By Daniel Hudson, and Heather Darby UVM Extension Agronomists
In times of high fertilizer costs there is increasing interest in using various fertilizer adjuvants. The goals of the adjuvants are to enhance the effectiveness of a fertilizer and to reduce fertilizer losses to the environment. Agronomists often get asked about whether fertilizer adjuvants are worthwhile. It will probably come as no surprise that research has shown that some are economically beneficial sometimes, others rarely, and some almost
Here are a few beginning principles:
- Products that you are considering should be backed by rigorous objective research. Research reports should include exactly how, when, where, and by whom the research was conducted, along with contact information and disclosures of conflicts of interest.
- Products should have a proposed mechanism that is accepted by experts in the field. In this case, unbiased soil chemists ought to be able to explain how/why/when the product works.
- Fertilizer additives should NEVER displace good management practices.
Nitrification inhibitors such as nitrapyrin (N-Serve® and Instinct®) and cyanoguanidine (Super-U® and Guardian®) slow the conversion of ammonium (NH4+), which is held by soil particles, to nitrate (NO3–), which is easily leached below the reach of plant roots. Since nitrification inhibitors influence only the conversion of NH4+ to NO3–, they only protect the NH4+portion of the fertilizer product. While these products do slow the nitrification process, yield results have been inconsistent among studies. Reasons for this inconsistency may include:
- The duration of efficacy might not be long enough and nitrification eventually happens anyway.
- Leaching does not happen to the same extent in all soils, so nitrate leaching may be minimal even if nitrification occurs.
- Nitrogen may not have been limiting in some or all years.
- Drought conditions may have obscured potential nitrogen effects.
In well-drained soils, these products seem to reduce leaching in some cases. In poorly drained soils, preserving nitrogen in the ammonium form could slow the denitrification process (where soil microbes remove the oxygen from NO3– and the N portion of that molecule is released as N2).
Urease inhibitors such as Agrotain® are intended to inhibit the ability for urease (an enzyme found in the soil and on plant surfaces) to degrade urea. When urease is fully active, it causes ammonia to be released in a process commonly called volatilization. If urea is incorporated into the soil by tillage or by about ½ inch of rain, volatilization losses can almost be eliminated without the use of a urease inhibitor. Urease inhibitors will be most effective in no-till situations or in instances where urea is surface-applied and rain is not expected for several days. Volatilization losses are favored by heat, wind, moist soils, and alternating wetting-drying cycles. An Illinois study where conventional tillage was used demonstrated a three bushel per acre benefit over 12 years. A no-till study in Kentucky where urea + Agrotain® was used demonstrated an 11 bushel/acre yield benefit over 12 years.
A family of products from Specialty Fertilizer Products (SFP) includes NutriSphere-N®, Avail-P®, and More than Manure® (MTM®). These products are polymers of maleic and itaconic acid. The company suggests that they tie up copper and nickel ions in the soil, ions which are necessary for the function of enzymes involved for urease activity and denitrification. They also suggest that the product binds calcium ions in the zone around the fertilizer particle that could potentially immobilize the phosphorus. At this point, objective data that suggests that these products have a significant positive effect on yield is nearly nonexistent, while studies that suggest no yield effect are more numerous.
Fertilizer application timing, source, application method, soil texture, and tillage are some of the factors that should be considered to determine whether a nitrification or urease inhibitor may be beneficial in a given situation. Before buying a product claiming to be a fertilizer enhancing adjuvant, be sure its efficacy is supported by solid scientific data. If you decide to try one of these products, consider contacting a UVM Extension Agronomist to help you set up a statistically valid research plot on your farm.