Choosing the right liming product is more complicated than it seems. James Andrews asks an industry expert for advice.

Applying agricultural lime helps maintain correct soil pH, improves plant nutrient uptake and generally enhances soil biology and structure.

Thousands of tonnes are applied to farmland every year, but choosing the correct product and making sure it is properly applied requires some thought, says Brian Annis, a consultant for the Agricultural Lime Association.

Price is an important driver behind product choice, but quality and value for money should be the main considerations, as the product must be able to react with the soil, he says. “Not only must it spread evenly and accurately, but it must be fine enough to be able to mix with the soil.”

The harder the product is, the finer it must be to fulfil this requirement, so softer chalk does not need to be as fine as the harder limestone products, he says. Farmers may be offered cheaper, coarser products, but these could take years to achieve the desired liming effect.

When purchasing any liming product, the neutralising value is another important figure to note, says Mr Annis. “The higher the neutralising value, the more effective the product will be. When a liming material is sold, it is a statutory requirement to declare the neutralising value.”

The reactivity number gives an indication of the speed of reaction in the soil, and materials which are soft, porous and finely ground will have a much higher reactivity and be more effective than hard, coarse materials, he says.

To assess the value of a product, growers should consider the neutralising value, the fineness of the material and the reactivity. With ground materials and those with a high reactivity, take the neutralising value and divide it by the total of the combined purchase price and spreading charge. This will give the best base for comparing products, he explains.

Contaminants are rarely a problem with natural lime products, but growers should consider the risks associated with some of the recycled by-products from industrial processes. Some recycled by-products will have a high neutralising value and reactivity, and are good value for money, but according to Mr Annis growers should weigh up the risks before committing. “It is vital that these products are carefully assessed and that farmland does not become the dumping ground for products that would otherwise attract an expensive disposal charge.”

There are a number of recycled products available, which are primarily soil conditioners, but have some liming value. These can be used as maintenance applications, but will often not be sufficient for correcting large pH imbalances.

To determine which product offers the best value for money, growers should start by measuring soil pH, he says. “Remember that the pH can vary over the whole field, so map each test result so that any lime required can be applied in the appropriate areas and at the appropriate rate.”

Growers should also look out for “bulked samples” where pH is recorded as an average, because acidity can occur in patches in a field, he adds. “Where possible, use a FACTS-registered adviser who can also calculate the lime requirement based on the soil pH and the target pH.”

Users should also consider sourcing locally available materials, as liming operations usually require large tonnages, making the haulage cost particularly important, he says.

Liming legalities

Agricultural lime is sold under the Fertiliser Regulations 1991, which gives specifications for the range of agricultural liming materials available in the UK. The liming materials covered by the regulations fall under two categories:

Natural products

Material prepared from naturally-occurring deposits and include calcium carbonates such as limestone, chalk and sea sand, and calcium/magnesium carbonates including magnesian or dolomitic limestone. This category also includes the oxides (burnt lime) and hydroxides (hydrated lime) produced from the carbonate materials.

Recycled products

Materials produced from industrial processes which include sugar factory lime, silicate liming materials (slag from the iron and steel industry), various ashes from the burning of wood and poultry litter, and the by-products from waterworks treatment, the paper industry and the manufacture of cement.

UK fertiliser regulations are due to be revised and updated. There is also a proposal to extend the scope of the European fertiliser regulations to include alternative liming materials, says Mr Annis. “While none of this is going to happen overnight, the end result will obviously affect the agricultural lime industry, and it could make farmers more aware of the regulations affecting the use and quality of the wide range of liming materials available.”

Lime calculators

• Rothamsted Research (RothLime) www.rothamsted.bbsrc.ac.uk/aen/rothlime/

• ALA Lime Calculator www.aglime.org.uk/limecalculator01.htm