Minding Your Phosphorus - PulsePoint
April 16, 2019
Research into nutrient uptake and removal provides valuable information for pulse growers
by Trudy Kelly Forsythe
It is no secret that pulses need nutrients to grow. However, determining the right formula of nutrients required to optimize yield and quality in pulse crops can be a challenge. Research looking at nutrient uptake and response may help with the solution.
Dr. Jeff Schoenau, Professor of Soil Science and the Saskatchewan Ministry of Agriculture Soil Nutrient Management Chair at the University of Saskatchewan, says it is important for growers to know details such as nutrient uptake and removal in harvest, because it can help them develop long-term fertility plans to replace fertilizer nutrients removed when crops are harvested. This, in turn, can help prevent the eventual depletion of nutrients from their soils.
“Growers need information on how much fertilizer they need to add to meet the nutritional needs of the crop they are going to grow this year. Also how much nutrient the crop removes in grain harvest, thinking ahead about fertilizer needs for subsequent crops grown down the road in the rotation,” Schoenau says. “Knowing the potential yield response of the crop to fertilization will enable economics of fertilization to be assessed and the appropriate rate of fertilizer to add to address any nutrient limitations in the short‑term.”
Chris Holzapfel, Research Manager of the Indian Head Agricultural Research Foundation, is involved in research looking at nutrient uptake and nitrogen fixation by faba beans and soybeans in Saskatchewan. This research came about because, while faba beans require high amounts of phosphorus and export large amounts of it in the grain, what the actual nutrient uptake and removal by modern faba bean varieties in prairie soils is unknown.
Holzapfel’s research has shown that although pulse crops are considered somewhat unresponsive to phosphorus fertilization, responses can occur and, in some cases, can be quite substantial. That said, not all pulse crops respond equally.
“Faba beans have potential to be extremely large users of phosphorus and can respond to high rates of fertilizer,” says Holzapfel, further explaining that the work with soybeans illustrated both yield responses that can sometimes occur as well as further insights into how much phosphorus growers should apply with this crop, while taking crop removal and potential yields into consideration.
The effects of seed-placed phosphorus fertilizer on emergence were also of interest as this is the preferred placement method for a large percentage of growers.
“Although responses varied with environment, our results showed reduced stands frequently occurred with high rates of seed-placed phosphorus,” says Holzapfel. “However, growers could probably be reasonably comfortable with higher rates than previously thought. Our
results suggested that faba beans were more tolerant to high rates of seed-placed phosphorus than soybeans.”
He points out, however, that their work with faba beans was not as extensive so is less conclusive.
While the researchers did not directly measure phosphorus removal or uptake in the faba bean trials, Holzapfel believes that phosphorus uptake and removal with faba beans has the potential to be much higher than for other pulses, depending on their yields, according to recent work at the University of Saskatchewan.
Faba beans can remove approximately 1.2 pounds of phosphorus pentoxide per bushel of grain harvested. In the better adapted regions of the Prairies, faba bean yields as high as 100 bushels per acre are possible, therefore it is quite possible for phosphorus removal to exceed 100 pounds per acre with this crop.
“Faba beans also appear to be more responsive to relatively high phosphorus application rates relative to other pulses,” says Holzapfel. “Additionally, our results and some past work suggest that faba beans are less sensitive to seed-placed phosphorus than other pulses. Growers still need to exercise caution and consider various soil and environmental factors if considering seed placing high rates of phosphorus fertilizer.”
Holzapfel says that while nutrient uptake in itself is not vitally important to growers, it can teach several things that are.
“First, by understanding what the total nutrient uptake of crops is, we can estimate the total amounts of the applicable nutrients that we must be able to supply, in order to prevent yields from being limited,” he explains. “The nutrients can be supplied by a combination of both the soil and applied fertilizer.
“Soil phosphorus includes both soluble, plant-available phosphorus, and also any phosphorus mineralized from organic matter during the season,” he adds. “Although yield responses to phosphorus fertilizer tend to be modest, many growers wish to either build or maintain residual phosphorus levels and the overall productivity of their fields.”
To do this, it is usually recommended that growers consider the amount of phosphorus crops typically remove in the grain and then apply fertilizer phosphorus at rates that are sufficient to at least replace what is removed.
“Ultimately, growers aim to maximize profits which means only utilizing inputs which are likely to provide a return on investment,” says Holzapfel. “Although pulse crop responses to applied phosphorus can be inconsistent, it is generally accepted that we need to maintain the soil’s ability to provide nutrients in order to keep land productive over the long term.”