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One large dose of phosphate can feed a crop for years, so if the price is right…

I got a phone call from Lynn McLean, a farmer from Zealandia, Sask., one hour southwest of Saskatoon, who wanted to know about the research on residual phosphorus that had been done in the 1970s and 1980s. There was a lot of data from Saskatchewan, Manitoba and Montana and all the studies showed essentially the same thing.

As this column has said for years, phosphorus fertilizer is an investment in the land. If you own the land and farm it for many years, P fertilizer investment will always be paid back in spades.

When the fertilizer companies got greedy in summer and fall of 2008, P fertilizer was $1,400 per tonne and dealers were saying “Hurry up and buy before it goes to $2,000.” At that time, this column said “Hogwash, at that price keep your fertilizer.”

We all know the results: farmers quit buying and the price came back to something more sensible. But it makes a farmer think: should we bank some P when cash is at hand and the price is more reasonable?

So I dug out the old data and then began to ponder a large dose of P fertilizer on my Dundurn farm. My analysis and summary of the data is in Table 1. I did a bit of rounding of the rates to make even numbers.

The data for Table 1 came from a study on a Sutherland Clay soil on the U. of S. Kernen Farm. The five years had check (no P) yields from 16 to 42 bushels per acre, and it included a good range of years — some good, some bad, some average. Broadcast urea was added each year based on soil test recommendations of the day, so likely about 50 to 60 pounds of nitrogen per acre.

As I look over the gross profit data, I conclude that a dose in the neighbourhood of 80 to 100 pounds of P2O5 per acre should be a good bet. I plan to look closely at doing just that in spring 2010. Who knows what the price of grain will be in the next few years or the price of P for that matter. But if the price of grain stays around $5 or better, it should be a good bet. Who knows what might happen to the price of phosphate, but if I have a few years supply in the ground, who cares.

Of course, my first cut analysis of gross profit is not a detailed financial analysis and does not consider the opportunity cost of the money. But with interest rates near zero I am just as happy investing in the fertilizer.

Of course the available cash and income tax situation of an individual farm will play a big role in any decision to use high single applications of P fertilizer. It is an expense in the year applied and goes on to pay back for several years.

The senior author of the reference that gave all this data is Brett Wagar, who did his M. Sc. degree at the U. of S. Soil Science Department then went home to the farm at Macklin, Sask. He is still there, so I phoned Brett to see if he had put the data to use. He had. He has used 80 pounds P2O5 per acre as a single broadcast before planting alfalfa and it supplied the P for several years of alfalfa. It was his opinion that 80 pounds was a safe rate to use, particularly for soils with three per cent organic matter or better and with a clay loam texture or better. In higher organic matter soils, the extra P stays around in the organic form and is metered out bit by bit over the years.

The experiment also included a comparison with annual seed placed rates at various rates up to 40 pounds of P2O5 per acre per year. Results showed that annual applications at rates higher than crop removal also lead to residual P that is available to future crops, even if it does not all show up in the standard soil tests. Many farmers have used P for 10 or more years now at rates that probably are providing residual effects. As our yields climb higher and higher, part of the reason is probably residual fertilizer P.

So I rest my case. P fertilizer is an investment in the land and for some farmers with extra cash and tax to pay, it may be a good investment to put on several years’ supply when the price is known.

J. L. (Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms near Dundurn, Sask.

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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