Will spreading lime solve all your soil fertility problems?

Lime is the most important and cheapest nutrient that farmers can apply to their soils to increase grass growth, according to Teagasc Dairy Advisor Michael Donoghue.

Teagasc research shows that along with addressing soil pH levels, lime applications have a positive impact on the Nitrogen (N), Phosphorous (P) and Potassium (K) availability.

According to Teagasc, the ideal pH for grassland swards is 6.3-6.5, below these levels weed grasses out perform perennial ryegrass and clover.

It estimates that applying lime to an acidic soil (pH 5.5) can release between 48-64 units of N per hectare each year for several years.

And, Donoghue said lime is the starting point to improve soil fertility on all grassland farms, but applications must be based on recent soil test results.

When farmers get their soil results they tend to focus on the P and K levels, as they are in vogue, but lime is the most important nutrient for soil fertility.

Donoghue, who contributed to the Aurivo and Teagasc Dairy Handbook, said that addressing the pH level should be the first thing farmers do once they receive their soil results.

Lime applications

Teagasc soil test results show that the majority of Irish grassland soils are lacking in lime and as a result they have lower than desired pH levels.

Each year, Teagasc carries out about 38,500 soil tests in Johnstown Castle and the 2015 results show that over 60% of grassland soils had a lower than desired pH.

To address this issue, Donoghue said that farmers should aim to apply lime on the fields that require it over a number of years.

Generally if a field requires lime, the farmer should spread 2t/ac in the first year.

“If the soil requires more than 2t/ac, they should spread a further 2t/ac in two years time, depending on the soils pH levels.

“A little and often approach is the safest bet to addressing a lime deficit,” he said.

Source: Teagasc
Source: Teagasc

But is lime enough?

However, Donoghue said that lime is only the starting point and if the soil has a requirement for P and K it should also be addressed through the use of farm yard manure and slurry.

He said that these are the cheapest sources of P and K and they are readily available on most farms.

“On farms that are building up soil P and K levels organic manures may not be enough and artificial fertiliser may be required.”

But, he also said that farmers who are building the P indexes should ensure that they stay within the cross compliance requirements.

Teagasc recommendations for building soil fertility levels:
  • P: 10-30kg/ha each year to rise one index (25-50% of a bag of 16% P/ac).
  • K: 15-30kg/ha each year to rise one index (25-50% of a bag of 50% K/ac).
  • Index 4 P: reduce or omit P application until next soil test.
  • Index 4 K: reduce or omit K applications until next soil test.

“The amount of artificial P that farmers are allowed spread will depend on the volume of meal feeding on their farms

“Farmers who import a lot of feed on an annual basis are only allowed spread low levels of artificial P. Some farms have very low P allowance,” he said.

He also said that there is no limit on the amount of artificial K that can be spread on grassland soils.

Addressing low soil fertility:
  • Soil sample the whole farm to see what nutrients are required.
  • Complete a nutrient management plan – Provides information on the levels of lime, P and K required.
  • Target low pH soils first – Grassland soils should have a pH between 6.3-6.5.
  • Use slurry and farm yard manure on fields low in P and K.
  • Artificial P and K may be requires – The farm must stay within cross-compliance requirements.