Nutrient Sources and Management Flashcards

An applicator is choosing a potassium (K) source for a crop that is sensitive to chloride and is being grown in soil that also requires sulfur. Which of the following fertilizers would be the most suitable choice?

Answer: Potassium sulfate (SOP)

Potassium sulfate (K2SO4), also known as SOP, provides both potassium and sulfur (about 18%) and has a very low chloride content, making it ideal for chloride-sensitive crops. Potassium chloride (MOP) is the most common and cheapest K source but has a high chloride content. Potassium nitrate supplies nitrogen and potassium but no sulfur. Potassium thiosulfate is a liquid source of K and sulfur but SOP is a more common granular choice for broadcast applications addressing both needs.

A farmer plans to use manure from their livestock operation as the primary nutrient source for a corn crop. What is the most critical first step to ensure accurate nutrient application and avoid environmental losses?

Answer: Sending a representative sample of the manure to a laboratory for nutrient analysis.

Manure nutrient composition is highly variable due to factors like animal diet, bedding, and storage methods. Therefore, the most critical step for accurate nutrient management is to have the specific manure analyzed by a laboratory. Relying on book values can lead to significant over- or under-application of nutrients. While a crop advisor is a valuable resource, they will need an analysis for a precise recommendation. Applying at the highest rate without knowing the content risks significant environmental pollution.

Which of the following is a primary advantage of using chelated micronutrients, particularly in high-pH (alkaline) soils?

Answer: They protect the micronutrient from being tied up and made unavailable in the soil.

In high-pH soils, micronutrients like iron, zinc, and manganese can become insoluble and unavailable for plant uptake. Chelates are organic molecules that bind to the micronutrient, protecting it from these soil reactions and keeping it in a plant-available form. While some slow-release products exist, the primary function of chelation is availability, not release timing. Not all chelates are organic-certified. They are specifically formulated to supply micronutrients, not secondary macronutrients.

An applicator uses Diammonium Phosphate (DAP) with an 18-46-0 analysis. What two primary macronutrients is this fertilizer supplying?

Answer: Nitrogen and Phosphorus

Fertilizer analysis is represented as N-P2O5-K2O. For DAP (18-46-0), the '18' represents 18% Nitrogen (N), the '46' represents 46% Phosphate (P2O5), and the '0' represents 0% Potash (K2O). Therefore, DAP is a source of both Nitrogen and Phosphorus.

When comparing Urea (46-0-0) and Ammonium Sulfate (21-0-0) as top-dress nitrogen sources, what additional essential nutrient does Ammonium Sulfate provide that Urea does not?

Answer: Sulfur (S)

Ammonium sulfate ((NH4)2SO4) is a source of both nitrogen (21%) and sulfur (approximately 24%). This makes it a good choice for soils or crops that are deficient in sulfur. Urea (CO(NH2)2) is a highly concentrated nitrogen fertilizer but does not contain sulfur.

A grower is applying a liquid fertilizer through their irrigation system (fertigation) to quickly correct a nutrient deficiency identified mid-season. Which form of nitrogen is most immediately available for plant uptake but also most susceptible to leaching?

Answer: Nitrate (NO3-)

The nitrate (NO3-) form of nitrogen is highly soluble in water and is immediately available for plants to absorb. However, because it is an anion (negatively charged), it is not held by negatively charged soil particles and can be easily leached below the root zone with excess water. Ammonium is held by the soil, and amide and organic forms must first be converted by soil microbes into ammonium and then nitrate.