The Great Phosphorus Debate: Much Ado About Nothing

With soils expected to be cold and wet this spring over much of the Prairies, phosphorus availability to the growing seedlings is going to be critical. If you share this concern, the next question is what product to use, at what rate and what’s the best way to apply it? By first understanding a few principles, we’ll try answer these questions.

WHAT PRODUCT?

The most common form of granular phosphorus fertilizer used on the Prairies is monoammonium phosphate (MAP, 11-52- 0), which provides P in the form of orthophosphate. The most common liquid form is a mixture of ortho-and ammonium polyphosphate (APP, 10-34-0), which provides both polyphosphate and orthophosphate. Plant roots take up phosphorus from the soil solution in the ortho form. However, in the soil the poly form converts rapidly to the orthophosphate form with the conversion rate greater than the plant uptake rate.

Under field conditions in Western Canada, there is normally no difference in effectiveness between P supplied as orthophosphate (11- 52-0) or polyphosphate (10-34-0).

So why the debate over poly versus ortho? Some Australian research suggests that fluid formulations of MAP and APP have greatly improved efficiency over granular P fertilizer. These studies, however, were conducted on dry, highly calcareous soils where precipitation of P with Ca was rapid. In these Australian studies, the soils were much more highly calcareous and drier than we see on the Prairies.

Work done by senior agri-coach Geza Racz and his co-researchers in the 70s found both liquid and dry P fertilizer performed equally well on most Canadian soils. It should also be noted that the Australian studies didn’t find an advantage for fluid formulations over dry formulations on neutral and acid soils.

CHOOSING A RATE

Regardless of where it comes from, all crop species require a certain amount of phosphorus to grow. Producing a bushel of wheat requires about 0.72 to 0.87 lb. P2O5. Whether it comes from added fertilizer or the soil, it has to be available in the amounts the plant requires at the right time. To ensure this:

1)At low soil test P levels, add P at levels greater than plant removal to provide plants with adequate P and to increase P levels in soil to increase long-term soil productivity.

2)AtmoderatesoiltestPlevels, add P at a level that equals crop removal to maintain soil productivity.

3)AthighsoiltestPlevels, add starter amounts of P to alleviate P deficiency during the seedling stage.

Even though the apparent efficiency of P fertilizer may appear low (25 per cent or less) in the application year, the reality is that over longer periods (more than 10 years), the efficiency can be as high as 90 per cent. This means your plants may not be taking up the P you applied this year but from years past.

APPLICATION

Phosphorus is immobile in the soil, this means P will not move very far from the fertilizer granule, usually less than one inch from the fertilizer band. Mobility of P in soil is very low because phosphorus is tied-up in soils with calcium, magnesium, aluminum and/or iron. Phosphorus moves to the root mainly through diffusion, meaning phosphorus ions move from a higher concentration (soil solution) to a lower concentration (root).

Because P availability at germination is critical and given that phosphorus is immobile we need to do everything possible to make sure that seed contacts phosphorus as soon as possible after germination.

The best-known methods of application for some commonly grown crops in Western Canada are:

Cereals: Apply P with the seed or side-band one inch beside and one inch below seed.

Canola, peas: Apply P side-band one inch beside and one inch below seed. Small amounts of P (20 lbs. P2O5 /ac. or less) may be added with the seed if P cannot be side-banded.

Small amounts of P (20 lbs. P2O5 /ac. or less) may be added with the seed on very P deficient neutral to acidic soil (likely in Alberta and Saskatchewan). Adding P with the seed on carbonated soil, usually results in yield decrease of about one bu/ac (a Manitoba consideration).

Low rates of P (less than 15 to 20 P2O5/ac.) may not provide sufficient granule or droplet sites in the fertilizer band for access by all plants in the seed row. We can alleviate this by using a low phosphorus content product, impregnating fertilizer or seed with phosphorus to maximize the number of feeding sites available to the seedling.

N and P are often placed together in bands at time of seeding. The high concentrations of N, particularly with N fertilizers such as urea, limits root growth into the fertilizer band and uptake of fertilizer P during the early seedling stage. Also these bands may be placed more than about one inch away from the seed row. Hence, “starter P” is usually needed to provide adequate P during the seedling stage. N and P dual bands plus “starter” (P with or near the seed) also maximizes benefits of P fertilization.

All of this is to say that on the Canadian Prairies, it’s not the P source that’s critical, it is the availability. The ideal P fertilizer management scenario is to add P fertilizer to a soil specific critical response level and then maintain that level by replacing P removals in harvested portions of crops with equal amounts of P fertilizer.

The importance of availability, and not form or application, is why we refer to the phosphorus debate as “much ado about nothing.” We hope we’ve tamed this tempest in a teapot!

GezaRacz,Ph.D.,andDougPenneyaresenior agri-coacheswithAgri-TrendAgrology

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