Phosphorus 101: The basic BMPs

Here are the basic best management practices for phosphorus fertilizer

Soil tests

The Olsen (bicarb) test is effective across a wide range of soils, including the high pH calcareous soils common in Manitoba and Saskatchewan, while the Bray test is effective only in neutral to lower pH non-calcareous soils, as are found in parts of Alberta and Saskatchewan. Kelowna and modified Kelowna tests are also considered effective in the pH range on most of the soils in the Northern Great Plains.

Rates

Canola takes up 1.31 to 1.63 pounds of fertilizer phosphorus (P2O5) for each bushel, and around one pound per bushel is removed in the seed, on average. This is based on estimates from the Canadian Fertilizer Institute. A 50-bu./ac. canola crop will therefore remove about 50 lb./ac. of P2O5 (equivalent to 100 lb./ac. of monoammonium phosphate). The rest stays in crop residue and is returned to the field. Soybeans remove about 0.90 pounds of P2O5 per bushel, on average. Peas are 0.69 pounds per bushel, spring wheat is 0.59, corn is 0.44 and barley is 0.42.

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Average phosphate fertilizer rates have lagged crop removal for years unless fields are getting regular treatments of livestock manure. “The majority of soils in Western Canada are testing low for phosphorus unless they have a history of manure application,” says soil scientist Cindy Grant, who recently retired from Agriculture and Agri-Food Canada in Brandon, Man. “That is why farmers can expect a response to phosphorus fertilizer in soils with low test results.”

Farmers tend to follow one of these three philosophies when it comes to phosphate fertilizer rates:

  • Sufficiency approach. Apply just enough to meet the bare minimum for yield, with the crop taking up the rest of what it needs from soil reserves. This can maximize net return from fertilizer in the year of application but will drain soil reserves. Once soil reserves are medium to low, this approach will hurt yields and profitability (see table at top).
  • Removal approach. This aims to match application rate to expected crop removal based on yield targets. This will not build soil phosphorus amounts, but should not reduce them either.
  • Build and maintenance approach. Apply fertilizer at rates in excess of crop removal to slowly increase soil phosphorus levels to an adequate level. Research by Cindy Grant showed it took about 16 lbs./ac. of P2O5 (32 lbs./ac. of monoammonium phosphate, for example) to raise soil test phosphorus by one ppm on a slightly acidic sandy loam soil at Carman, Man., but it took 30-40 lbs./ac. of P2O5 on a calcareous clay loam soil at Brandon. If crop removal is 40 lbs./ac., farmers would have to use a rate of 52-80 lbs./ac. of P2O5 to satisfy removal and increase soil reserves by one ppm.

Product

Most phosphate fertilizer products will do the job. Whether monoammonium phosphate (11-52-0/12-51-0) or liquid polyphosphate (10-34-0), the key is to apply enough to match crop removal or, as a short-term alternative, meet the sufficiency requirement.

A 40-ton-per-acre application of feedlot manure can provide about 600 lbs./ac. of phosphate. Swine and poultry manure will have higher concentrations, and phosphorus and nitrogen can be in almost equal parts. Test manures to avoid over-application of phosphate. Soil with manure applications may not require phosphate fertilizer (or potassium, sulphur and micronutrients) for five years or more, depending on application rates and type of manure applied.

Placement

Seed-placed rates that exceed 15-20 lbs./ac. of P2O5 can start to reduce plant counts. Dry conditions and light-textured soil increase the risk of stand thinning from seed-placed fertilizer. Increased soil moisture lowers the risk of seedling injury. This rate also ensures all seeds are in fairly close proximity to phosphate prills or droplets. Because this rate is not enough to match crop removal, the ideal practice is to split the application and put the rest in a side or mid-row band. Another option is to put higher rates with other crops in the rotation, achieving the crop-removal balance with a whole-farm approach applied over time.

Deficiency symptoms

These are very difficult to identify with phosphorus. Often severely deficient fields don’t show any major outward symptoms, which is why Jeff Schoenau, professor of soil fertility at the University of Saskatchewan, uses the term “hidden hunger” to describe phosphorus deficiencies. Growth rate and vigour are often stunted, but this only becomes obvious when compared side by side with plants that are not deficient — which is what Scott Keller observed with his strips of missed phosphate applications.

— This content is from a Canola Watch article at canolawatch.org.

Jay Whetter is communications manager for the Canola Council of Canada. Email him at [email protected].

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Jay Whetter is communications manager for the Canola Council of Canada.

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