Almost all soil phosphorus originates from the parent material, but much of this phosphorus remains unavailable to plants, so for a crop to have a sufficient supply of plant available phosphorus, some additional phosphorus needs to be put down.
There are five main types of commercial phosphorus fertilizers: monoammonium phosphate (MAP), diammonium phosphate (DAP), triple super phosphate, orthophosphate and polyphosphate.
Of these, MAP, orthophosphate, and polyphosphate are used in Western Canada, with the most widespread fertilizer type being MAP, a granular fertilizer which includes blends such as 11-52-0.
DAP tends to be slightly more toxic to seedlings than MAP, and triple super phosphate, though popular in Europe and Asia, is too expensive to distribute in the Prairie provinces when most of it is produced overseas.
According to Dr. Jeff Schoenau, a professor in the Department of Soil Science at the University of Saskatchewan and Saskatchewan Agriculture chair in soil nutrient management, “The main thing to keep in mind with phosphorus, regardless of form, is that it’s relatively immobile in the soil; it won’t move very far from where it’s placed to another location in the soil.”
The key, then, is to place the phosphorus close to the plant roots so that it will be available to the plants early in the growth cycle and to do so without exceeding the recommended rates of phosphorus. Crops sensitive to high fertilizer rates, such as flax or pulses like peas, cannot tolerate the high salt content as well as cereals, so placement of the fertilizer is important.
Because of its limited mobility in the soil, broadcast phosphorus is not very effective unless incorporated and is considered inferior to other placement methods at lower rates (typically 10 to 30 lbs. of phosphate per acre). If broadcasting, the rate should be increased to at least 50 to 60 lbs. phosphate/acre.
Most often, phosphorus is seed placed or side-banded (often one inch below and one inch to the side) or both, with the distance from the seed in side-banding meaning that more phosphorus can be put down at once than with seed placed phosphorus. Other placement methods — below the seed (typically about one inch) and deep banding (four to 12 inches) — are uncommon in Canada. Dr. Rigas Karamanos, manager of agronomics solutions with Viterra, believes that “the most efficient way is to apply [phosphorus] in a band.”
A large application of phosphorus, typically broadcasting between 100 to 150 lbs. phosphate/acre, has been used in the past to take advantage of the residual phosphorus effect and to build up the phosphorus reserves in the soil.
In the 1970s and ’80s, some research showed that such applications could improve yields substantially. Today, however, as pointed out by Dr. Don Flaten, a professor in the Department of Soil Science at the University of Manitoba, the land tenure situation is different, since many farmers are working on rented land, and “the economics are a lot less favourable than years ago.”
Furthermore, when a large amount of phosphorus is put down in the soil, the mycorrhizae are killed. Because these soil microorganisms will beneficially infect plant roots and increase their ability to take up certain nutrients, such as the micronutrient zinc, new deficiencies can be caused. Therefore, if a farmer seeks to improve the soil fertility on his land, perhaps even just on eroded knolls, it’s better to look for manure or a similar low cost, useful amendment which will increase organic matter as well as phosphorus.
Ultimately, the type of phosphorus fertilizer applied does not matter, as it will convert to the plant available form of orthophosphate by the time the seed has germinated and the plant can take up nutrients. Karamanos has found that after four to eight days, “by the time the plant starts utilizing [phosphorus], [the products] are all the same.” This phosphorus will not all remain available, with some of it adsorbing to soil particles, but it is likely retained in the soil as residual phosphorus for the coming years.
According to Schoenau, some say that phosphorus fertilizer “is inefficient because in the year of application, [we] might only get 20 per cent of it recovered by the crop,” but “in the longer term, phosphorus fertilizer and recovery can be quite efficient because it’s not readily lost from the system.”
Consequently, there are few differences between liquid and granular or non-aqueous fertilizers.
Granular versus liquid
While the forms of phosphorus differ initially, with granular fertilizers often being orthophosphate and the liquid fertilizers ammonium polyphosphate, the polyphosphate will rapidly convert to orthophosphate.
Liquid fertilizers may be easier to handle, and separation of fertilizer from seed can be readily achieved to avoid damage. Like granular fertilizers, however, spacing remains an issue.
Karamanos says, “People think that when you apply liquid, it’s a constant stream, and yet, it’s not.” Rather than a steady stream of liquid, the fertilizer hits the ground in individual droplets, so the supply would depend on the rate of application, the kind of dribble system used to apply the fertilizer, and the specific equipment figuration therein. The higher the rates applied, the smaller the spaces between the droplets. At lower rates, such as 15 lbs./acre, studies have found that the distance between the fertilizer droplets is greater than the distance between granular fertilizer particles.
For some farmers, it makes sense to apply manure to meet the necessary phosphorus requirements and to use fertilizer to meet the plant’s remaining nutrient requirements. However, it can be difficult to evenly distribute the manure, particularly since it must be applied in tonnes per acre to meet a crop’s phosphorus requirements.
Schoenau says, “Solid manure still might only be a half to one per cent phosphorus by weight, so it still means that you have to have a lot of it in terms of volume of manure.” As such, manure is usually applied in excess of what crops require, which can lead to increased phosphorus loss and has prompted some places, like the province of Manitoba, to instigate soil-based guidelines for the application of phosphorus.
There are no major differences between solid or liquid manure, so the product of choice would depend on the equipment and supply available. For example, solid manure requires the use of a manure spreader and liquid manure that of an injection system. In both cases, “it’s good to get [the manure] into the ground, [to] get it close to the roots where the roots can take the nutrient up,” says Schoenau. However, since some of the phosphorus in manure is in the organic form and must be broken down and mineralized by microorganisms, not all manure phosphorus is available in the year of application. Compared to a commercial fertilizer source, there is only about 50 per cent phosphorus availability.
If possible, it is best to do a soil test and apply phosphorus based on those recommendations, especially if the land management history is not known.
“Recognizing… what your cropping system is taking out of the system, out of the soil, over the long term is something useful to keep in mind,” Schoenau says. “Economics are important to consider.”
With crops usually removing more phosphorus from the soil than is being applied, a soil test can help farmers determine the probability of a yield response to additional phosphorus fertilizer and whether or not they are currently applying at rates that will deplete, maintain, or build up soil phosphorus.
“Once a soil has an established history of phosphorus application, the method [becomes] a bit more irrelevant,” observes Karamanos. “What determines the method to a great extent is the amount of phosphate you can place safely with the seed.” †