Generally, any rate of seed-placed nitrogen will reduce stands and can potentially impact crop yields, says John Heard, crop nutrition specialist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI). It’s often a case of balancing stand loss with flexibility of nitrogen application. “Previous studies with cereals indicated stand losses of 15 per cent can be accommodated without affecting final crop yield,” he says.
Cereal crops may compensate for fewer plants by producing more tillers, and are generally less sensitive to seed-placed nitrogen than crops like canola, flax and soybeans. These crops can suffer significant seedling injury if fertilizer is placed too close to the seed and at rates that are too high.
Potential consequences of stand loss include severely reduced yields, delayed maturity and reduced weed competitiveness. In short season areas the delayed maturity of cereals associated with stand thinning may impact grade.
The role of nitrogen
Nitrogen is highly mobile and can easily be lost from agricultural systems, especially under wet conditions or on poorly drained soils. Nitrogen can be leached, can move in runoff, or can convert to nitrogen gas or nitrous oxide gas. Nitrogen applied at the soil surface can also volatilize. These are all permanent losses.
Seed injury from seed-placed nitrogen fertilizer can occur with rates that are too high or when seed bed utilization is low. Nitrogen fertilizers that convert to ammonia in the soil — like urea containing fertilizers and the ammonium portion of ammonium sulphate and ammonium phosphate — have potential to harm seedlings. Small-seed crops like flax and canola can be more susceptible to injury. Sandy soils that are low in organic matter and high pH, carbonated soils increase the risk of injury from nitrogen fertilization, which is exacerbated by dry conditions after application.
Guidelines for seed-placed nitrogen fertilizer rates shouldn’t be seen as set in stone, says Heard, because they largely depend on the moisture in the seedbed. As soils dry out, the same rate of nitrogen fertilizer can cause more stand thinning than under moist conditions. Safe seed-placed nitrogen rates tend to be higher for clay soils than sandy soils and may be more toxic on high pH soils. As soil pH increases, so does the portion of free ammonia (NH3) compared to soil bound ammonium (NH4+).
For a second opinion on safe seed-placed fertilizer options Heard recommends an Excel spreadsheet calculator developed by South Dakota State University (Find this online at www.sdstate.edu/ps/extension/soil-fert, look for “Fertilizer seed decision aid” on the page.)
This spreadsheet calculator allows farmers to assess the level of stand thinning they are willing to accept using different fertilizer types, rates and application systems. Heard used the calculator to assess the stand risk from Manitoba seed-placed nitrogen rate guidelines and found that it predicted higher stand thinning in canola than what is commonly presumed. “This should be of particular concern to canola growers who are shaving seeding rates,” says Heard.
New seeding equipment may also be contributing to thinner stands. Many farmers now use seed drills that place seeds more precisely and have allowed them to reduce seeding rates. “Ten years ago when we were dealing with a small-seeded, open pollinated canola, the seeding rates were seven to eight pounds per acre, and you could probably sacrifice a bunch of that stand and still get good yields,” says Heard. “Now farmers are seeding large-seeded, hybrid canola and they are trying to shave the seeding rates down and are using drills that have even tighter placement. So with all of those things the last thing we need is more stand thinning because of (seed-placed) fertilizer.”
Many studies have been conducted into proper placement, timing and sources of nitrogen, and generally it’s accepted that, to reduce the risk of seedling injury, nitrogen should be placed away from the seed-row. Separation of the seed and fertilizer reduces the concentration of nitrogen near the seed and allows for more effective seed-bed utilization.
Around 75 per cent of nitrogen in Canada is banded. Side or mid-row banding both provide effective placement, although the risk of injury increases with wider row-spacings.
Agrotain is a urease inhibitor that can be added to either urea or UAN and reduces losses of nitrogen to volatilization, denitrification and leaching. Many studies over the past few years have shown that Agrotain provides a safe and effective way to apply starter fertilizer without the risk of significant seedling injury. But Koch Agronomic Services, which recently acquired Agrotain International, issued a press release in April 2012 stating that it no longer recommends the use of Agrotain stabilizer at seeding or in what may be referred to as seed safety fertilizer applications.
“We are concerned that some growers who use the product in such applications (i) may not be observing proper procedures for treatment and handling of their fertilizer, (ii) may not be applying the product or the fertilizer at labeled rates, (iii) may not be taking into account environmental conditions and other factors that affect seedling safety when applying their fertilizer or (iv) assume incorrectly that fertilizer containing Agrotain stabilizer can be substituted for other products used at seeding and/or applied at the same rates as other products. Such misuse of fertilizers can result in damage to seedlings, especially in the current weather conditions,” states the press release.
Controlled release products such as ESN, a polymer coated urea have also been shown to reduce seedling damage from seed-placed nitrogen and provide an additional tool for farmers.
More and more studies are showing that reducing seed-placed fertilizer to avoid seedling injury in sensitive crops like canola and soybeans is contributing to phosphorus depletion.
“In Manitoba we grow high yields of canola and high yields require high rates of nitrogen. There’s no way you can safely seed-apply all the nitrogen required with canola in this province,” says Heard. “Most have recognised that and have made alternative decisions about how to fertilize canola.”
Nitrogen often tends to be applied to compensate for losses. It’s estimated that the use efficiency of applied nitrogen is less than 50 per cent because of losses and immobilization in organic matter through microbial action.
But this nitrogen is not gone forever. Tied-up nitrogen can later be released once again through mineralization over the growing season. Immobilized nitrogen in the soil is not lost, but builds soil organic matter, improves soil quality and provides a buffer to cycle back for following crops in years when yield potential is high.
The key to the recycling of carryover nitrogen in the soil through mineralization is the soil organic matter content. More soil organic matter increases the potential for nitrogen release under the right conditions. Soil organic matter building practices such as reduced tillage and high crop residue return crops increase stored nitrogen. Under good moisture and temperature conditions release of this nitrogen may occur.
There is a need for more research to be done in Western Canada into how management practices and fertilization strategies can make the best use of stored nitrogen and phosphorus across long term rotations. †