Work to strengthen the UK’s ability to manage soil more sustainably is starting to bear fruit. Andrew Blake reports
That adding organic matter (OM) increases soil fertility has long been appreciated. But determining the best sources of that material, how much is needed on specific soils and how it should be applied to maximise crop yields is the thrust of ongoing research, part-funded by AHDB.
Adding OM not only supplies plant nutrients – it helps improve soil structure, explains Andy Whitmore, of Rothamsted Research, who is leading the work.
“It does this, partly we think, by feeding earthworms, fungi and other organisms to create a better environment for crop roots to explore the soil for water and nutrients.”
Practical spin-offs include lower fuel consumption and longer working windows, especially in wet seasons, he adds.
The four-year research project*, begun in 2012, has triggered further work involving other funding sources, for example from Sustainable Agriculture Research and Innovation Club (SARIC), to see how long the benefits of added OM last once applications stop.
“One of the things we’ve found is that a little organic matter can go a long way,” says Dr Whitmore.
This has been highlighted by a draft sensor fitted between tractor and plough working plots receiving various types and rates of added OM in long-term experiments on Rothamsted’s Broadbalk field.
“As we expected, the plots which had a lot of organic matter had lower draft than the plots which had had none in their history,” says Dr Whitmore. “But it was interesting the plots which had had only a little bit, from crop residues and things like that, also had quite low draft forces – almost as low as those on plots with very high rates of added organic matter.”
The Broadbalk experiment, which started in 1843, amends soil with quite large amounts of farmyard manure (FYM), he adds.
The AHDB series of experiments on another Rothamsted field, Fosters, have tested a wider range of materials – anaerobic digestate (AD), compost, and crop residues, as well as mixtures of FYM, AD and compost with residues.
Application rates ranged from about five tonnes/hectare to levels more like those added annually on Broadbalk, namely 35t/ha.
“In the first year there was no statistical difference between the experimental plots which were amended and the ones which were not, but in subsequent years adding OM increased yields by 0.5-1.5t/ ha.
“FYM consistently increases yields the most, but compost and AD are almost as effective.”
Feeding into the project is another field experiment funded by Defra examining reduced tillage, plus pot experiments, on different soils. A network of eastern counties growers is also exploring various OM amendments in practice.
“Yields appear to increase by more than might be expected from the extra nutrition supplied when adding OM to soils. We’re trying to quantify these benefits and find out how the OM benefits crops.
“We’ve tried to answer the question: ‘How do organic amendments increase yields?’ to better advise farmers how best to use them.
“This hasn’t proved as straightforward as we expected.
“From all results it appears benefits are greatest with spring-sown crops and often in years when the rooting environment is challenging – either very wet or very dry. But effort to deduce more than this has proved inconclusive.
“Amended soils are no more stable. They contain more – but not significantly more – earthworms, and water filters in more quickly though not significantly so; and contrary to what we had expected penetrometer resistance was no less.”
Dr Whitmore is surprised by the results and hopes X-ray style CT scanning may help to explain them.
“It’s also surprising emissions of nitrous oxide were not greater from the amended plots.
“We’ll look at nitrate emissions this autumn as part of the SARIC work. If these too aren’t excessive it will imply that farmers can derive benefits from OM without risking additional pollution from the added N – at least in the short-term.”
Because transport costs differ from farm to farm, one aim of the AHDB project is to find the minimum cost at which maximum benefits from adding OM can be achieved.
“We will be making an economic analysis to see what the financial benefits are, but haven’t done this yet,” he says.
Guy Thallon, head of research, development and innovation at Produce World Group, says: “This project has changed the way we look at and understand organic matter management in agricultural production systems.
“For decades the focus has been on increasing or maintaining the amount of organic matter in soil rather than on its role as the fundamental food-stock for the hugely complex microbiological community which makes soil alive and healthy.”
The differences between organic amendments are particularly important, he explains.
“Compost and FYM are rich mixtures of energy-packed organic material, bacteria, fungi and nutrients, whereas anaerobic digestate has been stripped of its energy by extensive processing, leaving few calories as a food source.
“It’s akin, in human nutrition, to having porridge for breakfast or a slice of white toast.
“This project shows we need to shift our mindset away from focusing on merely increasing the level of soil organic matter, a Herculean task which can take 10-20 years and extensive monitoring.
“It prompts an organic amendments management plan which ensures fresh organic material is returned to the soil at every opportunity within the rotation through a rich and varied diet of straw, compost, manure and cover crops.”
Integrating this approach with the broader farm strategy is important, Mr Thallon believes.
“The increasing use of cover crops has made this simpler and moves away from the dilemma of having livestock on one side of the country and arable on the other.
“On-farm or local sources of organic material are the most cost-effective and should provide the highest confidence in quality and provenance.”