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Fertilizer toxicity can kill a seed

Too much fertilizer too close to the seed can create a toxic environment. Here’s how to recognize the problem if you have it

photo: patrick mooleki, saskatchewan agriculture

The last thing any farmer wants to do is create a toxic environment at seeding time by applying too much fertilizer so close to the seed that it prevents normal germination and establishment.

Fertilizer is not the only culprit when it comes to poor emergence. Seeding too deep, poor seed quality, disease, insects, dry soil or flooding can cause problems for seed emergence. However, with modern seeding equipment and one-pass seeding regimes, it pays to be aware of the havoc toxicity from fertilizer can wreak.

Most nitrogen used in Western Canada is applied in the form of either granular urea, compressed anhydrous ammonia (AA) gas or liquid urea ammonium nitrate (UAN). When anydrous ammonia is injected into the soil, the pressure is lost and the AA reverts to its gaseous form, releasing the ammonia (NH3) into the soil. Ammonia reacts with water in the soil to form ammonium ions (NH4+) that adhere to soil organic matter and soil particles.

Both ammonia and ammonium ions at high concentrations are very toxic to seed and seedlings. And so, AA must be banded into moist soil and the furrow must be closed to prevent ammonia loss. If side-banding, ensure that there is good separation between the AA and the seed row which should prevent ammonia from moving into the seed row and injuring seed or seedlings.

When urea is applied to the soil, it is transformed by enzymatic reactions to form ammonia gas, which reacts with water to form ammonium ions — as in the case of AA. However, urea is also a salt, which poses another danger to seed and seedlings: salt toxicity.

If a high rate of urea is applied in the seed row, seed mortality can be increased due to high salt concentrations. The seed and seed-placed fertilizer are competing for moisture around the seed, with the seed likely to lose the battle and become dehydrated or even killed. Seed-placed urea poses risks of salt, ammonia and ammonium toxicities to seed and seedlings.

Liquid UAN is 50 per cent urea and 50 per cent ammonium nitrate. The urea half behaves as already described with regards to formation of ammonium and the salt toxicity. The ammonium nitrate portion separates into the ammonium ion (NH4+) and the nitrate ion (NO3-). Studies show that nitrates at the typical concentrations of applied fertilizers are not as toxic to seeds and seedlings. Consequently, if UAN is placed close to the seed it may also pose salt as well as ammonium toxicity to seeds and seedlings.

Signs and symptoms

Typical signs of fertilizer toxicity are poor germination, large gaps in the rows and poor-looking seedlings. If you dig into the soil, you will find swollen dead seeds or seeds with very short or no radicles at all if these problems are due to fertilizer toxicity. If the seed did imbibe enough moisture to start germination and a radicle and a plumule formed, you will find brownish roots with stubby or blunt tips. In healthy plants, the roots should be whitish, narrow and long.

This image shows hot spots (gaps) in the rows and not so hot areas with better-looking plants.
This image shows hot spots (gaps) in the rows and not so hot areas with better-looking plants.

Due to poor root formation or lack of root formation, the shoot that forms runs out of nutrients and energy and may fail to push through the soil. The extent of the damage depends on the concentration of the salts in the soil. The seed may germinate and shoots might be able to emerge but they’ll look as if they were trampled on and there can be gaps in the field or areas with weak, sickly plants.

If it rains enough before the seeds die, the salt concentration is reduced and the damage minimized. If the damage is severe, there are no effective remedies, except possibly reseeding if there is sufficient time left to do so. There may be a significant volunteer problem. However, there are strategies that can help growers prevent this sort of damage in the first place.

Toxicity prevention

“Farmers should be aware of the guidelines for seed placed fertilizer,” explains Patrick Mooleki, provincial soil and nutrient management specialist for Saskatchewan Agriculture. “We have fact sheets that provide guidelines for safe rates of seed-placed fertilizers, which are dependent on soil moisture and soil texture.”

