Soybeans are efficient feeders for soil phosphorus (P) and in most cases growers are not going to see an increase in yield from additional P fertilization, regardless of the soil test P, according to research trials in Manitoba and Saskatchewan.
Since 2013, Dr. Don Flaten and Gustavo Bardella of the University of Manitoba, together with John Heard of Manitoba Agriculture, Food and Rural Development, have been assessing the short-term and long-term effects of P fertility rate and placement on soybean stands, dry matter production and seed yield.
Soybeans remove large amounts of phosphorus. A 40 bushel per acre soybean crop removes 34 pounds of P205 per acre. Current recommendations are that Manitoba growers apply no more than 10 pounds of P205 per acres in the seedrow, to avoid seedling toxicity.
Flaten presented some findings from their trials at Ag Days in Brandon this January. The key message is that even in soils with very low soil test P levels (down to three ppm Olsen P) soybeans can uptake enough P from the soil during the growing season to produce high yield without any additional P fertilization.
“The lack of response in soybeans to P fertilizer is probably related to our high pH soils on the Prairies, and the fact that there are sufficient reserves of certain forms of P in these soils that our soybean crops are able to tap into, that other crops can’t tap into,” says Flaten. “We have evidence from research done 50 years ago that soybeans have a unique capability, much greater than cereals or canola, to make use of soil P reserves in Manitoba soils.”
That’s not to say that there aren’t other circumstances where P fertility might be beneficial. “Within the short term of this experiment, we weren’t able to detect yield responses to any rate or any placement of P in any one of our 28 sites, but we haven’t tested every place in the Prairies,” he says. “There may be some soils and situations where that may not be true.”
For example, in acidic (low pH) soils, soybeans can respond quite substantially to phosphorus fertilizer. Flaten has also done a lot of research demonstrating the pop-up effect of starter P in other crops, and although he didn’t see that effect in the soybean trials, there may be situations, particularly when planting into wet, cool soils, where starter P might be beneficial. “There are probably a couple of factors that explain why we didn’t see any pop-up effect in soybeans in our trials in Manitoba. We planted after the soil temperatures had warmed up to at least 10 C, which is the standard recommendation, and soybeans large seeds have substantial reserves of P in them, and that may be sufficient to get the soybean plants off to a good start,” says Flaten. “If growers wanted to plant soybeans when the soils are below 10 C, under those conditions there might be starter P responses that we weren’t in a position to measure.”
No yield increase in response
Flaten’s team’s first experiment dealt with responses to P fertilizer applied at different rates and placements, while the second experiment looked at the overall effect of P fertility in the soil.
The researchers spring applied monoammonium phosphate (MAP 11-52-0) in the seed row, as a side-band, and broadcasted, at rates of 20, 40 and 80 lbs. P205/acre. The soybean variety used in the trial was Dekalb 24-10RY seeded to give a plant population of 210,000 plants per acre. Researchers replicated the treatments three or four times and assessed plant stands at two, three and four weeks after planting. They collected and measured midseason biomass at the R3 stage, and assessed seed yield and quality.
None of the trial sites had any increase in yields regardless of the P treatment, and even when plant stands were reduced at five sites because a high rate of P fertilizer — 80 lb./ac. — was applied, only two had a drop in yield. “Plant stand density overall didn’t play a prominent role in this study,” says Flaten.
The results indicate that soybeans are a lot less sensitive to seed row placed P applied as MAP than was previously thought, says Flaten, but added seedling damage is still a risk especially in medium to coarse textured, and sandy soils in some years, and when using wider row spacing, which can increase fertilizer concentration in the seed row. “The probability of reduced stands from typical agronomic rates of seed-placed P is small and the risk of reduced seed yields is even smaller,” he says. “Overall, seed row placement of P for soybean is a small risk, with little, if any reward.”
While soybeans may not respond to P fertilizer, because they are so good at tapping into soil P reserves, at some point productivity will suffer because of the lack of P fertility, says Flaten.
“We have historical examples where legume-based production eventually crashes after 10 to 12 years if there is no P fertility added. Even legumes, with a substantial capacity to take up P will eventually mine out those reserves and their yield will suffer because of phosphorus deficiency,” he says. “Other crops in the rotation are probably much more sensitive than soybeans to low P in soils, so in the long-term it’s not going to work for soybeans and in the medium term, it’s going to start hurting other crop yields before it hurts soybean production.”
Balancing P fertilizer applications with crop removal is crucial to prevent an over-accumulation or depletion of P in the soil, and many crops — such as canola, soybean and corn — remove more P in a season than can be safely added as fertilizer. As growers have switched from low P removal crops such as cereals to grow more acres of canola and more recently, soybeans, in several areas of Manitoba, soil test P levels are declining. Compounding the issue is that P fertilizer recommendations haven’t changed since the early 1990s and don’t take into account today’s higher yielding crops.
Flaten says growers should be focusing on maintaining P fertility using rotational fertilization strategies to sustain long-term productivity in their soils, adding that more and more land rental agreements include a P balance requirement.
“There are several options for farmers to consider to maintain P fertility,” says Flaten. “They could continue to add some P with soybeans in the rotation but that may not be the best use of that P fertilizer. It is likely better to apply more P with another crop, like canola or wheat, which makes better use of that fertilizer P. For example, in wheat you can place up to 50 lbs. of phosphate in the seed row with very little risk of seedling toxicity. For both wheat and canola, there is lots of room for high rates of P if you’re side-banding or mid-row banding fertilizer.”
Growers may seriously want to consider using a livestock manure application, if that’s available to them, as an alternate strategy, adds Flaten. “If they apply enough manure to meet the nitrogen requirements of the upcoming crop, they are putting on several years’ worth of P at the same time. Livestock manure is a fantastic resource for maintaining your P fertility in these sorts of cropping systems.”
The Manitoba Pulse & Soybean Growers website has an Interactive Phosphorus Fertilizer Calculator spreadsheet and a factsheet entitled “Phosphorus Recommendation Strategies for Manitoba” to help growers determine P removal rates of different crops and manage P fertility to achieve a maintenance range over the rotation.