Jarrod Miller, Extension Educator, Somerset County
Nitrogen is a tricky nutrient, it can exist in several different forms in the soil. The plant available forms are nitrate (NO3) and ammonium (NH4), and farmers have to find methods to keep those forms near plant roots. A recent review article in the journal Nature Plants discussed how plants may manage nitrification through root exudates. Nitrification is the transformation of NH4 to NO3 (again, both are plant available, so its OK). This is an issue for farmers because NO3 leaches out of the root zone easier than NH4. Researchers have observed that field crops like sorghum, rice and wheat may all produce biological nitrogen inhibitors (BNI) to prevent the transformation of NH4 to NO3. In the case of wheat, the authors were hopeful that the trait in two cultivars could be bred into other wheat, increasing their nitrogen use efficiency. This may become another tool for field crop production one day, along with cover crops, sidedressing and no-till in methods to manage nitrogen.
While BNI’s may come around in the future, there are nitrogen additives available that perform a similar task. Nitrification is driven by soil biology, and we have had a bactericide called nitrapyrin (N-Serve, Instinct) available for many years. Another chemical called dicyandiamide (Agrotain Plus, Super U, Guardian) prevents the an enzymatic reaction that convert NH4 to NO3. Both of these products are proven to reduce nitrification, but if you don’t have the conditions that promote nitrification, their added cost may not be useful. In Arkansas soil type has played a role in the effectiveness of these additives, and may need more research to be understood. The University of Arkansas also suggests that split application of N reduces the need for these additives, although sandy soils and pre-plant N may benefit. You can call your county extension office to see if these products may benefit your soil types and situations.
Another product you may be familiar with is N-(n-butyl) thiophosphoric triamide (NBPT), also know by the trade name Agrotain. Since NBPT went off patent in 2002, I have given you the scientific name in case you want to check your labels. NBPT performs a different task. Instead of denitrification, we are worried about ammonia (NH3) volatilization. That one additional hydrogen (NH3 + H = NH4) makes a big difference. Ammonium (NH4) can attach to soil particles, but ammonia (NH3) is a gas that can volatilize from your soils. Due to our increased use of urea based fertilizers, NBPT is a popular additive that the slows the conversion of urea into NH4. Again, while it is proven to work, you should only use NBPT if you need it. If urea is incorporated into most soils within two days, by either rainfall or tillage, there is less need for NBPT. For clay soils you may be able to wait up to 7 days for rainfall and not lose as much N from the soil. Arkansas also suggested that ammonium sulfate is as effective as NBPT +urea. Ammonium sulfate promotes more acidic soils, which helps maintain N as NH4, and not the gas NH3.
While all of this nomenclature may be confusing, it is good practice to be able to recognize forms of N in your soil tests and fertilizer labels:
- Ammonium: NH4
- Nitrate: NO3
- Urea: CO(NH2)2
References:
University of Arkansas, Nitrogen Fertilizer Additives:https://www.uaex.edu/publications/PDF/FSA-2169.pdf
Nature Plants: https://www.nature.com/articles/nplants201774
