FARMERS are only seeing the 'tip of the iceberg' as far as Scotland's supply of digestate is concerned, with the growing popularity of anaerobic digestion making it likely that the material will be a major feature of the fertiliser market for years to come.

However, despite the burgeoning tonnage of digestate pouring out of the nation's AD plants, in particular the industrial scale operations tied to some major distilleries, there is an 'immaturity' to both the supply chain and to farmers' understanding of it – and a pressing need for better information.

Bill Crooks, soil scientist and senior consultant at SAC Consulting, predicted that most of the organic materials and wastes that farmers were accustomed to using as feed and fertiliser would increasingly pass through an AD process first, as businesses take advantage of the available renewable energy incentives.

But part of farmers learning to live with that new reality had, advised Mr Crooks, to be a fairly urgent change of attitude to digestate, its value, and the mechanics of its use.

"The first message, and the most important one, farmers must be very clear with digestate suppliers in asking specifically what is going to turn up at the farmgate," said Mr Crooks. "There is great variation in what is coming under that heading of 'digestate'. Farmers need to be asking detailed questions and suppliers, I think, need to be ready to answer them. After all, they need farmers to keep coming back. There is no long term worth in being vague about what farmers are getting."

"Upon request, the receiver of these materials should get a print out of the nutrient analysis, ideally with a comparison with other nutrient sources," he said. "Don't accept the line about digestate being a 'great soil improver' that will 'heal the land'. It's value to farmers is as a fertiliser that displaces bagged fertiliser, not as an additional input.

"As such, it needs compared like-for-like with bagged fertiliser, with proper P and K values, so they can break it down into pounds and pence, and take into account spreading and storage costs. It shouldn't be used without being part of a fertiliser plan based on proper soil testing."

Mr Crooks added that pre-delivery questions could be as simple as "is it a liquid or a solid", as some suppliers are separating out AD liquor from solids, while some digester feed stuffs are producing a liquid with enough high dry matter content to make for problematic spreading and storage.

"As farmers know in this climate, slurry storage space on farm is a valuable asset, and that asset has a cost. Nevermind that liquid digestate arrives on the farm for free, you have to be sure that its value to your business as a fertiliser is worth what it is going to cost you to store and spread."