16 The Business of Prairie Agriculture Better Farming | April 2024 crease. “I think of it as the microbes are woken up by the moisture and feast on the dead bodies of their mates and crop roots and residues of the previous year, mineralizing nitrogen.” Soil testing In some cases, farmers could benefit from both fall and spring soil testing. “Fall soil testing gives an indication of the amount of nutrition in the soil after removing crops, and is used by agronomists to come up with fertilizer prescriptions for next year,” says MacKenzie. Schoenau urges farmers to sample carefully to account for greater microscale and macroscale variability, explaining that spatial variability in available nutrients can be considerable due to residual fertilizer bands, and patchy regrowth and weed infestation from late-season rains. Farmers may also wish to re-check a few fall-sampled fields in the spring to assess the efficiency of nutrient carryover. “Residual phosphorus and potassium should carry over effectively as there is no potential for significant leaching or gaseous losses of these nutrients. Residual nitrate, however, is susceptible to loss by leaching and denitrification in spring, especially if wet. Sulfate could be leached, but no gaseous losses,” Schoenau says. Tenuta says farmers with fields stressed by drought in 2023 ought to expect slightly higher soil test nitrate and more mineralization in 2024. He adds that when a fall and spring are very wet, fall soil nitrate test accuracy can be off. Geisel further explains that depending on when and where fall soil tests are taken, findings can change due to mineralization of unavailable nutrients increasing levels, or losses of nutrients reducing levels. “Fall soil samples are an important step in helping farmers and agronomists get an idea of end-of-season nutrient levels to help with winter planning. However, depending on the length of fall and winter conditions, these numbers can change by spring,” he says. Plan for success by utilizing historic applications and yield results, crop nutrient uptake needs, and making adjustments based on either fall or spring soil testing, Geisel says. For farmers having spring soil samples taken to verify what their fall tests indicated, Hammermeister recommends samples be collected in the vicinity of the original sample points, in order to limit the impact of natural variability that occurs across a landscape. BF Drought & soil nutrients RICHARD KAMCHEN Richard Kamchen is a veteran agricultural freelance writer based in Winnipeg, Man.
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