Soil Fertility Quiz #7

Urea (46-0-0) is surface-applied to a soil with pH 8.2 during a warm, moist week without incorporation. What is the primary risk?

Urease enzymes will hydrolyze the urea into NH3 gas, which will volatilize.

Which step is the rate-limiting step of nitrification?

The conversion of NH4+ to NO2- by Nitrosomonas.

In an Indiana no-till corn system with heavy residue, which liquid UAN application method is most efficient?

Injected at planting

Which material causes the most significant nitrogen depression (immobilization) if added to soil?

Sawdust (C:N 400:1)

What is the primary advantage of Sidedress (V4-V8) application in corn?

It aligns nitrogen with the 60-70% rapid uptake phase.

How does Polymer-Coated Urea (PCU) release nitrogen?

Water diffuses in, dissolves urea, and N diffuses out based on temperature.

What is a quality trade-off of over-applying N in horticulture?

Nitrate accumulation and delayed fruit maturity.

A corn grower in a high-rainfall region on a sandy loam soil (low organic matter) reports interveinal chlorosis on the youngest leaves. Despite applying 180 lbs of N as urea, the crop remains stunted. Based on the mobility of sulfur in the soil and plant, what is the most likely explanation for these symptoms?

Sulfate has leached below the root zone due to the soil's low AEC and high percolation.

What biological or chemical process must occur before elemental sulfur (S⁰) applied as a fertilizer can be taken up by plant roots?

Microbial oxidation to SO₄²⁻ driven by organisms such as Thiobacillus spp.

Atmospheric wet deposition of sulfate has decreased significantly across the United States between 1985 and 2018. What is the primary implication of this trend for modern agronomic fertilizer programs?

The risk of sulfur deficiency has increased, particularly in high-yielding cropping systems.

A vegetable grower is producing onions and garlic. Why is maintaining adequate sulfur consumption more important for these specific crops compared to a crop like wheat?

Sulfur-containing oils and compounds like allicin are responsible for the characteristic pungency and aroma.

A soil sample was collected from a field, but because of a delay, it sat in a warm, moist truck over the weekend before being dried and sent to the lab. How will this most likely affect the sulfate-S soil test results?

The results will be falsely high due to continued mineralization during the delay.

A soybean producer notices chlorsis and necrosis appearing on the margins of lower leaves. Based on the mobility of potassium in the plant, why do these symptoms appear on older tissue first?

Potassium is mobile in the plant and is translocated from old leaves to new growth during deficiency.

Buffering capacity is defined as the ability of 

the soil to resupply an ion to the soil solution.

Symplastic movement

requires pumps to move ions across a membrane

For every 1% of soil organic matter, the soil will mineralize

approximately 20 lbs. N per acre

Eutrophication of bodies of fresh water is often caused by the plant nutrient

phosphorus

Rainfall is a naturally acidifying process because H2O reacts with the naturally occurring atmospheric gas ______ forming _____.

50%; H+

A soil scientist is evaluating phosphorus availability in two different soils: Soil A is a sandy, acidic soil (pH 5.5) with low clay content, and Soil B is a calcareous soil (pH 7.8) with high clay content. If both soils receive the same rate of phosphate fertilizer, predict which soil will likely show greater P fixation and explain the dominant mechanism in each soil.

Soil B will show greater fixation through both Ca-P precipitation and higher surface area for adsorption

A soil test shows adequate phosphorus levels, but corn plants are still exhibiting P deficiency symptoms. The soil has high clay content and a pH of 5.2. Which factor is most likely limiting phosphorus availability to the plants?

Phosphorus fixation by iron and aluminum oxides