Seed-placed fertilizer is a common practice to increase seedling vigor and optimize fertilizer placement and crop response. This is a popular strategy to apply phosphorus for canola, corn, and wheat. However, the seed-placed fertilizer rate cannot exceed seed safety limits, otherwise seedling germination and plant population may be reduced. Sulfur is very important in canola growth and development, so farmers often try placing ammonium sulfate (AMS) with canola seed as well! This can create big problems.
A team of agronomists and soil scientists at the University of Manitoba conducted greenhouse and field studies, examining the effect of seed-placed ammonium sulfate on canola plant population and seed yield. The plant population loss was much greater on soils with pH > 7.5 (Figure 1). The high pH soils contained calcium carbonate (CaCO3), which reacts with ammonium sulfate to create calcium sulfate (gypsum) and ammonium carbonate. The higher reaction pH of ammonium carbonate produces free ammonia (NH3). Free ammonia (NH3) in soil is toxic to living organisms and kills germinating seeds. Acute ammonia toxicity is a major concern with fertilizer materials that liberate free ammonia (NH3) in soil, such as anhydrous ammonia (82-0-0) or urea (46-0-0), ultimately reducing plant population if you are not careful with fertilizer rate and placement.
Figure 1. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Soil carbonate content is 21% CCE and 0.5% CCE in knoll soil and hollow soil, respectively. Brandon, Manitoba.
Across the landscape, soil pH and carbonate content will vary. The well-drained lower landscape positions (swales, hollows) often have acidic to neutral pH and little carbonate. The upper landscape positions (knobs, knolls), suffering decades of soil erosion, often have high pH and ample carbonate (Figure 1). The risk of plant population loss is greater on eroded knobs where adding ammonium sulfate can create ammonia toxicity concern.
Considerable yield loss will occur if canola plant population is less than 70 plants per square meter. Even with low fertilizer rates, the interaction of seed-placed ammonium sulfate and phosphorus can greatly reduce canola plant population. In Manitoba, 25% plant population loss was observed with only 8 lb/acre S and 18 lb/acre P2O5 (Figure 2).
Figure 2. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Carman, Manitoba, 2011.
Sulfur is vital for successful canola production, but it must be applied safely. There are new air drill configurations with innovative seed and fertilizer placement options. Seed safety is paramount with seed-placed fertilizer. Ammonium sulfate should be broadcasted or banded away from seed (mid-row). Keeping ammonium sulfate away from seed will also allow you to maximize seed-placed phosphorus rates and efficiency without jeopardizing seed safety.
Placing ammonium sulfate with seed should be an emergency option only. Canola plant population loss should be expected, even at low ammonium sulfate rates, on soils with pH greater than 7.5 and calcium carbonate.