Farmers are advised to seed-place only safe rate of fertilizer based on the crop, soil texture, soil moisture, width of seed row openers and row spacing as stipulated by the guidelines. The rest of the fertilizer must be applied away from the seed either before or after seeding. During seeding, fertilizer should be applied in the side-band or in the mid row, depending on the equipment at hand. Farmers who want to apply more fertilizer with the seed should consider replacing part of the urea with a polymer-coated fertilizer such as ESN. Consult suppliers for guidelines.

Generally, if the season starts off very dry, it would be beneficial to reduce the amount of seed-applied fertilizer. “What a farmer might have applied in a previous season with the seed is not necessarily going to be the right amount to apply in the next season,” explains Mooleki. “Soil moisture needs to be assessed every year.”

Consider rates carefully in soils with low organic matter. Organic matter holds onto moisture and can be helpful in reducing both ammonium and salt toxicity of fertilizers to seeds. Light textured sandy soils don’t hold moisture as well as heavy clay soils, which offer more protection against ammonium toxicity.

“Seed mortality due to fertilizer toxicity is a huge issue,” says Mooleki. “It is something I try to raise awareness of in my work with farmers.” In fact, questions about fertilizer toxicity are some of the most frequently asked at Saskatchewan Agriculture’s Agriculture Knowledge Centre. Saskatchewan Agriculture demonstrated what happens with high fertilizer rates at its Crop Diagnostic Schools in 2013.

“We had plots in Swift Current and Indian Head to show that this damage can be significant, and how side-banding and the use of ESN can help reduce the damage when high rates are used,” says Mooleki. “In 2014 we are planning to include information on fertilizer rates at our Crop Diagnostic Schools (CDS) to be held at Scott and Melfort, Sask., but we won’t be focusing on fertilizer toxicity. Instead, for the soils segment of the CDS, we will be focusing on nutrient deficiencies and soil factors that limit crop productivity such as salinity and sodicity.”

In addition to nitrogen fertilizer toxicity, farmers should also be aware of toxicities that can be caused by phosphorus, potassium and sulphur fertilizers.

Polymer-coated fertilizers

“ESN is a polymer-coated urea granule,” says Ray Dowbenko, senior specialist, agronomic services at Agrium. “The coating acts to slow the reaction of the urea granule with moisture in the soil and in doing that, it diminishes the free ammonia and salt problems that can damage seed. Additionally, the slow release of nitrogen coincides better with the growing seedlings’ demand for nitrogen — the seed has on-board stores to power germination. As those stores run out, nitrogen from the ESN granule has, and is slowly becoming available. There is much less risk of damage to the seed or the concern of nitrogen getting washed away if ESN is used.”

ESN has more than 15 years of peer reviewed, vetted seed safety research behind it. “The first research trials were conducted in 1997, and it was commercialized in Canada in 2005,” says Dowbenko. “The guidelines that Agrium recommends for ESN applied with the seed are very conservative. Farmers can be assured that at the rates we provide guidance on, while they may seem high if one was thinking of straight urea, are well and truly tested and established in peer reviewed research.”

With uncoated, regular urea, seed safety is further compromised when soil is dry. ESN in dry soil, on the other hand, does not compete with the seed for what little moisture there may be in the same way. “The coating on the granule separates the seed from the fertilizer, and also reduces urea from reacting with moisture in the soil directly, acting as a barrier,” explains Dowbenko. Only when moisture crosses that barrier will the urea be able to react and release nitrogen.”

According to Dowbenko’s work with Agriculture and Agri-Food Canada and Alberta Agriculture, if growers use 100 per cent ESN fertilizer as their nitrogen source, they are able to exceed their provincial safe rate guidelines up to three times, and up to 1.5 times in a 50:50 ESN:urea blend. Dowbenko also advises growers to understand limitations in their own operations such as row spacing, row width, soil texture and soil moisture, as well as individual crop sensitivities.

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