Toxicity and salt effect from fertilizer can damage canola seed and seedlings, and reduce the return on investment for seed. One way to increase canola emergence rates is to keep all fertilizer out of the seed row – except for a starter rate of phosphate.
The risk of seed-placed fertilizer comes from the nitrogen component of ammonium phosphate, ammonium sulphate and all nitrogen fertilizers. Ammonia can damage crops through direct toxicity while nitrate will damage seedlings by desiccation through the salt effect. Potassium fertilizers also have a high salt index and should not go in the seed row.
Why is phosphorus the exception?
Phosphorus supply during the first two to six weeks of canola growth is critical to achieve optimal yield. Given that phosphorus availability is reduced under wet, cold conditions, canola benefits from a seed-placed supply of phosphorus fertilizer.
But not too much. Apply just enough to give each seed equal access to starter phosphate prills or droplets but not so much that the ammonium reduces emergence. A seed-placed rate of 20 lb./ac. of phosphate (~40 lb./ac. of monoammonium phosphate) will balance those two objectives. This approach shows the interactions of the Right rate at the Right time in the Right place as part of 4R Nutrient Stewardship.
How to reduce the nitrogen risk
Under some circumstances, growers can safely apply a little extra ammonium phosphate or nitrogen fertilizer with canola seed. Factors that reduce the risk from seed-placed nitrogen are:
- Seedbed moisture. Water dilutes the concentration of nitrogen molecules around the seed and seedling, and disperses nitrogen molecules throughout the soil. This moisture reduces fertilizer concentrations around the seed.
- Fertilizer source. Polymer coating or urease inhibitors slow the release of ammonia and ammonium from urea.
- Seed bed utilization (SBU). High SBU – which can be achieved with wider openers and narrow row spacing – will lower the risk because seed and fertilizer are spread over a larger area.
- Soil pH. Lower soil pH reduces the risk from seed-placed nitrogen. That is why safe rates are generally higher in Saskatchewan and Alberta than in Manitoba.
- Soil texture. Heavy (clay) soils lower the risk and light (sandy) soils increase the risk.
Test the safety of seed-placed rates
These factors don’t eliminate the risk entirely, so a large percentage of fertilizer will still have to go outside the seed row to give canola seeds a safe place to emerge and contribute to yield.
To test the effect of seed-placed fertilizer, turn off seed-placed fertilizer runs for a 100-foot strip in some fields. Mark these areas and then go back and do plant counts early in the season to compare treated and untreated strips.
The Nutrient Management chapter in Canola Encyclopedia has more on the placement of fertilizer (under each macronutrient heading), and a chapter on the 4R stewardship practices – Right source of fertilizer used at the Right rate, at the Right time and in the Right place. For a more detailed version of this article, read the “Right rates for seed-placed fertilizer” Canola Watch fundamentals article. While there, please sign up to receive our timely Canola Watch agronomy emails.