Top Crop Manager

Features Fertilizer Seed & Chemical
When does top-dress fertilizer make sense?

June 17, 2013 – Canola can benefit from a top dress of nitrogen or sulphur fertilizer if soil reserves are short and likely to prevent a canola crop from reaching its yield potential.

“Ideally, growers want to have ample fertilizer in place at or before seeding so an extra top-dress pass isn’t necessary,” says Dan Orchard, Canola Council of Canada agronomy specialist. “However, a top-dress will pay off in certain circumstances.”

Nitrogen top dress can provide an economic return if soil conditions and crop yield outlook have improved since seeding, if excessive rain has caused high rates of nitrogen denitrification and leaching, or if canola is already showing signs of nitrogen deficiency.

Nitrogen deficiency symptoms first show up in older leaves as pale green to yellow colouring, and sometimes purpling. These older leaves tend to die early, turn brown and drop off prematurely. Overall plant growth is slow, with short thin stems, small leaves, and few branches.

“Nitrogen is ideally applied before the 5-leaf stage so it’s in place when nitrogen uptake really starts to take off,” Orchard says. “Sulphur should also be applied with the earlier-is-better approach, but as a rescue treatment can be applied up to early flowering.”

Sulphur top dress can make financial sense if growers could not put on the desired rate at seeding, if yield potential improved and growers had not been applying recommended rates of sulphur, or if canola shows signs of sulphur deficiency.

Canola plants short of sulphur tend to have yellowing, stunting and cupping of new leaves, while older leaves may look fine. “Deficiency symptoms are usually present on the newer leaves as sulphur isn’t mobile in the plant like nitrogen is,” Orchard says. Sulphur deficiency tends to show up on hilltops and sandier soil first, where soil sulphur content tends to be lower.

“Canola plants need sulphur to produce key amino acids. Without the sulphur to build these amino acids, a canola crop can suffer huge yield loss,” Orchard says. “When you know sulphur may be short or if you see signs of sulphur deficiency, a top-dress of sulphur fertilizer can rescue the crop and provide significant benefits.”

Growers have dry and liquid options for both sulphur and nitrogen. Ammonium sulphate (dry) or ammonium thiosulphate (liquid) provide sulphur that is immediately available to the crop. They also provide some nitrogen. For nitrogen only, top-dress with urea (dry) or UAN (liquid).

Urea and UAN need rain within a day of application to limit nutrient losses. Agrotain can help reduce nitrogen losses if rain is not imminent. Sulphur fertilizer also needs rain to wash it into the root zone, but sulphur is not volatile like nitrogen fertilizer, so while dry conditions may delay availability to the crop, losses will be minimal if rain is not immediately forecast.

“A small amount of nitrogen, say 20 pounds of actual nitrogen per acre, can provide an economic return. Rates can go up from there depending on the need,” Orchard says. “If you’re not sure about top-dressing, apply a nitrogen-rich strip in the field as a test and take it to yield. Comparing yield is important. A leaf colour difference between the two treatments does not necessarily translate into improved yield.”

Sulphur at a top-dress rate of 10 to 20 pounds per acre can provide an economic benefit if insufficient sulphur was applied at seeding and the field is known to have areas of shortage. Higher levels can provide a return in highly deficient soils.

“Top-dress fertilizer should be broadcast if dry or dribbled if liquid,” Orchard says. “Tank mixing liquid nitrogen with herbicide and applying through the sprayer tank cannot achieve the rates needed to make a difference.” Herbicide spray nozzles are also designed for leaf coverage, which is not what you want with fertilizer. Leaf coverage increases the risk of fertilizer burn on canola.