I have never been a huge fan of research station soil fertility work, and in my day never did an experiment on university or government land. My worry was that the residuals from unknown manure and other applications would make the data of doubtful use in “ordinary” farm fields.
This past winter I had the pleasure of hearing Reynald Lemke, research scientist with Agriculture and Agri-Food Canada (AAFC), present a summary of long-term experiments from AAFC’s Swift Current, Sask., research station. It was a fascinating talk and made me realize the value of these long-term field experiments.
The Swift Current long-term wheat plots were started in 1967 so have now run for 45 years — long enough to see significant trends.
The experiments were simple nitrogen (N) and phosphorus (P) applications on continuous wheat and fallow, wheat, wheat plots. The most striking statistic to me was recovery of N and P fertilizer expressed as the extra N or P harvested in the grain as a percentage of the N and P fertilizer that had been applied over the many years.
When Reynald Lemke tallied up the long-term results at Swift Current it turned out that 98 per cent of the P applied could be accounted for in the P that was hauled off the plots with the wheat. The lights came on.
In the intervening 40 years there have been many studies of residual P from adding single high rates and following the residual response. That data has long ago led me to state: “P fertilizer is an investment in the land. If you own the land and farm it for many years the money spent on P fertilizer will always come home in spades.”
The recovery of N fertilizer in the grain from long-term experiments at Swift Current was actually greater than 100 per cent. At first flush that may seem impossible but it makes a great deal of sense.
At the AAFC Research Station at Scott, Sask., there is a piece of ground that has been planted to wheat continuously for 100 years with no fertilizer applied. The five-year average yield is still about 15 to 20 bushels per acre. So, if it still yields that much with no N fertilizer there must be some natural additions. This comes from nitrogen fixation by free living microorganisms, the bit of N that comes down in rain and a bit of N still being released form the soil organic matter.
At the famous Rothamsted Research Farm in Jolly Old England the field named Broadbalk has plots that have grown winter wheat continuously since 1843 — yes, 1843 — with no fertilizer or manure. It still has a five-year average yield of about 15 bushels per acre. Now, in the U.K. the N that comes down in rain can be quite significant at times and in the wet winter they do lose N to leaching and drainage — all plots are tile drained. So, the positives and negatives of it all are different, but they still harvest some wheat with no fertilizer.
So, it makes sense that Swift Current could record more than 100 per cent recovery of N fertilizer (rates were modest) over the long term. That means that in the brown soil zone, if nitrogen fertilizer is used at a moderate to high rate (50 to 75 pounds per acre) and a dry year comes along, the nitrogen will be sitting there ready to use when the next crop is planted and when it rains.
We should not think that the Swift Current data can be applied everywhere. In the black soil zones where excess water is not uncommon — especially the Red River valley — losses of N to denitrification and leaching will occur and excess N will be lost. †