Fertilizer Toxicity
Fertilizer toxicity is an issue that is important for all farmers who use a one-pass seeding system, possibly an issue they don't realize is affecting them.  Blame to poor emergence is typically blamed on seeding too deep, poor moisture conditions, weed pressure, and other effects, but producers seldom look at how they are placing their seed and fertilizer in one operation, and the interaction between the two during germination and emergence.  Producers must gain a good understanding of the benefits and dangers of seed & fertilizer placement to avoid an environment that is toxic to the crop.
Toxicity of Fertilizer
All fertilizer salts are toxic to germinating seeds and to plant roots if applied in sufficient concentration near or in contact with the seed. Fertilizers vary in toxicity per unit of plant nutrient due to:
  • differences in the amount of salts contained in the fertilizer,
  • differences in the solubility of the salts in the soil,
  • the presence of specific materials or elements that are particularly toxic (for example, ammonia and boron).
  • Many nitrogen fertilizers, although they have a relatively low salt index, release free ammonia into the soil.
Information courtesy of the Ontario Ministry of Agriculture, Food, and Rural Affairs
Nitrogen Fertilizers
Various fertilizers differ in toxicity. Ammonium nitrate, monoammonium phosphate and ammonium sulphate are similar in toxicity and much safer than anhydrous ammonia, aqua ammonia or urea. Diammonium phosphate is more toxic than monoammonium phosphate but less toxic than urea. More care should be taken, particularly with sensitive seeds such as beans or peas, and on coarse-textured soil (sand and sandy loam), than is required with ammonium nitrate or monoammonium phosphate.
  • Because anhydrous ammonia and aqua ammonia are extremely toxic fertilizers, they should not be placed near seeds. It is preferable to make preplant applications crossways to the direction in which the crop will be planted. Stand reductions may still occur over the band in very dry soils or if planting takes place too soon after application. 
  • Urea is toxic when banded with or near the seed but is safe when broadcast at rates normally used.
Information courtesy of the Ontario Ministry of Agriculture, Food, and Rural Affairs

Nitrification
All nitrogen fertilizers begins form conversion and diffusion from the time it is placed into the soil. It is important to understand how each form in this conversion process affects the plants.
This process, called nitrification, is summarized in the stages below.
  • The initial form of nitrogen fertilizer is ammonia (NH3). In this form, it is very toxic to seed and plant, but not very mobile in the soil.
  • Ionic transfer from water and clay particles converts ammonia to ammonium (NH4). Ammonium is not as toxic as ammonia, but can cause seedling damage at too high of a concentration. Ammonium sensitivity is dependent on plant species. Ammonium is also not very mobile in the soil.
  • Microbial activity in the soil will cause conversion of ammonium to nitrate (NO3). Nitrate is the most effective form of nitrogen for plant safety and supporting robust plant development. Nitrate is very mobile in the soil.
Fertilizer Toxicity with Side-banded Fertilizer
In this example, fertilizer has been side-banded with the seed. As soon as the seed is in the soil, it begins absorbing moisture and whatever nutrients are nearby. Within 24 hours, the band of ammonia and ammonium (shown in red) will grow to typically 3” in diameter in normal conditions, and larger in dry, highly fractured or sandy soil. Absorbing ammonia (NH3) can severely injure the seed. Absorbing ammonium (NH4) can lead to development impairment and reduced plant vigor.
Minimize Risk of Fertilizer Damage
One-pass seeding with Bourgault Mid Row Banders® will place the nitrogen, regardless of form, far enough from the seed to protect it from the toxic effects of ammonia and ammonium, but well within reach of the plants to obtain the beneficial nitrate form.