Just 20% of Irish soils are at optimum levels for soil pH, phosphorus (P) and potassium (K). Getting soil pH right is the first thing that any farmer should try to do before tackling soil indices.

Sub-optimal soil pH levels will affect soil function and have a negative effect on the release of other nutrients from the soil.

Correcting soil pH will increase fertiliser efficiency and help on the road to fertiliser use reductions into the future. Lime is one of the cheapest investments you can make on your farm and the benefits are well worth it where soil pH is below optimum levels.

A soil test is essential to identify soil pH levels. This will determine the amount of lime required, but do not sample soil that has received lime within the past two years as the result may be distorted.

P availability will be significantly better where soil pH is correct. It is very hard to build P and K levels where soil pH is low and you will not be getting the most out of your fertiliser spend until it is corrected.

Leading on from that, where P and K levels are low (1 and 2), then the availability of nutrients from organic manures can also be reduced.

Fertiliser spend

In a year like this, when fertiliser prices climbed rapidly, the expenses really hit home. Ensuring correct soil pH, P and K levels can prepare farmers for high price years. If soil is at optimum fertility, you can afford to maintain indices and build in a year when prices are competitive. It may be the case that P and K from slurry or organic manure will fit the bill or most of the bill in high-price years.

For example, this season nitrogen and P prices rose, but K is reasonably competitive so this may be a year to focus on improving K levels.

Lime application

Lime can be applied throughout the year and is not limited to the shoulders of the year on grassland. On cropped fields, lime should be incorporated into the top 10cm of the soil ahead of sowing.

On grassland, reseeding is a good time for application, but on low-pH soils it is essential to get lime out whenever possible. Farmers should make the most of low grass covers after grazing in the spring, after silage harvesting in the summer and again on low grass covers in the autumn when the rotation is getting a bit longer and there is enough time to allow the lime to wash in.

Lime can be applied at a maximum rate of 7.5t/ha (3t/ac). Where more lime is needed, the requirement should be split in two and the second split applied in two years’ time.

On grassland, it is sometimes better to spread at lower rates, such as 5t/ha (2t/ac), so it is washed in well and doesn’t sit on the grass.

Lime application can also be taken a step further by spreading according to variable rate maps. Soil pH levels can vary hugely across fields and many lime spreading contractors can spread at variable rates. You may still spread the same amount of lime, but it will be spread at higher rates where it is needed most in the low soil pH areas.

It takes time for lime to work in the soil. Ground lime will have a quicker impact, but larger particles may last longer in the soil and maintain soil pH for longer.

Lime – quick facts

  • Aim for a soil pH of 6.3 on grassland.
  • On high-molybdenum soils aim for 6.2.
  • High soil pH can increase molybdenum availability, which can reduce copper intake in livestock.
  • Approximately 48% of grassland soils are below 6.2 for soil pH.
  • Applying lime will increase soil pH.
  • Lime can be applied at any time of the year once there is not a strong cover on the ground.
  • Lime should not be applied at rates over 7.5t/ha (3t/ac).
  • Use soil test results to decide on application rates. Soil test results should be carried out every three to five years.
  • Lime can increase ammonia losses – where slurry or urea is applied, wait at least 10 days before applying lime. If lime has been applied, wait three months before applying urea or slurry.
  • CAN, compound fertiliser and protected urea will not suffer ammonia losses as a result of lime spreading so can be spread at any time.