Practice Questions more exam 2

Which of the following best describes the process of "Mineralization"?

Microbial breakdown of organic nitrogen into ammonium (NH₄⁺).

What is the primary role of Rhizobium bacteria in soil fertility?

They fix atmospheric nitrogen into a form usable by legume plants.

Which environmental condition most strongly inhibits biological nitrogen fixation in legumes?

Low soil pH (acidic conditions) or lack of Molybdenum.

What is the chemical process called when Ammonium (NH₄⁺) is converted to Nitrite (NO₂⁻) and then to Nitrate (NO₃⁻)?

Nitrification

Which two genera of bacteria are primarily responsible for nitrification?

Nitrosomonas and Nitrobacter

Why does nitrification generally increase soil acidity?

It releases hydrogen ions (H⁺) as a byproduct of oxidizing ammonium.

What is "Immobilization" in the context of soil nitrogen?

The process where microbes consume available soil nitrogen to decompose high-carbon organic matter.

Which crop residue is most likely to cause temporary nitrogen deficiency due to immobilization?

Corn stalks or wheat straw (high C:N ratio).

What is the primary form of nitrogen fixed by lightning?

Nitrate (NO₃⁻)

Which factor most limits the rate of nitrogen mineralization from organic matter?

Soil temperature and moisture.

What is the primary reason Phosphorus has such low mobility in soil?

It binds strongly to soil particles (fixation) and is present in very low concentrations in the soil solution.

Which ion form of Phosphorus is primarily absorbed by plant roots in acidic soils (pH < 6.5)?

H₂PO₄⁻

In alkaline soils (pH > 7.5), which element primarily binds with Phosphorus, reducing its availability?

Calcium (Ca)

What is the typical concentration of Phosphorus in the soil solution?

0.01–1.0 ppm

Why is "Total Soil P" a poor indicator of plant-available phosphorus?

Total P includes vast amounts of insoluble, mineral-bound, and organic P that plants cannot access.

Which soil amendment can help increase Phosphorus availability in acidic soils?

Adding Lime to raise pH toward neutral (6.0–7.0).

What is the role of Mycorrhizal fungi in phosphorus uptake?

They extend the root system’s reach, accessing phosphorus in soil volumes roots cannot reach.

Which of the following is a major source of organic phosphorus in the soil?

Decomposing plant residues and manure.

What is "Phosphate Rock" primarily composed of?

Apatite (calcium phosphate minerals).

Why is Phosphorus loss via leaching generally considered negligible in most agricultural soils?

Phosphorus is highly insoluble and strongly adsorbed to soil particles.

Which fertilizer has the highest nitrogen concentration by weight?

Anhydrous Ammonia (82-0-0)

Why is Anhydrous Ammonia (82-0-0) typically injected deep into the soil (2–8 inches)?

To prevent volatilization and ensure it is placed in moist soil for dissolution.

What is a key advantage of using Ammonium Polyphosphate (APP) liquid fertilizers?

They can keep micronutrients like Zinc in solution, preventing precipitation.

Why might a farmer choose Monoammonium Phosphate (MAP, 11-52-0) over Diammonium Phosphate (DAP, 18-46-0) for starter fertilizer?

MAP creates a less alkaline reaction near the seed, reducing the risk of ammonia injury.

What is the function of a Urease Inhibitor (e.g., Agrotain) when applied to surface-applied urea?

It prevents the conversion of urea to ammonium, reducing ammonia volatilization.

Which fertilizer is a liquid solution of urea and ammonium nitrate?

UAN (Urea Ammonium Nitrate)

What is a potential risk of applying Diammonium Phosphate (DAP) directly in contact with germinating seeds?

It can release ammonia gas and raise local pH, injuring the seed.

Which manure type typically has the highest phosphorus content per ton?

Broiler Poultry Manure

Why is laboratory analysis of manure recommended before application as fertilizer?

Nutrient content varies significantly based on animal diet, bedding, and storage.

What is the primary benefit of using cover crops in nitrogen management?

They scavenge residual nitrate from the soil, preventing leaching over winter.

What does a "Soil Test" for Phosphorus actually estimate?

Potentially available phosphorus (solution + easily desorbable/extractable P).

Which soil test extractant is a "weak acid" buffer using sodium acetate and acetic acid?

Morgan Extractant

Why is the Morgan extractant considered a "universal" extractant?

It was designed to work reasonably well across a wide range of soil pH levels.

What is the main difference between Mehlich I and Mehlich III extractants?

Mehlich III is a stronger extractant that can handle soils with higher carbonate content and extracts more nutrients.

Why must soil test P values from different extractants (e.g., Mehlich I vs. Mehlich III) not be directly compared?

They extract different pools of phosphorus and have different correlation curves with crop yield.

A soil test indicates a high Cation Exchange Capacity (CEC). What does this primarily indicate about the soil’s nutrient management?

It has a strong ability to hold positively charged nutrients like ammonium and potassium.

Which of the following best describes "Net Immobilization" in soil fertility?

Microbes consume soil nitrogen to decompose organic material with a high C:N ratio.

Why is Phosphorus (P) considered an "immobile" nutrient compared to Nitrate (NO₃⁻)?

P diffuses only a very short distance (approx. 0.02 cm) from the fertilizer source.

A farmer applies Urea (46-0-0) to the soil surface during warm, humid weather without incorporation. Which loss pathway is most significant?

Volatilization

Which form of Nitrogen is generally preferred by most crops for uptake, and what is its charge?

Nitrate (NO₃⁻), negatively charged.

In acidic soils (low pH), which elements are primarily responsible for "fixing" (binding) Phosphorus, making it less available to plants?

Iron and Aluminum

What is the primary environmental risk associated with Phosphorus loss from agricultural fields?

Surface water eutrophication (algae blooms) via erosion and runoff.

Why is a "Starter Fertilizer" often placed in a band near the seed rather than broadcast widely?(phosphorus)

To take advantage of Phosphorus’s low mobility and ensure roots encounter it early.

Which soil testing extractant is described as a "buffered acid" that extracts multiple nutrients and is widely used in the Eastern US?

Mehlich III

A crop removes 150 lbs of Nitrogen per acre. The soil test suggests the soil can supply 40 lbs of Nitrogen via mineralization. How much Nitrogen fertilizer should be applied to meet the crop's full demand?

110 lbs

What happens to soil pH when a plant absorbs Ammonium (NH₄⁺) as its primary nitrogen source?

The soil pH decreases (becomes more acidic).

Which of the following conditions most promotes Denitrification (loss of N as gas)?

Saturated, anaerobic soils with high organic matter.

What is the main reason that "Total Soil Phosphorus" is not a useful metric for determining fertilizer recommendations?

Total P includes forms that are insoluble and unavailable to plants.

Which fertilizer contains both Ammonium (NH₄⁺) and Nitrate (NO₃⁻) and provides immediate nitrogen availability?

Ammonium Nitrate

Why might a farmer use a Nitrification Inhibitor (e.g., N-Serve) with ammonium-based fertilizers?

To slow the conversion of ammonium to nitrate, reducing leaching and denitrification risks.