Biosolids used to tackle soil constraints

Biosolids are created as a byproduct from treated wastewater and distributed from SA Water's Bolivar Wastewater Treatment Plant to primary producers as an organic matter used to improve soil quality.

Demand has risen considerably in recent years, with 90,000 tonnes of the byproduct collected for use by farmers this financial year.

SA Water Production and Treatment senior manager Lisa Hannant said this was a record amount, outstripping the 55,000t average and 86,000t distributed in 2007/08, with more supply collected from regional treatment plants in Port Pirie, Port Augusta, Murray Bridge and the Adelaide Hills.

"As proactive environmental leaders in SA, we're always looking at how we can reduce waste and increase the re-use of by-products from our wastewater processes, to benefit the community," Ms Hannant said.

"Biosolids are an increasingly popular method of improving soil quality by the state's primary producers, and by tapping into our accrued stocks of biosolids we've been able to significantly grow in our ability to meet this increasing demand."

Hart farmer Jim Maitland, who grows cereals, export oaten hay, durum and pulses, started applying biosolids in 2018 with the aim of replacing and building capital phosphorus.

"We have been using granular prosperous replacement based on removal since 2014," he said.

"The next step for us is to grid soil sample the entire farm to establish a reference point. Aside from phosphorous, the grid soil sampling allows us to characterise the soil and create data layers for pH, PBI, CEC and other exchangeable cations such as Potassium, Calcium, Magnesium and Sodium.

"We have been variable rate spreading the biosolids based on DGT-P levels, PBI and CEC. We're also trying to put back other elements and nutrients in an effort to not mine the soil."

Mr Maitland said they spread a small amount in 2018, then had a break from application before getting into it seriously in 2020.

"We've been objectively measuring soils since 2020 and we're starting to see movements in those measurements this year," he said.

"We don't see it as a silver bullet for spiraling fertiliser prices, but in years to come we think it will be a benefit if we can get P levels higher and be less reliant on granular fertilisers."

The amount of biosolids spread on paddocks generally comes down to logistics rather than a set rate.

"In our operation it's about having load capacity to get from one end of the paddock to the other and avoiding having trucks drive through paddocks," Mr Maitland said.

"We have the material tipped at one end and the average rate is based on the length of the paddock and being able to empty the spreader once it goes up and back.

"Paddocks that inherently receive a lower average rate due to the logistics will be spread more frequently."

APPLYING biosolids has become a regular part of the cropping program for Gulnare farmer Ashley Burgess and family.

They have been applying the organic matter to soils for about 15 years, carting to the farm by the end of March and spreading it at 5 tonnes a hectare during the month of April.

They target lighter soil types until supply is exhausted.

Mr Burgess said there have been noticeable difference in yields, especially in early years of application on certain soils.

"Everyone has an enormous range of soil types, including us," he said.

"It's about trying to get some guts back into our lighter soils. In my opinion, biosolids aren't a replacement for fertiliser.

"We still use fertiliser to replace what we've taken out (at harvest time) and then the biosolids still go on top."

Chicken manure was trialled at the same time as biosolids when the Burgess's first applied them, but biosolids won out due to easier spreading.

"Our spreader wasn't effectively spreading chicken manure and leaving lines in paddocks," Mr Burgess said.

"We think it's key to get an even spread, no matter what you're spreading."

Mr Burgess said any growers considering using biosolids would have to strongly consider freight costs, as well as the cost of spreading once the material was on farm.