For more than 10 years there have been many studies done on Vesiculararbuscular mycorrhiza (VAM) fungi and the role they play in phosphorus availability to crops. Most of us tend to concentrate on things we know most about and soil microbiology was never my long suit. So I kind of listened at what I thought to be mostly an interesting academic subject but suspected it was not a big issue for many farms, including mine.
I observed something in my 2009 pea crop, however, that changed my mind.
For this story we have to go back to early May 2008 when my pea crop was seeded with a 45′ air drill. A drill width’s worth of seed check was accidentally left — the arrows in Photo 1 show the location of the strip after the wheat harvest in 2008. Seeding was east-west and harvest north-south. The seed miss was going to grow to weeds so I took a small cultivator and kept it clean — about three licks as I remember it.
In May 2009 Golden peas were seeded with no fertilizer of any kind applied. During field checking in July, I noticed an area that was noticeably poorer than everything else. As I walked around, did some soil probing and the like to figure out the cause, I noticed the poor area had no residue from last year’s wheat crop. Voila! I had figured it out; that was the area that had been summerfallowed in 2008.
It turns out, the poor area was phosphorus (P) deficient because the mycorrhizal fungi (VAM) could not do their job. The hair like growth (hyphae) of the fungi had been disrupted by the several tillage operations and the lack of a host crop. Where no tillage had occurred the fungi could do their job. The hyphae solubilize P and enter into a joint venture with the crop to provide available P in the immediate vicinity of the root.
I went back to the literature to look for other examples and found that the lack of a continuous host for the fungi can reduce P uptake of crop after fallow. A corn study in the lower Fraser Valley of B. C. showed that fallow gave a consistent 50 per cent or more reduction in fungi counts and an eight bushel per acre decline in grain corn yield in each of three successive years.
For any CCA or P. Ag. types who want to look up that reference it is in Agronomy Journal (2006), Volume 98, Pages 394-401. One of the authors was Grant Kowalenko, an old farm boy from the Arelee, Sask. area, agradof theUofS and a summer student of mine eons ago.
It all makes sense when we think about the history of fertilizer use on the prairies. Back to the 1950s it was found that P fertilizer gave large P responses on fallow land, and most crops were seeded on fallow in those days. When stubble experiments started, N fertilizer was the main nutrient of concern — and oddly enough P fertilizer response was harder to demonstrate on stubble.
Not all mysteries are as simple as this. Soil test P level, manure history, crop rotations and more all play a role in the hits and misses of crop production. But it was a source of great interest for me to see such a graphic example of the soil organisms doing their thing. And it gave me incentive to be more attentive to the soil organisms we all to often take for granted and do not understand as well as we should. I have always said, when I quit learning it is time to get planted and let the soil organisms make me part of the soil.
As this issue hits your mailbox seeding will be in full swing. Remember my usual warning — be careful out there so you are around and healthy to take the crop off that you are busy seeding. Here’s to big crops in 2010!
J. L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask.