Some 35 per cent of UK arable soils fail to meet the target pH of 6.5. Gus Merrick of Omya UK offers some guidelines on rectifying imbalances.

The target 6.5 soil pH is essential for maximising crop yield potential and is optimal for achieving full nutrient use efficiency, said Mr Merrick.

“A soil pH of 6.0, for example, might seem like a small variation from the target pH, but phosphorus availability in the soil will only be at 52 per cent compared with the target 6.5 pH, at which point the nutrient is 100 per cent available,” he said.

“A neutral pH will also benefit earthworms. Research has shown that correcting the soil pH from 5.5 to 6.0 and above will increase the earthworm population by as much as 35 per cent. Earthworms have many positive roles in soil health, including the digestion of organic matter. They will also enrich the soil with exchangeable calcium, magnesium and potassium, as well as available phosphorus and nitrogen in the form of nitrates.”

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The soil pH has an impact on the microbial population, including nitrifying bacteria present in the root nodules of legumes like peas and beans, he added. These actively participate in the conversion of nitrogen from the air into nitrogen available to plants. From pH 6.2 to 7.0, there will be 55 times more nitrifying bacteria present and they will promote plant nitrogen uptake and prevent macro-nutrient leaching.

The pH measurement uses a logarithmic scale, he explained, with a pH of 5.0 ten times more acidic, compared with a pH of 6.0. The most common method of reducing soil acidity and reaching the target soil pH is to use lime, usually in the form of calcium carbonate, said Mr Merrick.

“A common misconception is that calcium affects soil pH, when in fact it is the carbonate that reduces soil pH. The calcium element within a standard soil lime application is a valuable nutrient in its own right. Calcium is present in every cell of the plant. It strengthens the cell wall and helps to protect the plant from pest and disease threats.”

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The starting point for addressing soil pH is to test each field, he pointed out.

“Soil pH can be variable within fields, so a grid system is preferable for soil sampling. A ten-hectare field should be divided into five, two-hectare sections, while a smaller field can be split in half, with 25 samples taken at a depth of 15cm from each grid.”

He recommends using the online calculator provided by the industry organisation, the Agricultural Lime Association, for calculating lime application rates. Users can enter the starting soil pH and soil type, to find the amount of lime required in tonnes per hectare for the various types. The figures are expressed in percentage terms.

“An arable field with a sandy loam soil at a pH of 6.3 with a target of 6.5 would need two tonnes per hectare of high-quality ground limestone. If the plan is to apply a fast-acting granulated lime, the recommendation is for 0.3 tonnes per hectare for a sandy loam. Clay soils have higher buffering capacity and may require higher rates of lime to alter pH.

“There are many different sources of liming material in the UK, with ground limestone and screened lime the most commonly used products. They are readily available and tend to be competitively priced. It is important to note that not all liming materials are equal. In general, the finer the particle size and higher the purity of calcium carbonate, the more reactive the material.”

Other high-quality liming materials include products like Limex, a co-product of the sugar beet industry which also has additional nutrient values. Meanwhile, granulated lime is made from ultra fine particles that are produced using a soluble binder.

"Granulated lime can be spread with a conventional fertiliser spreader to 36m at any time of year. It acts rapidly, due to the ultra-fine particle size, and is used at lower application rates compared with conventional lime.

“Most lime is applied by contractor to the crop stubble after harvest, but it can be used at any time of the year, ground conditions permitting. Common practice is to lime land every five years and wait for the pH to fall before reapplying.

"However, it makes far more sense to raise the soil pH to an optimum of 6.5 using ground limestone and maintain the pH at this level using small doses of granulated lime annually. This method will achieve optimum pH and give 100 per cent nutrient use efficiency.

"Lime is a relatively inexpensive input compared with fertilisers. Given the risks associated with crop performance and nutrient use efficiency on acidic soil, it is worth making sure that fields are maintained at the correct pH level,” concluded Mr Merrick.