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Declining fertility after 98 years of wheat

Wheat yields on long-term plots at Agriculture and Agri-Food Canada’s (AAFC’s) research station at Scott, Saskatchewan, just may be the canary in the coal mine. As an early warning, these plots are showing that while wheat yields have increased during the last century, they have levelled off and even declined in recent years. “What concerns me the most is that the yields peaked and haven’t come back completely,” explains Stu Brandt, a recently retired research scientist at AAFC Scott. Brandt supervised the trials while he worked at Scott and says that declining organic matter has reduced the natural fertility of the soil, and yields are suffering.

The data comes from a fallow-wheat-wheat rotation called Rotation C, which was set up at several locations on the Prairies, but discontinued at most other locations. The comparisons are between wheat-on-stubble versus wheat-on-fallow, with and without fertilizer.

A third treatment, phosphorous fertilizer for wheat-on-fallow, was added in the 1930s. In the mid-1980s, a nitrogen/phosphorous fertilizer treatment was added to wheat on stubble. 

The data points are 10-year moving average yields. The first data point is the average from 1912 to 1921. Brandt used the 10-year moving average to smooth out much of the year-to-year yield variability and to provide a clearer indication of trends over time. Without this smoothing, trends are still present, but the lines move up and down so much from year to year that trends are much harder to visualize.

Brandt says that leading up to 1940, the wheat yields, regardless of rotation and fertilizer, declined from the early years of production. He says this was attributed to increasing pests, declining soil fertility, increasing drought during the 1930s and soil erosion. This era was characterized by intensive tillage with plows, discers and harrows, and pests were culturally controlled.

During the second quarter of the century, yields increased due to better wheat varieties and improved weed control. Phosphate fertilizer also improved fallow wheat yield. Intensive tillage was complemented with stubble mulch tillage, where more crop residue was left on the soil surface.

The third quarter of the century saw more stubble mulch tillage and eventually a move to minimum tillage. Wheat yields continued to increase due to improved pest control, better management practices and improved varieties.

Since 1985, yields began to decline in the absence of fertilizer, but continued to increase when fertilizer was used until the severe drought of 2001 to 2003. While the yields have recovered slightly, they still have not reached their peak yields of the 2000 growing season. This is, despite improvements in integrated pest control and the move to no-till. “I think what we are seeing is that soil fertility has bottomed out, and we aren’t getting many nutrients from the soil anymore,” explains Brandt. 

During that 25-year period, the N/P fertilized stubble plots have yielded far more than the unfertilized wheat on stubble plots, and a similar amount to the fallow plots.

At first glance, the yield of wheat-on-fallow with P fertilizer added is puzzling. Brandt offers the possible reasons; “There are a couple of things that would explain this. First, fallow does store some moisture, and during the past 10 years, moisture has been very limiting some years. Second, the treatments with P fertilizer added have been higher yielding over a long period of time. This means more organic matter returned to the soil, so if organic matter is becoming a yield-limiting factor, treatments that add more organic matter would support higher yield. I suspect it is a combination of the two.”

Green soilage trial proves point
Brandt ran a separate trial on part of a fallow plot to see if he could improve organic matter, soil fertility and wheat yields. In 2008, 40 tons per acre of chopped green alfalfa were applied and incorporated (called soilage) into part of one of the long-term fallow plots. That addition produced an increase in yield from 43 to 56 bushels per acre. “By increasing the soil organic matter, we were able to significantly increase yield. That increase came solely from better soil fertility, so it really provides a warning that these soils are lacking in organic matter and soil fertility,” explains Brandt.

Two years later, the evidence of the soilage application was still visible, with healthier, more productive wheat stands during the summer of 2010 at the AAFC Scott Experimental Farm’s 100th anniversary.

Brandt also pointed out that during the wet 2010 growing season, the fertilized stubble wheat was the best plot, while parts of the fallow plots were flooded out. As a result, he expects the fertilized stubble plots to produce the higher yields. 

Looking forward, Brandt says that while past improvements came from better cultivars, better pest control and fertilizers, soil productivity will need to increase to support increased wheat yields. He explains that eliminating fallow and tillage, using more N-fixing crops in the rotation, and replacing the nutrients that are removed by the crop are all practices that will need to be implemented. 

Brandt explains that the real culprit in declining soil productivity in the trials is fallow tilled every third year. He says fallow does not add any organic matter while hastening the decomposition of soil organic matter. This means that over time, the practice of fallow continues to deplete the stores of nutrients in the soil, particularly where fertilizers are not used to replace what is removed in the harvested grain. “Because we have reached that point where our soil organic matter has declined so much that our yields are depressed, I tell farmers that if we don’t change our farming practices to rebuild soil fertility, yields will stay depressed and may even decline further,” says Brandt.