Using low rates of phosphorus fertilizer is good for the environment and the farmer’s bottom line.
But cutting corners too much can affect yields and impact long-term soil fertility, said John Heard, a soil fertility specialist with the crops branch of Manitoba Agriculture, Food and Rural Initiatives.
“There’s really only four rates of phosphorus application: zero, starter rate, crop-removal rate, and buildup,” Heard said in a presentation at Ag Days.
The total amount of naturally occurring phosphorus in the root zone could range from 400 to 2,000 pounds per acre. But about 99 per cent of that is insoluble — combined with calcium, magnesium, iron or aluminum; or not in a form that plants can use. At any one time, only one to two pounds of P per acre are readily available to the crop. Topping it up to ensure good yields is good agronomy.
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“An active crop can use up all the phosphorus that’s available in the soil two times per day,” he said. “So, it’s very important that soil-test reserves of readily available phosphorus meter out at a rate that meets crop demand.”
Replenishment ability is the figure captured in a soil test for P205, the plant-available kind that also comes in trucks or bags from fertilizer dealers.
Manure boosts P
To monitor soil fertility, about 40,000 samples are taken around the province every five years by the International Plant Nutrition Institute. In 2010, 57 per cent of the samples in Manitoba were found to be low or very low.
Similar tests done in 1990 found only a quarter of soils were very low in phosphorus. But after 2005, soils in some areas began testing very high, especially where farmers had access to large quantities of manure.
From eight to 15 parts per million of P205 is the “sweet spot” in terms of the environment and agronomy. At the medium to high test range, a farmer has more flexibility because when fertilizer prices are high, corners can be cut over the short term. When prices soared during the past three to five years, many Manitoba farmers did exactly that. Statistics show that more P was exported in the form of grains than was applied.
But falling behind on P soil reserves puts the farmer at the mercy of fertilizer prices that fluctuate from year to year, because bumping up the soil test by one part per million requires an application of 20 to 40 pounds per acre.
“If you keep the soil down, I guess that means you think that phosphorus prices are going to be cheaper in the future,” said Heard.
“But I’m kind of a ‘pay-as-you-go’ person. You may as well pay with the productivity you have now rather than future productivity.”
Good fertility management is based on four principles: source, rate, time, and place.
“Pop-up” effect
There’s a reason why most new seeding rigs are designed to place fertilizer near the seed in the seed row, with the goal of achieving the “pop-up” effect on seedlings while minimizing fertilizer costs.
“You get the same yield response with about 25 pounds of seed-placed phosphorus as what you get with 50 pounds broadcast and incorporated,” said Heard.
Starter P can supply the “pop” but still fall short of crop removal rates. Using more in the seed row makes seed toxicity a concern, particularly with canola, although less so with wheat.
A rate of 20 pounds per acre of starter P placed in the seed row with canola would only match crop removal rates for a 20-bushel-per-acre crop. Using more to put the amount in line with actual yields would jeopardize seed safety due to the ammonium that is usually included with starter fertilizer types.
“It doesn’t mean that yield stops at 20 pounds of phosphorus, it just means that you’ve got to put it on in some other way,” said Heard.
How to determine actual application rates in the real world? One technique, which works with both liquid and granular types, is to lay down a tarp and drive over it with the seeder running and then look at the result.
Standing still, the liquid fertilizer appears to be pouring out at a steady stream. But at seeding speed of five to eight miles per hour, it should be distributed one drop at a time. At a rate of 15 pounds of P per acre, there should be a space of about 3.5 inches between drops.
“With crops like canola, for every bushel that you remove and take to the Cargill elevator, you are also hauling them back one pound of phosphorus fertilizer that you bought from them six months earlier,” said Heard.
