Indian Head, Sask. – High yielding pulse crops can be grown without applying phosphorus fertilizer.
Jeff Schoenau drew that conclusion before producers attending the Indian Head Agricultural Research Foundation’s zero till field day.
Schoenau, a soil scientist at the University of Saskatchewan, said it’s due to a combination of three factors: no-till for a good number of years; previous use of phosphorus at recommended rates; a variety of different crops in the rotation.
“Soil erosion removes a lot of nutrients, including phosphorus, from your field. No-till eliminates erosion, so the phosphorus stays in the soil. With multiple crops in the rotation, the microbes do a better job of recycling the phosphorus in the soil,” he said.
Read Also
VIDEO: Green Lightning and Nytro Ag win sustainability innovation award
Nytro Ag Corp and Green Lightning recieved an innovation award at Ag in Motion 2025 for the Green Lightning Nitrogen Machine, which converts atmospheric nitrogen into a plant-usable form.
“The use of phosphorus fertilizer at recommended rates over a number of years has built residual supplies in the soil. Even with the best placement of phosphorus money can buy, we only get about 20 percent recovery through the crop in the year of placement. The third thing is that P is immobile.”
He said phosphorus tends to become fixed in the soils and if it’s not close to a plant root, it can’t be taken up. The low recovery rate contributes to high residual phosphorus.
“Our pulse crops such as peas, lentils and chickpeas have a very good ability to access some of this residual phosphorus. They accomplish this because they have the ability to acidify their rhizosphere.”
Schoenau admitted there are a number of mysteries when it comes to figuring out what’s really happening in long-term no-till fields.
“In these fields that have been no-till for longer periods, there are other microbial processes that we don’t understand so well. I think one of these is the increase in mycorrhizal fungi,” said Schoenau.
“These are the fungi that infect the roots of most crop species, and they are favoured by continuous cropping. In the soil, these mycorrhizal fungi act to extend the root system of the crop. They actually physically increase the root area – the absorbing surface of the root. This has all kinds of positive implications for the crop, because it gives the roots better access to everything, including the immobile nutrients like phosphorus.”
The soil scientist told the tour that the list of factors not understood seems to be growing faster than the list of factors that have been mastered.
“Here’s another thing we don’t understand very well. As you build up organic matter in these long-term no-till soils, I have a hunch that we also see greater phosphorus mineralization.”
But is there enough phosphorus in the soil by nature to sustain more than one year of crop production?
“I would say that in pastures and long-term no-till, phosphorus recycling through the organic fraction can be an important supply of available P. However, we don’t know as much about that as we do the inorganic phosphorus,” said Schoenau.
“It’s an important question that needs more research. It’s my intention to do more research on the role of organic P in the supply of phosphorus in no-till systems.”
Producers and researchers talk about mature no-till fields. However, Schoenau said because no-till is relatively new, nobody knows what constitutes a mature field.
“We’re looking at soils now that have been in no-till for 25 years and longer, and they’re still changing, still evolving. We still can’t say if these fields have reached any sort of equilibrium. Probably not. They’re certainly moving toward some sort of equilibrium, but we can’t call them mature no-till fields.
“We know now that it can be hard to see these important kinds of changes in the soil in a five-year or even a 10-year no-till field.
“But when we analyze a 20 year or 25 year no-till field and compare it to conventional management, then we see major improvements.”
All of which leads to the theoretical question of whether a field can become self-sufficient in phosphorus or any other nutrient. Schoenau said no-till can improve the recycling of nutrients, but there is no free ride and there is no perpetual nutrient machine.
“If you take a crop off the field, you are removing nutrients. Invariably you must replace what you remove, all the nutrients not just phosphorus.
“Nitrogen is an example. If you put legumes in your rotation, N can be fixed from the atmosphere.
“It can be an important contribution to your nitrogen budget. But N fixation cannot replace nitrogen fertilizer in your overall rotation.”
The long-term no-till fields that Schoenau has been studying for the past 15 years are located on Jim Halford’s farm south of Indian Head. The tour included those fields this year.
Pulse crops without phosphorus are nothing new to Halford. He has grown green field peas for three years without applying phosphorus fertilizer.
He said the cost saving per year is between $9 and $10 an acre.
“So far we’ve had no problem without the phosphorus,” said Halford. “We’ve been averaging 35 to 50 bushels per acre. That’s over one dry year and two wet years. If we ever had a normal year, those numbers may be higher.”
Halford agreed with Schoenau on the importance of soil microbes.
He said if the soil goes undisturbed, then it’s logical that the rhizobia also go undisturbed.
“Every time you cultivate, you disturb them and they have to start all over again to rebuild their house.
“If you don’t bother the rhizobia, if you let them get on with their lives they build a healthy community. And the healthier they are, the more productive they are when it comes to providing nutrients to your crop.
“In our test strips here, we’re getting no response from applying phosphate fertilizer to pulses. I think it’s gradually becoming accepted that you can grow pulses in no-till without adding phosphorus.”
