CCA - Certified Crop Advisor Fertilizer Sources and Application Questions and Answers 1 — Questions and Answers
Question 1: A producer plans to topdress urea (46-0-0) on an established cool-season grass pasture. Warm, breezy conditions are forecasted with no chance of rain for several days. Which of the following describes the primary nitrogen loss pathway of concern and the best management practice to mitigate it?
- Denitrification of nitrate; incorporate the fertilizer immediately with tillage.
- Leaching of nitrate; apply a nitrification inhibitor with the urea.
- Volatilization of ammonia; use a urease inhibitor or time the application just before a significant rainfall event. (Correct answer)
- Immobilization by soil microbes; switch to a different nitrogen source like ammonium nitrate.
Correct answer: Volatilization of ammonia; use a urease inhibitor or time the application just before a significant rainfall event.
Urea applied to the soil surface is converted to ammonium by the urease enzyme. In warm, moist, and breezy conditions, especially on high-residue surfaces like a pasture, this ammonium can rapidly convert to ammonia gas and be lost to the atmosphere. This process is called volatilization. A urease inhibitor slows the enzymatic conversion of urea, allowing more time for rainfall or irrigation to move the intact urea into the soil profile where the resulting ammonium will be protected from volatilization.
Question 2: Which of the following statements accurately compares the initial soil reaction of monoammonium phosphate (MAP, 11-52-0) and diammonium phosphate (DAP, 18-46-0) when applied as a granular fertilizer?
- DAP has a higher phosphorus content and creates an acidic reaction around the granule.
- Both MAP and DAP have the same neutral effect on soil pH around the granule.
- DAP is less water-soluble than MAP, leading to slower initial phosphorus availability.
- MAP creates an acidic reaction in the soil immediately around the granule, while DAP creates an alkaline reaction. (Correct answer)
Correct answer: MAP creates an acidic reaction in the soil immediately around the granule, while DAP creates an alkaline reaction.
When MAP dissolves, it creates an acidic reaction (pH around 3.5-4.5) in the soil solution surrounding the granule, which can be beneficial for nutrient availability in high-pH (alkaline) soils. Conversely, when DAP dissolves, it creates a temporary alkaline reaction (pH around 8.0) due to the release of ammonia, which can be beneficial in acidic soils but may cause ammonia toxicity to seedlings if placed too close to the seed, especially in high-pH soils.
Question 3: A grower is planting corn into a cool, high-residue, no-till field. A recent soil test indicates low phosphorus (P) levels. Which fertilizer placement method would be most efficient for providing phosphorus to the young corn plant?
- Banding the phosphorus fertilizer 2 inches to the side and 2 inches below the seed at planting. (Correct answer)
- Topdress application of granular phosphorus fertilizer over the growing crop at the V3 stage.
- Broadcast application of phosphorus fertilizer in the fall prior to planting.
- Foliar application of a low-salt phosphorus product at the V5 growth stage.
Correct answer: Banding the phosphorus fertilizer 2 inches to the side and 2 inches below the seed at planting.
Phosphorus is relatively immobile in the soil, and its availability is often reduced in cool, moist, no-till conditions. Banding the fertilizer in a 2x2 placement concentrates this immobile nutrient near the developing root system of the seedling. This ensures the young plant has access to adequate phosphorus for critical early-season growth, which often results in better plant vigor and can lead to increased yield.
Question 4: A grower is selecting a fertilizer for an in-furrow (pop-up) application at planting. Which of the following common fertilizer sources has the highest salt index, posing the greatest risk of seedling injury if applied at excessive rates?
- Monoammonium Phosphate (MAP)
- Potassium Chloride (KCl) (Correct answer)
- Triple Superphosphate (TSP)
- Urea
Correct answer: Potassium Chloride (KCl)
The salt index measures a fertilizer's effect on the osmotic potential of the soil solution. A high salt index can cause 'fertilizer burn' by drawing moisture out of seedling roots. Potassium chloride (0-0-60), also known as muriate of potash, has one of the highest salt indices among common fertilizers (around 116) and must be used with extreme caution for in-furrow applications. By comparison, Urea is high (75), MAP is low (30), and TSP is very low (10).
Question 5: A farmer incorporates a large amount of mature cereal rye cover crop residue, which has a high carbon-to-nitrogen (C:N) ratio, into the soil shortly before planting corn. What is the most likely immediate effect on nitrogen availability for the corn crop?
- A rapid increase in available nitrogen due to fast mineralization of the residue.
- A temporary decrease in available soil nitrogen due to microbial immobilization. (Correct answer)
- No significant change in nitrogen availability, as the corn crop cannot use organic N.
- An immediate increase in atmospheric nitrogen fixation by free-living bacteria.
Correct answer: A temporary decrease in available soil nitrogen due to microbial immobilization.
Residues with a high C:N ratio (typically >25:1 or 30:1) do not contain enough nitrogen to meet the needs of the soil microbes that decompose them. The microbes will take up available inorganic nitrogen (nitrate and ammonium) from the soil to break down the carbon-rich residue. This process, called immobilization, temporarily 'ties up' soil nitrogen, making it unavailable to the cash crop until the microbes die and the nitrogen is re-released (mineralized).
Question 6: What is the primary agronomic advantage of using a chelated form of a micronutrient, such as zinc EDTA, compared to an inorganic salt form, like zinc sulfate, particularly in high pH or high phosphorus soils?
- Chelated forms are significantly less expensive per pound of actual nutrient.
- The chelate lowers the soil pH in the application zone, increasing general nutrient availability.
- Chelated forms provide a much slower, season-long release of the nutrient.
- The chelate protects the nutrient ion from being tied up by adverse soil reactions, keeping it available for plant uptake. (Correct answer)
Correct answer: The chelate protects the nutrient ion from being tied up by adverse soil reactions, keeping it available for plant uptake.
A chelate is a large organic molecule that surrounds and chemically bonds to a metallic micronutrient ion (like Zn++, Fe++, Mn++). This protective 'claw' prevents the positively charged nutrient from reacting with negatively charged soil components (like hydroxide or phosphate ions), which are abundant in high pH or high P soils and would otherwise cause the micronutrient to precipitate and become unavailable. This protection keeps the nutrient soluble and available for plant root uptake.
CCA - Certified Crop Advisor Fertilizer Sources and Application Questions and Answers 1
A producer plans to topdress urea (46-0-0) on an established cool-season grass pasture.
Warm, breezy conditions are forecasted with no chance of rain for several days.
Which of the following describes the primary nitrogen loss pathway of concern and the best management practice to mitigate it?