Dairy: Boosting soil pH can yield big results

Lime is the cheapest fertiliser available in this country.
Lime is the cheapest fertiliser available in this country.
Joe Kelleher

Joe Kelleher

Up to 60 units of free background nitrogen will be released by spreading lime on low pH soils -This was one of the key messages at a recent open day on the farm of Hugh Brennan, Dromcollogher, Co. Limerick.

When spreading compound fertiliser in a field which has a soil pH of 5.5 you are losing two thirds of the phosphorus and one third of the nitrogen due to fact that these nutrients are tied up in the soil and are not made available to the growing plant.

This means that if you spread a bag of 18.6.12 in a field with a soil pH of 5.5, then the reality is that you are really only getting the benefit of 12 units of nitrogen and 3 units of phosphorus. This is why correcting soil pH always comes ahead of correcting phosphorus and potassium deficiencies.

With all this in mind, trials have shown that by increasing soil pH from 5.5 to 6.3, an extra 2ton of dry matter per hectare can be grown.

Nationally we are spreading approximately 750,000t of lime annually. However, to rectify the fact that two thirds of our soils nationally are deficient in lime, this figure needs to be doubled. In the 1970s, our parents were spreading far in excess of 1.5 million tonnes annually. So why is this generation so reluctant to spread lime?

Ger O'Sullivan, Teagasc, Killarney recently carried out an in-depth study to find out the reasons as to why today's farmer is cautious about applying lime.

The study showed that the four main barriers that were preventing farmers from applying lime were: weather, paddock availability, interactions with other fertilisers and finances.


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Traditionally, the autumn was seen as the ideal time for spreading lime. However, lime can be spread in any month of the year. The exception to this is on silage ground where the recommendation is to spread lime when the last cut is taken. Very little lime is spread in the summer months when soils are most trafficable. For example there was no lime spread in June 2015 in the whole of Co Kerry. Ground that has been cut for first cut silage (and is intended for grazing for remainder of the year) is the ideal ground to receive lime now.

Paddock Availability

Lime typically comes in 20t loads, which means that at 2t/ac you need to have 10acres of ground available to spread one load of lime. Most farmers know the paddocks that require lime. If you don't, then you need to soil test your paddocks.

With a bit of planning and a bit of forward thinking most farmers should have no issue in making 10ac available for spreading lime. The key here is to have the lime ordered and the contractor booked. Lime is not harmful to cows. In a worst case scenario, it may cause some slight scouring. Once lime is washed off the grass, there is no risk.

Interactions with other fertilisers

Spreading urea after lime can lead to nitrogen losses through volatilization. Use compounds or CAN to overcome this. Lime can be spread one week after slurry. However, doing the reverse can lead to losses of nitrogen in the slurry. But as the amount of nitrogen contained in slurry spread in the summer months is low, getting the lime out may be more valuable.


One might think that I am mad to be suggesting that we increase our expenditure in a year when milk price is at an all-time low. But perhaps this is the year we need to release that 60 units of background nitrogen. One tonne of lime generally costs €23/ton (spread and delivered) or €460 for a 20t load. If this load of lime was spread across 10ac, then it should grow an extra 6t of dry matter over a 12 month period. Replacing this 6t of grass with concentrate would cost €1,500 approximately. Most of the major banks are now offering loans for the purpose of improving soil fertility.

Lime is the cheapest fertiliser available in this country. Perhaps we should spread more of it.

Joe Kelleher is a Teagasc advisor based in Newcastle West, Co Limerick

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