Lime is vital to neutralise acid soils, improving pH to provide crops with ideal growing conditions.

For arable crops that’s pH 6.5 – for grassland it’s only 6.

There’s plenty of documentary evidence that low pH stunts growth, mainly through poor root development. This inhibits nutrient uptake and reduces crop potential.

Lime promotes optimum nutrient supply for plants by making nitrogen, phosphate, potash and sulphur more available. It also improves soil structure by encouraging better particle formation.

If you don’t lime an historically acid field you may as well throw your money down the drain.

The effects of acid rain must be rectified, but the impact of applying inorganic fertilisers, which intensifies this acidification, is far more important.

According to the Agricultural Lime Association 5-7kg of calcium carbonate is needed to counter the effect of each 1kg of applied ammonium sulphate.

For ammonium nitrate and urea the figure is 2-3kg.

So an annual application of 200kg/ha N as a 34.5N product for wheat would need 571kg, creating a lime requirement of 1t/ha a year.

No one limes annually – it is applied rotationally. So in a five-year rotation you would need 5t just to stand still, and double this if you are using ammonium sulphate.

A good dressing is about 5t/ha annually until pH reaches the target.

If you can’t afford this, do it every other year, but remember that all the time the pH remains below optimum it reduces potential.

I cannot over-stress how important regular soil testing is.

Look for areas that may have take-all, and test these first.

Kits from gardening websites and garden centres can be useful guides. But to ensure pH testers are reliable they must be calibrated. If in doubt leave the task to the experts.

If you apply sewage sludge you must prevent the soil pH falling below 5.5. Beyond that level toxic elements such as aluminium are released which can end up in watercourses.

In essence a longer term strategic approach to liming is needed.

Lime taken off in crops must be replaced. Every 1t/ha of fresh barley straw removed takes away 10kg/ha of calcium carbonate.

There are two good websites for calculating the amount of lime you require.

The ALA website gives an excellent overview of lime, with its impressive book type approach. It offers a calculator at www.aglime.org.uk/limecalculator.

RothLime, a lime requirement model at www.rothamsted.bbsrc.ac.uk/aen/rothlime/, does a similar job, but with acid rain correction.

The user inputs the county, the current pH, the target pH, the crop type and the intended product details of which your quarry should be able to provide.

At Creeton Quarry in Lincolnshire, the screened limestone has a neutralising value (NV) of 50% and in a test run the programme suggested that raising pH from 5.5 to 6.5 in Oxfordshire would require 11t/ha (4.5t/acre).

Obviously the cost of the product, haulage and spreading costs would make such an application prohibitive, but the fact remains that if you are not at the optimum pH you are affecting yield.

Be aware that there are several lime sources in the UK ranging from calcium limestone and magnesium limestone, chalk and gypsum, and think about what you are trying to achieve.

In the Cotswolds magnesium lime might be more appropriate, but in Scotland a calcium lime is better able to displace the large quantities of magnesium built up from historic use of magnesium lime which inhibits potash uptake.

There is a concept called the reactivity index which combines the NV and softness of the material. Chalk scores quite highly, but if you are a long way from its source the transport costs of hauling it large distances will prohibit its use.

It’s worth noting that at present agricultural lime is exempt from aggregate tax (£1.95/t from April 2008). But this could change, and if the exemption was taken away, a significant rise in the price of this key input would occur.

fwarable@rbi.co.uk