ONTARIO GRAIN FARMER AGRONOMY 34 From optimizing fertilizers to cropping practices, there are many long-held standards for growing soybeans. But recent research trials are challenging some of these long-held conventions. At the 2026 Southwest Agricultural Conference in Ridgetown earlier this year, Adrian Correndo, an assistant professor at the University of Guelph, and Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA), provided their own ‘secrets’ for growing soybeans. Among their observations were updates to phosphate (P) and potash (K) recommendations, excitement over double-cropping soybeans, and insights on sulphur applications. Correndo presented data from research trials on P and K levels in soybeans from more than 28 trials across Ontario (Table 1). The goal was to determine whether OMAFA recommendations still apply, and to create a dashboard available for growers before the planting season. A beta version was planned for feedback in February. “We’re still building the database, and we have a lot of data,” said Correndo, stressing that results from the research are preliminary. “We’re also compiling all the historical data and trying to make it available.” His research addressed P and K levels according to Soil Test (STP and STK) values and compared them to Critical Soil Test Values (CSTV). In phosphate, 15 ppm was the recommendation for P with a range of 10 to 20 ppm labelled “acceptable”. “The average response of soil below 15 ppm is 10 bushels per acre (bu./ac),” said Correndo, adding that the range of yield loss is between three and 15 bu./ac, which is a reasonable response in soils below 15 ppm. “We may have cases where you have 30 bu./ac (response). But if we go above 15 ppm, you see that the response is an average of around 5 bu./ac.” MORE NOT ALWAYS LESS Relating that to application rates, Correndo used a graph that employed a ratio of pounds of soybeans and pounds of P2O5 while comparing them to soil test levels of P (STP) above and below 15 ppm. He found the optimum ratio was five (lbs of soybeans to lbs of P2O5). Using current pricing – which he said is a bit high compared to historical figures – that ratio would show up at 50 to 60 lbs of P2O5/ac. According to the Most Economic Rate of P (MERP) from all the trials, that application rate still looks reasonable, with an Olsen-P rate of five ppm at 0 to 15 centimetres (six inches) depth. “Overall, the recommendations wouldn’t change much, depending on price,” said Correndo. “They may change about 10 pounds or so – not a drastic thing. What’s driving most of the recommendations is not the price of fertilizer, it’s the soil test.” He used the same logic for researching potash. To achieve 95 per cent of relative yield, the critical value is around 90 ppm with Ralph Pearce Rethinking the rules From fertilizer economics to maturity group selection, fresh data is refining best practices for soybean production New Ontario trials are refining fertilizer rates, maturity group selection and sulphur use—highlighting where small management shifts can protect yield and improve returns. Table 1 – Revisiting P&K recommendations P2O5 (lbs/ac) K2O (lbs/ac) T1 0 200 T2 22 200 T3 53 200 T4 85 200 T5 116 200 T6 116 0 T7 116 22 T8 116 66 T9 116 133 T10 0 0 P source: 0-46-0 Broadcast (inc.) K source: 0-0-60 pre-plant Source: Adrian Correndo University of Guelph
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