18 January 2002

Digested sludge cake to answer input concerns?

By Charles Abel

CONSUMER concern about excessive food miles could help make digested sludge cake from local sewage treatment plants a more desirable source of crop nutrients than bagged fertiliser.

As consumers and lobby groups increase pressure on supermarkets for more sustainable food production, local sludge could become the preferred option, Severn Trents Chris Rowlands suggested at an industry conference in London last week.

Retailers are under increasing pressure as potatoes, for example, are moved from south-west England to East Anglia for packing, then a central warehouse for distribution, before finally returning to the south-west for sale on supermarket shelves.

Input miles could be the next focus, said Mr Rowlands. "Think about rock phosphates being ripped up overseas, shipped, crushed and processed before being applied by farmers in a specific operation."

Local sludge, by contrast, cuts input miles and brings an increasingly wide range of benefits. The message could soon be: "Ladies and gentlemen, you have produced it locally, so why not make use of it locally".

As well as supplying major nutrients and organic matter, digested sewage sludge can help improve soil water retention, cut erosion and supply potentially deficient nutrients such as sulphur, copper and boron.

Research on light land at ADAS Gleadthorpe, Notts, shows standard sludge applications can boost available water capacity by 5%, lifting potato and carrot yields by 0.75t/ha and 0.39t/ha, to give £60/ha and £36/ha more profit, respectively.

Further work shows sludge can also meet annual sulphur needs for most crops on most soils. Appropriate use could also alleviate copper deficiency on the 30% of cereals in Scotland and 5% in England and Wales that are vulnerable.

Boron benefits can also be had in vegetables, sugar beet and oilseed rape.

"There is lots of anecdotal evidence we need to look at more closely," added Mr Rowlands.

In five years of Southern Water trials at Horsham, West Sussex, digested sludge brought a marked reduction in take-all and mildew in cereals and avoided the need for pea and bean weevil sprays, for example. &#42

Unease over heavy metal directive

Draft EU legislation setting heavy metal levels for farmland could cause problems for some soil types in the UK. The limits are set arbitrarily and are not based on science, say UK experts. "We know from our own science that there are no adverse effects to soil life or crop yields at the levels we have in the UK," says crop nutrition expert Brian Chambers. But if the proposed limits are imposed they are likely to include wide ranges to accommodate sites where levels are higher due to local soil characteristics, observers believe. Meanwhile, the UKs much-delayed matrix for the safe use of sludge could finally be enshrined in DEFRA legislation later this year. The move would ensure growers get more information from sludge suppliers.