-6.5).

On mineral soils we aim to maintain soils in the range of pH 6.3-6.5 with grassland at 6.3 and tillage soils at 6.5. Peaty soils are ok in the pH range of 5.5-5.8 due to high organic matter levels.

Soil acidity is a major limitation to the productivity of our soils as it reduces the availability of major soil nutrients such as nitrogen (N), phosphorus (P), and potassium (K), along with the uptake and efficiency of applied nutrients in manures or fertilisers.

In addition, in grassland swards it will reduce the persistency of productive rye grasses and clovers. Some counties such as Kildare and Offaly have a higher percentage of soils above pH 6.5 due to the underlining parent material which is limestone.

Soils with naturally high pH levels of over pH 7, or fields that are over-limed, will result in reduced availability of major nutrients, especially P, along with minor nutrients such as manganese. That's why lime should only be applied on the basis of a recent soil test report.

Soil fertility and grass production

Recent research from Johnstown Castle demonstrates the importance of lime in relation to soil P availability and the improved efficiency from applied P fertiliser.

Figure 1 shows the change in soil test P levels when lime is applied by unlocking stored soil P (purple bar) and increasing the efficiency of freshly applied fertiliser P (green bar) compared to applying high quantities of P fertiliser alone (red bar).

This clearly shows that soil pH optimisation is the first step to consider when setting out to build up soil P levels.

Figure 2 shows the grass yield response to lime and P fertiliser in grassland.

The application of 5t/ha ground limestone (purple bar) produced approximately 1t DM/ha additional grass and had similar grass yields compared to the application of 40kg/ha of P fertiliser (red bar).

However, the addition of lime and P fertiliser in combination (green bar) produced the largest grass yield response - the equivalent of 1.5t/ha more grass than the control plot.

These results show how effective lime is for increasing the availability of both stored soil P (from previous fertiliser and manure applications) and freshly applied fertiliser P.

Return on investment

As with any business, achieving a positive return on investment is critical when using inputs. Optimising the pH of grassland soils increased the annual output of grass by at least 1tDM/ha.

In addition to P and K release from the soil, increased N supplies worth up to €80 may also be achieved, boosting spring growth in particular.

If this extra grass production is utilised by the grazing livestock it has the potential to reduce farm feed bills by at least €180/ha year.

On the basis of a grass value of €180/t, and a lime cost of €25/t, 1t of additional grass production each year over a typical five year liming period using 5 t/ha represents a 7:1 return on investment in lime. This is before the potential for reducing fertiliser costs into the future are factored in.

Mark Plunkett and David Wall work for Teagasc's Crops Environment and Land Use Programme based at Johnstown Castle, Wexford

How to apply lime on grassland

Apply lime based on the soil test report. Where lime recommendations exceed 7.5t/ha it is best to split the total application rate and apply up to 7.5t/ha initially and the remainder in year three.

Do not over-lime soils since it will reduce the availability of both major and minor nutrients.

Lime can be applied at any time of the year. However, mid-summer and autumn are ideal as soils are dryer and firm, there are increased spreading opportunities after silage and grazing, and there is less interference with slurry or nitrogen (N)fertiliser applications.

Ground limestone is the most cost effective source of lime. Ground limestone will start to work once it is applied and is washed into the soil. The finer fractions of the ground limestone will adjust soil pH upwards to target soil pH over the first three months, while the coarser components will maintain this pH adjustment over the following 12-36 month period.

Use magnesium limestone where soil magnesium (Mg) levels are low.

Granulated limes are a finely ground limestone (<0.1mm) that is designed to aid the reaction with soil acidity to increase soil pH in the shorter term. Recent research shows that these products, which are usually used at much lower application rates than ground limestone, are more suitable for maintaining soil pH where soil pH is already close to the target pH of over 5.9.

Maintaining soil pH will result in increased annual release of soil N from organic matter up to a value of €80/ha. This N release usually occurs in spring and contributes to better early season growth facilitating earlier stock turnout.

On heavier and organic soils there is often a hesitance to applying lime for fear of "softening the sod" or increased poaching due to rapid break down of soil organic matter. On these soils it is best not to apply more than 5 t/ha, but to do so on a more regular basis to control soil acidity.

It is recommended to leave at least three months between liming when following on with a urea or slurry application to reduce the risk of N loss through volatilization. To avoid such losses occurring apply urea or slurry first and apply lime 10 days later.

On grassland soils with high molybdenum (Mo) levels increasing soil pH above 6.2 can lead to increased Mo levels in the herbage. High intakes of Mo in ruminant animals can lead to an increased risk of copper deficiency. It is therefore recommended to maintain soil pH at 6.2 on these soils. The alternative is to supplement animals with copper.