Big Yields. Are They Impacting Soil Fertility?

Ontario crop yields have exploded over the last 35 years. Corn yields have increased by 80% during this time and soybeans are not that far behind at 65%.

Those big yield gains create a much greater draw on soil nutrients. In the case of corn, for example, the average corn crop now removes 63 lbs of phosphorus per acre compared to 35 lbs in 1981. Potassium removal rates have grown from 25 lbs to 45 lbs over the same time period.

In Ontario, current P and K soil fertility recommendations are now 30 years old and many producers wonder whether it’s time to revisit these long established input levels. University of Guelph’s Dr. David Hooker says it’s a good conversation to have.

He addressed the situation at the SouthWest Agricultural Conference earlier this month. One way to assess whether there’s a need for change is to look at soil test trends. He says soil test P and K levels are indeed trending down into regions not seen in the past 50 years.

In many cases, growers are not applying enough fertilizer to meet crops needs, says Hooker. “There are many situations where soil test levels are being drawn down so far that corn, soy and wheat yields are suffering because of these low soil test levels.”

Hooker says University of Guelph/OMAFRA long-term fertility studies are starting to unearth some of the implications of excessive removal. 2015 soybean trial data from the Elora Research Station, for example, compared yields on soil testing low for background fertility (P less then 10 ppm, K less than 80 ppm) and moderate to high background fertility (P greater than 20 ppm, K greater than 120 ppm). In this case, the soybeans planted into higher background fertility soil yielded an additional 17 bushels.

“It’s hard to find a soybean response to anything over five bushels,” says Hooker who believes growers stand to take a significant yield hit if they plant soybeans in low fertility fields without starter fertilizer.” He hopes trials in 2016 will provide further insights.

So how do you pump up these low background fertility levels? Hooker believes farmers need both long-term and short-term strategies. Short term, he says, “lots of crops respond to optimized placement – banding a small amount of fertilizer tends to give a much bigger response than broadcast.”

Long-term strategies depend on a number of factors, notes Hooker. The big question is whether you rent or own the land and what’s the return you can expect on fertilizer investment.

Overall, Hooker says current OMAFRA fertility recommendations are sufficient to meet current crop yield expectations, but he indicates there will likely be some tweaks in the future. In the meantime, he’s recommending growers soil test regularly and keep an eye on background fertility levels.