Digestate can offer useful nutrients for growing crops, and a Suffolk arable farm has invested in equipment to make best use of it and precisely quantify its value. Jane Carley reports.
Upton Suffolk Farms is employing the latest application technology to get more value from digestate.
The 700-hectare arable enterprise near Red Lodge, Suffolk, grows potatoes, onions and sugar beet, plus maize, forage rye and triticale to feed a 5MW digester operated by Bay Farm Power, supplying gas directly to the main.
Land is rented out for carrots and parsnips, and also for free range pigs, which perform an important role to control ‘groundkeepers’ in the potato crop.
Farm manager Nick Sheppard explains: “The AD plant has been operational for four years and we have an agreement to use 36,000cu.m of liquid and 6,000 tonnes of solid digestate annually.
“The farm is mainly on light Breckland soils, so one of our aims in using the digestate is to improve soil organic matter.”
Enhancing soil health is also the goal of the farm’s cover crop programme. As little land as possible is left bare over winter, and the established cover crops are then either grazed by sheep or cultivated.
“The farm is 100 per cent irrigated, so if we can improve the water holding capacity of the soil and potentially reduce irrigation, that is a significant cost saving,” he says.
Solid digestate is applied with hired-in spreaders in January and February on a ‘little and often’ basis, at rates of 20-30t/ha using GPS-equipped tractors to vary application rates according to forward speed, and to get an even a spread across the soil surface as possible.
Liquids have traditionally been spread using an adapted irrigation boom system, working from the farm’s underground network of irrigation pipes and by transporting digestate four to five miles from the AD plant’s lagoon.
“There were a number of issues, including run off due to poor infiltration,” Mr Sheppard explains.
“In the wet spring of 2018, ponding was evident in some fields. We were also losing ammonium nitrate due to volatilisation, and as we want to get the maximum nutrient value from the digestate, we started to look at alternatives.”
The proportion of stone in the Breckland soils means a traditional umbilical system is not suitable due to the potential for pipe damage, but in discussion with UK importer Agri Industry Solutions (AIS), the Veenhuis Rotomax looked to be a strong candidate.
Digestate is moved through the underground irrigation system to a small nurse tank, with an additional pump to ensure the Rotomax has a consistent supply of material.
The Rotomax is mounted on a trailed chassis which is towed by the farm’s Case IH Puma 240.
“The pipe is laid down and picked up by the reel rather than dragging behind the tractor, as with a conventional umbilical system,” he says.
“Digestate is applied via a 12m disc injector mounted on its chassis, which puts it into a slot so the liquid infiltrates into the soil.”
As the pipe is not dragged across the soil there is less damage from the pipe, allowing the use of a Rotomax in growing crops. The reel is capable of carrying up to 700m of 11cm pipe, offering up to 25ha coverage in one set up.
Using a Kverneland Tellus control, Mr Sheppard reports that the Rotomax is straightforward to operate and rates are easy to adjust up to 50cu.m/ha, the maximum that has been applied so far. The display shows application rate and nitrogen levels applied, varying as the digestate fluctuates. When working at a set kg/N/ha, forward speed is varied to maintain the rate.
In autumn 2019, 160ha was treated with the Rotomax with a further 430ha to come in spring 2020.
Yet to try the Rotomax in growing cereal crops, he reports good results injecting into grass, into cover crops post-drilling, and into seedbeds pre- and post-emergence.
“We have seen no detrimental effects on germination or any plant losses, so for the 2019/20
season, we plan to use digestate as a nitrogen source, applying in growing cereal crops from February onwards,” he says.
The tractor is part of a hire agreement, so is probably more powerful than strictly needed, says Mr Sheppard, but he comments that it treads lightly enough.
Extendible axles and central tyre inflation on the Rotomax chassis also minimise compaction.
Irrigation water is extracted from boreholes, so the pipework system has a series of gate valves, allowing sections to be blanked off, allowing irrigation and digestate spreading to take place at the same time.
The Veenhuis Nutri-Flow Near Infra-Red (NIR) nutrient sensing system, which analyses manure composition in real-time as it is applied, has also been specified, and will be used from the spring.
“It will allow us to apply nitrogen within the digestate at a set rate of say, 20kgN/ha and I will experiment to see what rates will deliver the best benefits,” he says.
“Rotomax also has section control to avoid overlaps, so the application is very accurate.
“Application data can then be downloaded via a USB stick to Gatekeeper and integrated with our Soyl yield maps.
“By studying the analysis, we could also apply nutrients variably across the field using the Soyl maps.”
Mr Sheppard suggests that the technology could also be useful for high value crops such as potatoes, to fine tune nutrient applications.
Soils at Upton Suffolk Farms are high in phosphorous, so nitrogen and potassium are the key nutrients, with trace elements also valuable for all crops, especially vegetables.
He points out that with little experience from his arable team on handling organic fertilisers, Veenhuis’s hands-on assistance has been very helpful in getting used to the Rotomax equipment.
“We have seen even application, so if we can manage and improve nutrient supply to the crops this will hopefully increase yields,” he says.
“There is a possibility we may also cut costs by reducing mineral fertiliser applications, but I am tentative about this especially for potatoes and onions.
“If the nitrogen is not available when the crop needs it, it could hamper yields,” he explains.
“We grow predominantly salad potatoes, with a growing period of less than 90 days, so poor nitrogen availability can seriously impact yields.”
He admits the farm is on a bit of a learning curve.
“We applied digestate to grass in August 2019, but because the conditions were so dry there was no immediate uplift, although it looked so much better by the autumn,” he says.
“It is a technique with lots of potential and we look forward to exploiting it further in the future.”
Near Infra-Red (NIR) creates an invisible spectrum of the composition of a material and thus, the NIR-eye in the Nutri-Flow unit translates the manure composition into a spectrum. This spectrum and laboratory references are used to create a calibration set giving a measurement for nitrogen, phosphate, potassium, ammonium and dry matter (N, P, K, NH4-N Ammonium and DM percentage). This applies to slurry or digestate and this data is recorded in real time as the material is applied.
The Nutri-Flow system communicates with tractor or self-propelled applicator IsoBus control systems, and the dose can be based on kg/ha of the selected nutrients using prescription maps uploaded via a USB, giving site specific application of organic fertiliser.
GPS-guided automatic section control is utilised on the applicator in order to prevent overlaps, and further increase the accuracy of application.