Time to lime?

UK farmers may be under-liming their soils says Professor Keith Goulding, Sustainable Soils Research Fellow at Rothamsted Research

Maintaining a correct soil pH is vital to enhanced crop yields and sustainable soil health. Farming soils can make them acidic, so applying lime can help push their pH up and create optimum growing conditions. But this comes with a cost – and with today’s tight margins it is a cost many farmers struggle to bear. Consequently, much less lime is being applied in the UK than required, and many arable and grassland soils are below the ideal pH.

UK agricultural soils usually have a pH of between 5 and 7.5. Soil acidification is caused by several factors including acidic rain and acidifying gases and particles from the atmosphere, such as sulphur dioxide, ammonia and nitric acid. However, since the acid rain headlines of the 1980s, governments have acted, and toxic emissions reduced. As a result, acid deposition has been declining: There is now 95% less sulphur dioxide in our air than in 1990.

Nevertheless, problems persist. The most important current causes of acidification on agricultural land are the application of ammonium-based nitrogen fertilisers and urea. Coupled with the use of sulphur fertiliser and the rising popularity of nitrogen fixing legumes, these continue to make our soils more acidic than they should be. Crop growth, nutrient uptake and the mineralisation of soil organic matter also make a small contribution.

If soils are not buffered by naturally-occurring chalk or limestone, acidification causes the loss of base cations such as calcium and magnesium. It can also lead to an increase in aluminium saturation. The overall result is a decline in crop yields. Severe acidification can cause non-reversible dissolving of clay minerals and a reduction in cation exchange capacity – the ability of soil particles to “hang on to” useful cations. Ultimately the soil structure begins to decline.

Such weathering will not be reversible except over geological timescales and so represents a serious and costly degradation of soil quality.

Soil acidity can be tackled by applying lime as chalk or limestone, or other acid-neutralising materials. The amount of lime required to neutralise soil acidity depends on the soil pH and soil type. This can be easily  estimated from lookup tables in the AHDB’s Nutrient Management Guide or models such as ROTHLIME.

Amounts of lime needed to correct severe acidity can be up to 10 tonnes per hectare on sands, loams and clays and 16 t/ha on peaty soils. Methods to apply lime more precisely with pelletised lime and using Variable Rate Application are gradually being taken up.

However, there is no one-size-fits-all solution. Crop plants vary in their tolerance to acidity and plant nutrients have different optimal pH ranges. Target soil pH values in the UK are set at 6.5 (5.8 in peaty soils) for arable crops and 6.0 (5.3 in peaty soils) for grass. This optimises plant tolerance to acidity and nutrient availability, improves soil structure, helps to restore degraded soils and displaces aluminium. Get all these factors right and crop yields and agricultural productivity can increase.

Prof Keith Goulding will be speaking at the crop nutrition seminar at Croptec 2019

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