If there was a throne meant for the king of plant nutrients, I bet nitrogen will be sitting on it, unopposed. The nutrient is by far the most important in crop production. It’s also the most studied, and the most talked about. I’ve even found several songs dedicated to it, from pop to rap…that’s quite cool. What is not cool, however, is the fact that it is difficult to get nitrogen to stay where you want it to – close to plant roots. It seems to always be on the move, change forms and find pathways to leave the soil. Knowing these forms and pathways is important in minimizing nitrogen loss.
When nitrogen sources such as urea, anhydrous ammonia or manure are applied to the soil, they rapidly convert to the ammonium form. Ammonium (NH4+) is positively charged and can, therefore, be held tightly to the surfaces of soil or organic matter, which are mostly negatively charged (opposites attract; likes repel). But, under favourable conditions, soil bacteria convert ammonium to nitrites and finally to nitrates (NO3–). Nitrate is negatively charged and is repelled by the surfaces of soil and organic matter and therefore susceptible to leaching – movement of nitrate below the plant’s root zone by percolating water. Leaching is more prevalent in coarse-textured soils such as sandy soils because these soils have a lower water holding capacity. Nitrate-containing fertilizers, such as urea ammonium nitrate (UAN) and ammonium nitrate, are susceptible to leaching loss as soon as they are applied.
When soils are very wet or waterlogged for a couple of days, soil microbes starved of oxygen will strip the nitrate molecule of its oxygen. With the oxygen stripped from the nitrate, the remaining nitrogen is ultimately lost to the atmosphere as nitrogen oxides and dinitrogen gas. The process is called denitrification. Denitrification rates can range from 5 to 20% of applied nitrogen. Denitrification can be significant when nitrogen is applied in the fall, before a wet spring. It most commonly occurs in heavy clay soils because of poor drainage.
Urea-based nitrogen fertilizer products such as UAN, or dry urea are susceptible to ammonia volatilization if surface-applied and not incorporated. Ammonia is an intermediate form of nitrogen during the process in which urea is transformed to ammonium by urease enzymes. The risk of volatilization loss is high in moist soils and increases with temperature, soil pH and windspeed. Up to 64% of applied N can be lost as ammonia.
Plants take up nitrogen from the soil solution mainly as nitrates and ammonium ions. If you can delay or prevent ammonium from converting into nitrate, you will reduce nitrogen loss by leaching or denitrification. Nitrogen stabilizers can help with that. Some stabilizers can also slow down the conversion of urea to ammonium which allows more time for the nutrient to move into the soil, thereby reducing loss through volatilization. Your SynergyAG rep can help you pick out the right products.
-Ikenna Mbakwe, PhD, PAg
Head of Research