Where has all the phosphorus gone?

Phosphorus levels in Saskatchewan soil have taken a shellacking by the big yields made possible by contemporary crop varieties.

“We have been mining this nutrient out of our soils ever since we started breaking the land; we are continuing to do it today,” Stewart Brandt from the Northeast Agricultural Research Foundation said during his Agri-ARM presentation during Crop Week in Saskatoon.

“It was probably appropriate to do it initially, but in a lot of fields right now, that is no longer the most appropriate strategy to be using.”

Eighty-one percent of the soil samples sent to testing labs in Saskatchewan in 2015 tested below the critical level of 15 parts per million, according to a survey by the International Plant Nutrition Institute.

The median soil test in Saskatchewan was 14 parts per million, 21 in Alberta, 19 in Manitoba and 35 in Ontario, Brandt said.

“If the soil test is less than 15 parts per million, the critical level, we should be adding more than we remove in order to elevate it up to that approximately 15 parts,” he said.

“Thereafter we just need to offset removal with annual applications of fertilizer phosphate.”

Brandt said the high levels in Ontario are a cause for concern because losses into the environment increase when high levels of fertilizer phosphate are available.

“They’ve (Ontario farmers) actually reduced their soil test levels over the past five years by about five parts per million, whereas Saskatchewan has been maintained over time,” he said.

Brandt examined changes in yield potential of crops grown in the province by creating estimates based on variety registrations and the comparison of current varieties to those commonly grown in 1995.

“If you look at canola, the yield improvement has been in the order of about 90 percent, field pea up to 60 percent, wheat it’s about 33,” he said. “If you look at some of the other crops we grow in the province, I suspect you would see similar progress.”

The provincial yield increases achieved during the same time did not keep up to the increased yield potential of crop varieties, and poor phosphorus management is likely part of the reason.

However, newer varieties have helped increase bushel per acre yield averages in the province, which has further challenged phosphorus levels because application rates of the nutrient have remained stable.

“If you look at fert trends in Saskatchewan, over time we have increased our nitrogen application rates, but we certainly haven’t done the same for phosphate application rates in the province,” he said.

“In fact, from the mid ’90s to the present time, it has really remained quite stable.”

Brandt said prairie soil contains hundreds of pounds of phosphorus per acre, but very little of it is made plant available through natural processes.

“I think the most sobering thought is, yes, this nutrient does get converted from these highly unavailable forms into these available forms, but typically that process would only support a wheat yield of about three bu. per acre,” he said.

“Even less for the high P use crops like the oilseeds and pulses.”

Corn and soybean, which are increasingly grown in the province, have higher critical minimum levels of phosphorus at 20 parts per million, which may be further impetus for Saskatchewan growers to bring up their phosphorus levels.

Brandt said long-term studies at Swift Current, Sask., found that increasing depleted phosphorus levels increased phosphorus use efficiency, which indicates it’s more efficient to maintain phosphate than to attempt to replace it after it’s been depleted.

“As we deplete this nutrient from our soils, the response that we can expect from fertilizer phosphate is going to go down, “ he said.

“And the reason is that the crops aren’t the only things in the soil that want phosphate, and as you deplete it you increase that competition. I’ve seen the same thing with nitrogen on N-depleted soils as well.”

Placing phosphate with the seed is the most efficient way for growers to meet the current crop’s phosphate requirements, but the amount of phosphorus that can be placed with the seed is limited.

As a result, growers may also need to look at side banding or mid row banding.

“(They should use) a band placement because that isolates that fertilizer material in a very small area, and it’s less prone to being fixed by the soil itself,” he said.

“But when you’re talking about building soil phosphate levels, I suspect that broadcast is maybe every bit as efficient as banded.”

Brandt said there is a greater chance of losses to the environment when phosphorus is broadcasted without incorporation, compared to when it’s banded in.

Some growers choose to do a one-time correction to elevate levels above critical amounts when phosphorus prices are lower.

Other growers choose to in-crease phosphorus levels by banding in more than replacement values while seeding over a number of years.

Growers can also use manure to increase phosphorus levels in areas where it’s depleted.

“If you take the philosophy of buying and building P levels when prices are low, then you probably are in the situation that you can afford to cut back your annual application rates when phosphate prices are relatively high,” he said.

Brandt said it works well to vary application rates across fields because phosphate varies considerably across the landscape.

“We need to take the philosophy to build only on fields, and on parts of fields, where we need to do phosphorus building,’” he said.

“For some of those depression areas, we can probably continue to mine phosphate out of those, as portions of fields for a long period of time into the future.”

Some products, often foliar ones, claim to be more efficient and will help compensate for phosphate deficient soils.

He said these applications might work as a recovery measure in an emergency, but they can be counter-productive in the long-term because the strategy does not correct low soil phosphate levels.

“If you are removing a pound of phosphate, the only way to balance that equation is by applying a pounds,” he said. “Anything that suggest that you can apply less material than you are removing, if it works, it just makes you more efficient at mining the soil. It’s a stop gap measure.”

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