By using soil conductivity and topographic data, SWAT mapping helps farmers make more informed decisions about seeding, fertility, and crop management.
Soil mapping and precision agriculture are helping farmers make more informed decisions about seed populations, fertilizer applications, and crop protection. During a SWAT mapping demonstration, Certified Crop Advisor Patrick Lynch and agricultural specialist Mike Sullivan explained how soil characteristics, topography and field-level data can guide variable-rate applications and improve return on investment.
Lynch described how his experience with soil sampling and variable-rate fertilizer applications led him to recognize the limitations of treating entire fields uniformly.
Lynch explained that the SWAT system combines soil electrical conductivity measurements with real-time kinematic (RTK) elevation data to identify differences in soil texture, water-holding capacity, and landscape features.
The system uses an electromagnetic sensor to measure soil electrical conductivity at depths of approximately 0.5 and 1 meter. The information is combined with topographic data to create maps that divide fields into management zones.
Service providers then verify the maps through field inspections and soil sampling to confirm soil characteristics and identify areas requiring adjustments.
SWAT maps typically classify fields into 10 zones based on their characteristics. Lower-numbered zones generally represent hills, eroded areas, and sandy soils with limited water-holding capacity.
Middle zones represent average field conditions, while higher-numbered zones commonly include lower slopes, depressions, heavy clay soils, and organic soils.
Sullivan explained that these maps support soil sampling and variable-rate applications of seed, lime, fertilizer, fungicides, and plant growth regulators. Recommendations are adjusted according to field conditions, crop requirements, and management objectives.
For corn, variable-rate seeding generally involves populations ranging from 27,000 to 37,000 seeds per acre, depending on the field and hybrid. Lower populations may be used in drier areas to reduce competition for available water, while more productive zones can support higher populations.
Soybeans require a different approach because plants in favorable conditions may branch more extensively than those in drier areas.
The demonstration also highlighted fertilizer management. Fields receiving uniform fertilizer applications can accumulate excess nutrients in lower-productivity areas because crops remove fewer nutrients there.
Meanwhile, productive zones may experience greater nutrient depletion. Soil testing and variable-rate prescriptions help address these differences and avoid unnecessary input costs.
Technology can also support more targeted fungicide applications. In soybean fields, mapping and crop-monitoring cameras can help identify areas where a second fungicide treatment may provide limited economic benefits, reducing application expenses where appropriate.
Ground-truthing remains essential to verify map accuracy and evaluate results. Farmers and advisors can compare variable-rate treatments with standard applications using population counts and yield data.
The presenters emphasized that equipment compatibility, software integration and accurate field observations remain important considerations.
With input costs continuing to rise, SWAT mapping offers farmers a practical approach to matching crop inputs with field conditions, improving resource efficiency and making more informed farm management decisions.
Watch the full Patrick Sullivian demo presentation from the 2026 Great Ontario Yield Tour below.
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