Another installment in the 4 Rs of crop nutrition
When asked if urea is superior to ammonium nitrate, I usually respond with one of three words—yes, no or maybe. Last issue, I mentioned the 4 Rs of crop nutrition currently being promoted by the plant food industry. The Right source at the Right rate, Right time and Right place. Although the four are completely interconnected, let’s discuss the right source. I’ll use common nitrogen sources as a brief example. Other nutrients have similar but unique discussions of their own. All major nitrogen materials can be effective, if used in a wise and material-specific manner.
Anhydrous ammonia contains the highest nitrogen analysis available (82 percent). It is consistently the lowest cost nitrogen fertilizer in our system. Anhydrous turns gaseous at atmospheric pressure and must be incorporated into the soil to avoid volatile losses. Application on rocky, cloddy, or excessively dry/wet soils may induce volatility. The obvious rule of thumb is that if you can see it or smell it, adjust your equipment or wait until better application conditions exist.
When applied properly, anhydrous is extremely efficient. It converts to plant-available ammonium rapidly upon contact with the soil. Its incorporation requirement advantageously places it in the root zone near crop roots. For a few weeks post application, anhydrous reduces soil bacterial populations responsible for nitrification. It therefore provides some “stabilization” properties which can lower potential nitrate losses.
Depending upon crop and weather conditions, anhydrous ammonia can be applied in the fall, spring or at sidedress during the growing season. The nitrification inhibitor N-Serve (Nitrapyrin) is a wise choice to delay fall and early spring applied anhydrous ammonia from nitrifying.
At 46 percent nitrogen, urea is the highest analysis dry nitrogen fertilizer available. This generally lowers its cost compared to other dry nitrogen fertilizers. Urea must be converted to ammonium before becoming available to plants. Some urea naysayers feel that a delay in ammonification makes urea an inferior fertilizer product. In reality, urease, the enzyme responsible
for converting urea to ammonium is one of the most common enzymes present throughout nature. Our soils and crop residues easily supply enough urease to rapidly convert urea to ammonium within a few days of application.
Urea’s biggest negative is that during the urease hydrolysis process, gaseous ammonia forms, making volatility losses possible. In cool weather (reduced urease activity) or when rainfall/irrigation moves urea below the soil surface, volatility becomes minimal. Under high urea volatility conditions (70+ degree temperatures, wet-drying soils and heavy surface residue), urea can be protected using the urease inhibitor Agrotain.
Unlike urea, ammonium nitrate contains nitrogen in the two forms that are immediately plant-available. It is also not subject to volatility losses. Since it is only 34 percent nitrogen, it is generally higher priced than urea. Under high rainfall/soil saturation conditions, the nitrate component of ammonium nitrate can be lost immediately via leaching or denitrification. I generally use this as a counter balance to the negatives associated with urea. Like most cropping decisions, the environment dictates which nitrogen material will perform best.
Urea ammonium nitrate (UAN)
UAN generally contains 28 or 32 percent nitrogen. It is a liquid material made from mixing urea, ammonium nitrate and water. Therefore, it possesses the strengths and weaknesses of both materials. Being a liquid, it fits weed-and-feed programs. Liquids also work well with knife delivery, especially during sidedress application. Some producers believe rate calibration and delivery accuracy are superior with liquid materials.
However, liquid materials do have a few negatives. You are transporting a lot of water, which often increases the cost per unit of nitrogen. In no-till or heavy surface residue situations, broadcast liquid
nitrogen materials can get captured and “tied-up” by residue before reaching the soil.
Ammonium sulfates/ ammonium phosphates
Ammonium sulfate, diammonium phosphate (DAP), mono ammonium phosphate (MAP), and the ammonium polyphosphates all provide plant-available nitrogen. Since they contain relatively low nitrogen contents (11 to 21 percent), their cost per pound of nitrogen can be limiting. Therefore, we recommend using them to meet crop phosphorus or sulfur needs. In most cases we credit the nitrogen in these materials toward the total crop need and supply the majority of the nitrogen through one of the higher analysis fertilizers.
When selecting a fertilizer material, consider their physical and chemical properties, combined with environmental
conditions present and the other 3 Rs. Visit your MFA Certified Crop Adviser to help evaluate which nitrogen material fits each field, each year, within your farming operation.
Paul Tracy is Director of Agronomy for MFA Incorporated.
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