The real value within

Brian Chambers of ADAS argues for greater promotion of biosolids as organic fertiliser

UK research on the beneficial aspects of biosolids additions to land has, in the past, concentrated on nitrogen and phosphorus supply, and more recently on pathogens, but many other soil quality benefits have been claimed. It was to assess these additional benefits that UKWIR funded an ADAS led study, along with WRc and the Scottish Agricultural College (SAC), on seven established experimental sites throughout Britain where biosolids had been applied for at least four years.

Value-packed
There has been a lot of anecdotal evidence on the soil conditioning benefits of biosolids, but no one has quantified the real benefits to UK soil quality before. The aim of the study was to better inform regulators, water companies and farmer customers of the value of biosolids as an organic fertiliser. The ADAS studies with biosolids took place on five research sites across England and Wales, using a range of products including digested liquid, digested cake and thermally dried granules. The studies also included results from liquid biosolids research by SAC in Scotland.

Soil benefits
The biosolids applications resulted in improvements to physical soil conditions, increasing the infiltration of water into the soil and reducing the potential for surface run-off and the susceptibility to water erosion and associated sediment losses. The biosolids also helped the soils to hold more water, which is vital on light land and in the absence of irrigation water was estimated to result in increased potato outputs of £60/ha and carrots £36/ha. Furthermore, there was evidence that biosolids had improved soil structure, resulting in better soil aeration and improved resistance to compaction by vehicles and grazing livestock. The only potentially adverse soil affect noted was reduced spring soil temperatures, probably as a result of the increased water holding capacity of the soil.

The benefits from crop-available nitrogen and phosphorus are well known. Indeed, in terms of environmental economics the recycling of biosolids nitrogen and phosphorus to land, replacing the need for manufactured and imported fertilisers, is the favoured option on sustainability grounds. Biosolids also provide useful amounts of sulphur and magnesium, which is particularly helpful to sulphur sensitive crops such as oilseed rape, and potatoes and sugar beet which are susceptible to magnesium deficiency. Biosolids also contains many minor nutrients and trace elements like copper, boron, cobalt, selenium and iodine, which can be useful for crops and grazing livestock.

Modern biosolids have a lot to offer and if applied in accordance with the Safe Sludge Matrix are very valuable to farmers and growers. With improvements to industrial processes and better sewerage treatment, long-term risks from heavy metals are minimised. All in all, biosolids offer a low-cost and valuable material for farm use. A copy of the complete research report Beneficial Effects of Biosolids on Soil Quality and Fertility by Chambers BJ, Nicholson FA, Aitken M, Cartmell E and Rowlands C (UKWIR reference 01/SL/08/2; ISBN 1 84057 216 7) can be obtained direct from the printers: Westward Documedia, 37 Windsor Street, Cheltenham, GL52 2DG, Tel: 01242 283100, email: matthew.chapman@documedia.co.uk. The report can also be ordered through: www.ukwir.org.uk


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