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New wrinkles to adding phosphorous

If you own the land, adding phosphorus fertilizer will pay off in the long run

On February 8, 2010, my column looked at the economics of large single applications of phosphorus fertilizer. The economics are good, but the practice is still rare.

Most graphs showing fertilizer use in Western Canada start in 1960. Before that, fertilizer use was sporadic and included only a few acres with a modest application of ammonium phosphate (11-48-0). Almost all fertilizer in the early days was produced at the Consolidated Mining and Smelting plant at Trail B.C. The first fertilizer manufacture was actually a pollution control measure. CM&S was spewing gobs of sulphur into the atmosphere. They cleaned up their act and used the sulphur to produce the sulphuric acid needed to add to phosphate rock to make phosphate fertilizers.

Each time I look at the graph of fertilizer use I think of crawling off the Cockshutt 132 combine at Milden, Sask., August 31, 1960. Harvest over, I had two weeks to prepare for the grand experiment at the University of Saskatchewan. No fertilizer had been used on that farm from breaking in 1906 until I left in 1960. Dad was up in years when I left so my brother-in-law helped seed the 1962 crop. They seeded one field with two discers. Dad’s discer had no fertilizer attachment but Roy was able to apply 11-48-0 fertilizer. What a sight — every other round looked great!

At that time Don Rennie and others were using Radioactive P32 labelled fertilizers to find out how much of the phosphorus actually got into the wheat plant. The results were disappointing — only 25 to 30 per cent of the fertilizer phosphorus ended up in the plant.

When I went back to Milden, coffee row had a question. “If only 25 per cent of the fertilizer phosphorus gets into the plant, what happens to the rest? Is it available for future use?” Then my sputtering started.

Phosphorus does not gas off or wash out like nitrogen, but it gets tied up with soil minerals and we need to use it every year. That was the answer of the day. We knew that seed-placed was much more efficient and broadcasting was a waste of time. But that was all with very low rates.

Annual applications of 40 pounds per acre of 11-48-0 to summerfallow wheat was the practice for years. Nitrogen fertilizer use only took off when we finally realized that all that summerfallow was a huge waste.

1970s experiments

In the 1970s the many experiments with high rates of broadcast phosphorus showed residual effects. Phosphorus fertilizer is an investment in the land. If you own the land pour on the phosphorus and it will pay back in spades. But the practice of soil building has never been adopted on a big scale.

Recent analysis has made it clear. A Swift Current experiment started in 1967 shows that over time, 98 per cent of the phosphorus fertilizer added can be accounted for in extra phosphorus hauled to the elevator.

An article* has put it together in very succinct terms. That paper combined small plot data from fertilizer experiments in Jolly Olde (England) to statewide data on phosphorus use, crop removal and soil test summaries in Montana, North Dakota and South Dakota.

After decades of fancy technology, countless wet chemistry procedures and endless field experiments it comes down to simple math: Using the Olsen P test, 10 to15 ppm is the level to be maintained and 60 ppm is the upper limit after which environmental considerations hold sway. Excessive soil phosphorus levels are usually associated with long-term manure applications.

There are several different phosphorus soil tests. They all work well and should be part of long-term planning.

If we haul more phosphorus to the elevator than we put on in fertilizer or manure, the phosphorus soil test will go down and with it the crop yield potential. If we add more phosphorus than we haul away, over time, the soil test phosphorus will go up along with the crop yield potential.

Speaking of long term, the long-term field experiments have given us this information. Unfortunately, our decision makers seem to be in the business of shutting down anything but short-term research. We will pay the price down the road.

*Johnny Johnston, Paul Fixen and Paul Poulton. 2014. The Efficient Use of Phosphorus in Agriculture. Better Crops With Plant Food. Volume 98: Page 22-24. Better Crops with Plant Food is a publication of the International Plant Nutrition Institute.

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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