What should you do about phosphorus rates?

Cindy Grant, who co-authored a 255-page report on basically everything we know about phosphorus fertilizer in Canada, sighs when she acknowledges that with all that research, the key practice boils down to one line: Apply enough fertilizer phosphorus to meet crop removal, preferably as an in-soil band near the seed row.

“In the short term, crops can still yield well if they get just a small starter rate with the seed,” says Grant, who recently retired from a long career studying soil fertility with Agriculture and Agri-Food Canada in Brandon, Man., “but applying those low rates over time is reducing soil fertility and crop yield potential.”

Over the decades, farmers have been able to get away with rates well below crop removal because Western Canadian soils, which are relatively young, have large pools of phosphorus. Plant-available phosphorus in the soil solution accounts for only about 0.1% of soil phosphorus, as noted in “4R Management of Phosphorus Fertilizer in the Northern Great Plains: A Review of the Scientific Literature”, the report Grant co-authored with University of Manitoba professor and soil scientist Don Flaten. As plants take up this available phosphorus, the pools release more into the soil solution to maintain the balance.

The problem is that decades of lower-than-removal rates are reducing the size of the pools and the actual amount of available phosphorus is going down. Another factor is that canola, soybeans and corn are big phosphorus users, so as acres of these crops increase, the draw-down of soil reserves can be that much faster if rates are below the level of crop removal.

“The majority of soils in Western Canada are testing low for phosphorus unless they have a history of manure application,” Grant says. “That is why farmers can expect a response to phosphorus fertilizer in soils with low test results.” (See table 2.)

Warren Ward, agronomy specialist and fertilizer lead for the Canola Council of Canada, says, “The simple practice of setting rates to match crop removal helps to maintain soil health and ensures that available phosphorus is not limiting for yield.”

How much phosphorus does a crop need?

Canola takes up 1.31 to 1.63 pounds of fertilizer phosphorus (P₂O₅) for each bushel, and around one pound per bushel is removed in the seed, on average. A 50 bu./ac. canola crop will therefore remove about 50 lb./ac. of P₂O₅, which, for example, is equivalent to 100 lb./ac. of monoammonium phosphate fertilizer. The rest stays in crop residue and is returned to the field.

This is based on old research done before the introduction of hybrid canola, and many suspect that hybrid canola has improved nutrient use efficiency. But John Heard, soil fertility specialist with Manitoba Agriculture, says that until a new removal ratio is verified, the best management practice is to stick with the current one to one.

Soybeans remove about 0.90 pounds of P₂O₅ per bushel, based on the mid point between the minimum and maximums shown in Table 1. Peas are 0.69 pounds per bushel, wheat is 0.59 and barley is 0.42. Corn is also fairly low, at 0.44 pounds per bushel of yield, but with its high yields, the actual amounts removed with corn are probably the highest of any crop grown in Western Canada. (See Table 1.)

The Effect of Soil pH

Soil pH is a factor in phosphorus availability. “Availability of phosphorus is optimized with pH around 6.5. Above that, calcium and magnesium can tie up phosphorus in the soil. Below that, aluminum and iron can tie up phosphorus,” Grant says. She adds that farmers can expect a bigger benefit from higher phosphorus rates overall and from seed-row placement of starter rates when pH is at the extremes.

To get a proper picture of pH influence on soil test recommendations, make sure to use a test that is suited to the predominant soil pH in the area. Grant says the Olsen test is effective across a wide range of soils, including the high pH calcareous soils common in Manitoba and Saskatchewan, while the Bray test is effective only in neutral to lower pH, non-calcareous soil as are found in parts of Alberta and Saskatchewan. Kelowna and modified Kelowna tests are also considered effective in the pH range on most of the soils in the Northern Great Plains.

New Ideas on Placement

The pop-up benefit of seed placement is particularly common in cold soils that are low in available phosphorus. Cereals can tolerate higher seed-placed rates, which means the removal-matching rate of phosphorus for cereals can be safely seed placed. The same can’t be said for canola, which is much more sensitive to seed-placed fertilizer. The recommended practice for canola is to place no more than 20 lb./ac. of phosphate (40 lb./ac. of MAP, for example) with the seed and put all other fertilizer outside the seed row.

As Grant and Flaten describe, the seed and seedling damage from phosphorus fertilizer is related to damage from the fertilizer salt in the soil solution and to ammonia toxicity from the ammonium applied with the phosphate. “Increasing nitrogen in the fertilizer increases the risk of seedling toxicity,” they write.

Warren Ward knows that some farmers are using much higher seed-placed rates. “We often hear about farmers getting away with high-rates of seed-placed fertilizer,” he says. “Moist soils and high seed-bed utilization can reduce the risk, but thinning is likely happening – it just may not be noticed.”

A research project led by Grant and repeated at various locations in Canada showed that going from 20 lb./ac. to 40 lb./ac. of phosphate (liquid or dry formulations) in the seed row thinned stands by about 20 per cent. Adding 20 lb./ac. of sulphur as ammonium sulphate (dry) or thiosulphate (liquid) took it down by another 20 per cent or so. (These results are highlighted in the article “Product and placement”, which is in the 2013 Science Edition.)

“Canola can still yield well with a 40 per cent drop in stand – as long as seeding rates were high enough to keep plant counts above five per square root – so the real management decision is whether the reduced seed return on investment is offset by improved fertilizer logistics,” Ward says.

New research by Jessica Pratchler and Stewart Brandt with the Northeast Agriculture Research Foundation at Scott, Sask., suggests that one option is to put all the phosphorus fertilizer in a band outside the seedrow. That way farmers can apply the target removal rate without sacrificing seedling emergence. Overall, the researchers found no evidence of better responses associated with seed-placed versus side-band, even at low rates, and that high rates of side-banded phosphate were always equal to or greater than seed-placed. This could be especially effective if soils are warm, which means young canola plants are growing faster and better able to forage for the phosphorus it needs in those early days. (These results are highlighted in the article “Placement outside the seed row is fine for phosphate”, which is in the 2019 Science Edition.)

“We need phosphorus fertilizer rates to keep pace with our yield targets, and this is a good option,” says Ward. “It also helps from an environmental standpoint that the phosphorus is not being broadcast for the amounts over the safe seed-placed rate.”

A final consideration in setting rates that match removal is to take a whole farm approach. Phosphorus is fairly immobile in the soil, binding closely to soil particles. That means it won’t move much over time. “It also means you can band large single applications, say in the fall, in a year when the price is better or with a crop that is less sensitive to high seed-placed rates,” Ward says. This can maintain the overall crop removal balance across the whole rotation while not having to put high rates into the canola seed row.