Like most Canadians these days, farmers are looking to get the most bang for their bucks. That’s especially true when it comes to applying phosphorus fertilizers to their fields.
Jeff Schoenau, a professor of soil fertility at the University of Saskatchewan (U of S) and chair of soil nutrient management for the Saskatchewan Ministry of Agriculture, has been studying soil health and management for nearly four decades. He spoke about the influence of phosphorus fertilization practices on crops, soil and water at Saskatchewan Agronomy Research Update 2022, held Dec. 13-14 in person in Saskatoon and online.
One of the messages Schoenau delivered at the conference about phosphorus management is proper placement can be vital to ensuring the phosphorus nutritional needs of plants are being met.
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“When we’re thinking about plant nutrition, we consider how we can satisfy that crop’s nutrient needs efficiently and economically, and also with an eye towards keeping that phosphorus where it should be so that it is getting into the plant rather than losing it somewhere else,” he said.
“How can we get the best bang for our buck? How can we get that phosphorus into the plant and minimize the loss to water?”
Placement is critical
Schoenau said the placement of phosphorus fertilizers is important for the root’s accessibility to applied phosphorus and ultimately plant growth in a field. That’s because phosphorus is essentially immobile in soil and can only move a few millimetres from where it’s placed. It must be located near the roots of young seedlings so the plants can take up that phosphorus early in the growth cycle and be used in the photosynthetic process.
“That’s ideal for early phosphorus nutrition that gives rise to what is called the starter or pop-up effect, which is the increased early season vigour and growth that we see in the spring, especially when the soil is still cold, and the root growth is inhibited. Having some (phosphorus) close to the roots of the seedlings can really give that crop an early boost,” he added.
Too much of a good thing
Although phosphorus can provide a boost to plants, Schoenau cautioned that phosphorus fertilizers, like many other fertilizers, can also cause reduced germination and emergence if the rate applied within the seed rows is too high. The amount of damage depends in part on the type of plant it’s being used with since different crops have different tolerances to seed row-placed phosphorus, he added.
For example, he said research from a few years ago showed a significant reduction in germination and emergence in three different crop types when phosphorus application rates exceeded a certain level.
Cereal crops such as wheat and barley proved quite tolerant and were able to handle loads of as much as 40 or 50 pounds per acre of P2O5 but began to show signs of injury when levels exceeded that range.
Canola proved to be more sensitive with significant reductions in emergence when the amount of phosphorus applied exceeded 25 to 30 pounds per acre of P2O5. Peas and flax were most sensitive, with injury appearing when rates exceeded about 10 to 15 pounds per acre of P2O5.
Split application
While having phosphorus close to the roots early on is important, Schoenau acknowledged that what can be safely placed in the seed row with the seed may not be enough depending on fertilizer recommendations.
To avoid injury and have phosphorus close to the roots for uptake later in the season, growers may want to consider using a split application, he said. This involves applying some phosphorus directly in the seed row and the rest in a separate band. Banding helps reduce fixation in the soil and increases phosphorus availability compared with broadcasting.
“That split application provides some phosphorus close to the roots of the seedling early on and also some phosphorus for roots extending outward later on,” he added.
As part of that split application approach, some growers may be interested in a foliar application of phosphorus. Schoenau said it’s important to understand that leaves have a finite capacity to absorb phosphorus and foliar sprays are best used to “top up” earlier applications.
A recent study conducted by Schoenau and his team at the University of Saskatchewan showed that with a high proportion of the total phosphorus applied as a foliar application in mid-season versus the seed row at seeding, the yield response and phosphorus uptake was reduced.
“There’s only so much phosphorus that you can actually put through the leaves,” he explained.
In collaboration with AAFC researchers, another area of study Schoenau and the team at U of S have been looking into is the effects of seed bed utilization on phosphorus uptake. The effect of row spacing and opener spread on canola recovery of seed row-placed phosphorus was examined under field and controlled environment conditions. Field study results indicated row spacing and opener spread did not appear to have a significant effect on phosphorus fertilizer uptake and recovery in canola in most instances.
“Occasionally, we did see some significant effects where a higher seed bed utilization seemed to be associated with a little bit better recovery of the phosphorus, but, overall, it wasn’t large, it wasn’t huge,” he said.
Manure
U of S researchers also recently looked at how phosphate supply at the surface level of soil can be affected by the method of cattle manure application (broadcast, broadcast versus incorporated, sub-surface banding). Not surprisingly, the broadcast without incorporation application produced the highest phosphate levels at the soil surface. Unfortunately, surface application of manure was shown to increase the amount of phosphate that can interact with runoff water and be exported off of a field.
“It points out the importance of trying to get that manure with its nutrients below the surface into the soil where the roots are,” Schoenau said. “That will give the roots a better opportunity to absorb the phosphorus, and it will also reduce the potential for soluble phosphate to be moved off of that field in snowmelt or rainfall runoff.”
The university has also conducted simulated snowmelt runoffs using intact slabs of soil removed from replicated research plots to study the effects phosphorus fertilizer form, rate and placement can have on the concentrations of phosphate levels in snowmelt runoff.
One of the take-home messages from that study so far is that less soluble phosphorus forms did appear to result in less phosphate concentration in the runoff water, which Schoenau said was what researchers were expecting to see.
Schoenau also spoke about another recent research project the U of S was involved in that compared two different forms of phosphorus fertilizer (monoammonium phosphate and ammonium magnesium phosphate also known as struvite) and their effects on canola.
He said the results of the study conducted under optimum conditions in a growth chamber showed very little difference in the yield response or the phosphorus uptake between the two different fertilizers. In addition, crops that were grown after the canola in the chamber study showed little difference in terms of yield response or phosphorus recovery. Fertilizer response trials with different phosphorus fertilizer forms are continuing in the field.
Schoenau was asked following his presentation about how far from the seed side banded phosphorus can be placed in order for it to have the desired “pop-up” effect. He said he expects that as long as soil isn’t too phosphorus deficient, and the roots of the seedling can develop well, side banding of phosphorus a half of an inch or an inch away from the seed row should also provide early supplies of phosphorus.
He was also asked about deep banding phosphorus, especially during drought years. Recent studies at the U of S have started to look into this. Although at this point it’s difficult to definitively say whether or not deeper banding is effective, Schoenau pointed out that previous studies on the topic have suggested it can encourage root proliferation deeper in the soil and the better moisture at depth can enhance phosphorus movement to the roots and help produce a yield response.
