Biochar research finds limited yield benefit on Prairies

Research has found that adding biochar to prairie soil has little to no effect on soil conditions, plant nutrition or crop growth, says a University of Saskatchewan soil scientist.

However, farmers may realize other benefits from the product.

Dr. Jeff Schoenau told the recent Agricultural Producers Association of Saskatchewan annual meeting that biochar has long been recognized as an amendment to highly weathered soil, especially in the tropics, because of its ability to prevent nutrient loss.

Biochar can be made from a variety of materials, including straw, woody plants, manure and animal processing byproducts. It is made through a thermochemical or combustion process, using either a slow method called pyrolysis or the faster method of gasification.

Schoenau said pyrolysis results in a chunkier product similar to charcoal, while the faster method results in ash.

“It’s a rather stable material. It’s inert,” he said. “The one thing biochar has is a high surface area that gives it a very high adsorptive ability.”

That capability has made it a popular additive to tropical and acidic soils that don’t retain nutrients well.

Prairie soils already have good retention, Schoenau said, but about seven years ago researchers started to examine what effect it might have.

The first studies used willow biochar from the Saskatchewan Research Council and oat hull biochar from Titan Clean Energy at Craik, Sask., in a controlled growth chamber environment.

The biochar was used alone and with nitrogen and phosphorous fertilizers. The fertilizers alone increased the yield, as expected.

“Adding the biochar directly really didn’t have any effect on the yield of the canola,” Schoenau said.

The study moved out to the field and was done on a brown soil. Biochar was applied at one tonne per acre. He said there was a slight increase in the soil pH and the organic carbon concentration, a sort of carbon sequestration, but there were no effects on salinity.

“One of the interesting things that we are looking at now is adding biochars to saline soils and maybe they might perhaps provide a little bit better environment for germination of some salt tolerant grasses and crops.”

There was some evidence of sorption of available nitrogen in the soil, he said.

Another field trial using biochar from wheat straw and flax straw, in combination with two different rates of nitrogen and phosphorous fertilizer, on both brown and black soils, looked at yield effects on canola the first year and wheat the next year.

The biochar was applied at zero, one-half and one tonne per acre.

There were no effects on the brown soil sites.

“We did see, but only with wheat straw biochar, a significant yield benefit” at the moister black soil site south of Prince Albert, he said.

“This kind of characterized a lot of our work that followed. Sometimes we would see a benefit, sometimes we wouldn’t.”

He and students also looked at combining liquid hog manure and cattle manure with biochar to examine the effect on barley nitrogen uptake. The only trial that produced a yield increase was the hog manure on its own.

“When we combined that liquid hog manure with the biochar, interestingly, there was a trend for the nitrogen uptake to actually decline in that case.”

He said that is likely because the biochar was holding on tightly to the readily available nitrogen in the hog manure.

Schoenau said the results of the work are probably affected by rates of biochar application.

In most cases, the Saskatchewan applications were less than four tonnes per acre, compared to the 40 tonnes others apply.

The soils were already good to begin with, and the semi-arid to sub-humid climate means potential for losses aren’t as great.

However, the work yielded some other interesting observations.

“We did see the biochar amendment significantly reduce nitrous oxide emissions when combined with urea fertilizer,” he said.

Given that nitrous oxide is a greenhouse gas, there could be implications for sequestration.

Another impact is the effect biochar could have on soil-applied herbicides. One study using biochar and sulfentrazone, known as Authority, found that some biochar concentrations decreased the toxicity of the herbicide.

“What the char is doing is it’s binding with the herbicide and reducing or inhibiting its activity.”

This could have practical applications where soil residue is a problem. He also said biochar could present an opportunity for farmers to recycle waste organic material as soil amendments.

“I’m particularly excited about flax straw,” he said. “I really hate to see it burned. There may be some particular application for taking that and producing it into this adsorptive material that would then have a potential market.”

That might include contamination due to oil and heavy metals.

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