Foliar applied phosphorous: a “top-up” for hungry crops

By Janna Moats

Using soil-applied phosphorous fertilizers is a balancing act for Saskatchewan growers. Apply too much and there is a risk of causing damage to the seeds. Apply too little and yields could suffer.

But what if phosphorous wasn’t applied to the soil at all? Could foliar application methods help growers overcome issues of soil-bound nutrients?

Dr. Jeff Schoenau, a professor of soil fertility at the University of Saskatchewan, and graduate student Stephen Froese evaluated the possibility of using foliar applied phosphorous fertilizers on Saskatchewan grown crops.

Based on their findings, Dr. Schoenau said, “mid-season foliar application of phosphorous would be most suitable for a top-up, rather than as a substitution for seed-row applied fertilizers.”

Phosphorous is an essential nutrient that is required for all stages of plant development. Unfortunately, it is characteristically low in western Canadian soils, making fertilizer application an important aspect of crop management.

The mineral is highly reactive within prairie soils. “There’s a lot of opportunity for the phosphorous to get tied-up and become immobile in Saskatchewan soils,” said Froese. “There’s a lot of calcium carbonates that bind to it and make it unavailable for the plants to take-up.”

Studies from other areas of the world have seen benefits to using foliar applied phosphorous fertilizers, but the method hasn’t been tested under Saskatchewan’s unique growing conditions until now.

With support from Sask Wheat and other funding partners, Dr. Schoenau and Froese designed a scientific gauntlet to test foliar application methods. Through a series of trials, the team measured wheat’s response to different combinations of soil-applied monoammonium phosphate (MAP) and foliar-applied monopotassium phosphate fertilizers.

“Everything got the same amount of phosphorous,” explained Froese, which amounted to 20 kg of phosphate per hectare. “We just changed the proportions between foliar or seed-row fertilizer,” he added.

For foliar fertilizers to work, the plant needs to be able to absorb the nutrients through its tissues. According to Froese, “plants aren’t as eager to take up nutrients through the leaves as they are through the roots. They have a lot of defence mechanisms that can prevent uptake of nutrients through the foliage.”

But as it turns out, results from their chamber study showed that wheat could absorb the foliar applied phosphorous through the leaves, albeit in small amounts.

“We found there was a limit to how much phosphorous you can get through the leaves,” said Dr. Schoenau.

With a better idea of how the plants respond to foliar applied phosphorous under controlled conditions, Dr. Schoenau and Froese took their research out to the fields.

“We wanted to see how it would work in the real world,” said Froese.

The foliar-applied method did show some benefits in cases where the soils were especially deficient in the phosphorous. But the benefits were only seen when some level of soil-applied MAP was also available to the plant.

According to Dr. Schoenau, these results emphasize the importance of early crop nutrition.

“Having that phosphorous available to the plant in the seed row, when the roots can access it at an early stage, is very important for the nutrition of the plant,” explained Dr. Schoenau. “So, as a top-up is really where foliar phosphorous would seem to have a benefit.”

Overall, they found that crops perform best when phosphorous is available in the seed row during the early stages of development. Full details of their research are available in the Canadian Journal of Plant Science.

There are still other areas to study to improve industry’s understanding of foliar applied phosphorous.

“Timing is always important for any application of a nutrient,” said Dr. Schoenau. “It would be interesting to look at different application times of foliar phosphorous.”

Dr. Schoenau hopes to continue serving Saskatchewan growers through industry supported research initiatives, like this one.

“It’s great that we can get support from producer groups,” said Dr. Schoenau. “Research like this is all part of understanding what the best management practices are for getting the best agronomic and environmental benefits.”