Mixing green waste compost with biosolids could help cut artificial fertiliser inputs, improve soil structure and use two waste products, Velcourt says.
“Compost has a lot of advantages for improving soil organic matter, water-holding capacity and workability, but its major shortfall is in its nutritional value,” says technical director Keith Norman. “It is only about 1.2% nitrogen, so you need a lot of tonnage to get any useful contribution. Application within NVZs is limited to only 30t/ha.”
But mixing it with biosolids (ie sewage sludge) provides a much bigger and more immediately available “hit” of nutrients, as well as getting the longer-term soil structure benefits from the compost, he says. “With compost, a lot of its nutritional benefit doesn’t come through until year two, so the two work nicely together.”
His colleague Paul Cartwright agrees. “Sludge is often regarded as a slow-release fertiliser, but actually, once it hits the soil and gets wet, it does give an immediate release.” Biosolids also provide good levels of phosphate as well as nitrogen, he says [see table]. “If your soil’s at an extreme – sandy or heavy clay – you’re likely to see some benefit.”
Growers wanting to use either compost or biosolids must consider end-user requirements first, he warns. “In a nutshell, the safe sludge matrix says that you can’t use biosolids on anything that can be eaten raw. But, growers’ protocols may have additional restrictions and there may be other crops in the rotation to consider. If you’re growing vining peas, then harvest intervals of at least 30 months may apply if you use sludge in the rotation.
“There are fewer issues with compost, but you still need to know the end-user requirements,” he adds.
The recent introduction of a quality protocol for compost is a welcome boost to the use of compost on-farm, but Mr Norman says distance to collection sites will still prohibit many growers from using it. “The cost of haulage really kills it if you have to transport compost more than 20 miles, but there are local opportunities and there will be more local composting units in the future.”
In comparison, sewage sludge is generally easier to source, as water companies are keen to find ways of disposing of it, he notes. “There’s already more demand than supply nationally, but it does vary locally,” says Mr Cartwright. “There is a danger water companies could start charging farmers, but the price would have to be competitive, otherwise people would stop using it.”
While compost is typically applied using a standard manure spreader, companies such as Nutribio take on responsibility for soil testing, justifying the rate and applying sewage sludge, so it could be tricky if growers want to mix the two on a commercial scale, he notes.