Soil Fertility Quiz #4

A corn crop is exhibiting marginal chlorosis and necrosis on its lower leaves, along with weak stalks and poor grain fill. Which soil potassium transport mechanism is primarily responsible for the lack of K+ leaching the roots if soil moisture is adequate?

diffusion

Which of the following clay minerals possesses the highest potential to fix potassium in its interlayer spaces, thereby creating a large nutrient reserve but low short-term availability?

vermiculite

An agronomist is recommending a potassium fertilizer for a potato and tobacco rotation. Which fertilizer source is the most appropriate to avoid potential chloride toxicity while also providing essential secondary nutrients?

potassium sulfate (50% K2O)

In a sandy soil with low cation exchange capacity (CEC), what is the most effective management strategy to maximize potassium use efficiency in high-traffic turfgrass?

split applications of K2O during the active growth season

During a particularly wet spring, a soil test using the Ammonium Acetate (NH4OAc) method may show different results than a field-moist test. What is a significant disadvantage of the NH4OAc method that an agronomist should consider in high-fixing soils?

it does not extract nonexchangeable (fixed) K

Arrange the following soil potassium pools in order from the most readily available to the least available for plant uptake.

Soil solution K > exchangeable K > nonexchangeable K > mineral K

Which of the following factors would most significantly decrease the rate of K+ diffusion in a silty clay loam soil?

Significant soil compaction and poor aeration

A horticultural producer is using a fertigation system for high-value berries. Which potassium source is most suitable for this liquid application due to its high solubility?

Potassium Nitrate (KNO3)

A soil with a very high clay content and high cation exchange capacity (CEC) is described as having a high buffer capacity for potassium. In the context of the Quantity:Intensity (Q:I) concept, how does this soil behave compared to a sandy soil with low buffer capacity?

As plants remove K+ from the soil solution (Intensity), the high-CEC soil will resupply the solution from its reserves (Quantity) more effectively.

A farmer notices that corn plants in a field show stunted growth, purplish lower leaves, and delayed tasseling. Soil test results indicate a pH of 5.2 and moderate organic matter. Which management strategy would most effectively address the observed symptoms?

Band phosphorus fertilizer at planting and consider liming to raise pH

In a highly weathered tropical soil (Oxisol) with abundant Fe and Al oxides, a researcher finds that recently applied phosphate fertilizer becomes unavailable to plants within weeks. Based on the dominant fixation mechanism in this soil type, which amendment would be most effective at improving long-term phosphorus availability?

Incorporating organic matter such as compost or crop residues

A soil has a solution P concentration of 0.3 ppm and a high buffer capacity. After several weeks of crop growth, the solution P concentration remains at approximately 0.3 ppm despite significant plant uptake. What does this indicate about the soil's phosphorus dynamics?

The soil has adequate labile P reserves that are effectively replenishing the soil solution

Which transport mechanism is responsible for approximately 80-90% of phosphorus movement to plant roots in soil?

diffusion

In the lyotropic series for cation adsorption (Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺), which two factors primarily determine a cation's position in this sequence?

Charge and hydrated radius

A farmer notices that nitrogen deficiency symptoms appear first on older leaves of corn plants, while iron deficiency symptoms appear first on younger leaves. What explains this difference in symptom location?

Nitrogen is mobile in the plant and can be relocated from older to younger tissue while iron is immobile

Compare two soils: Soil A has 25% kaolinite clay and 3% organic matter, while Soil B has 25% montmorillonite clay and 3% organic matter. Beyond the difference in total CEC, how would these soils differ in their response to pH changes, and what mechanism explains this difference?

Soil A would show greater pH-dependent charge due to kaolinite's broken edge sites

Two agronomists debate fertilizer recommendations for a field with pH 5.0, high iron oxide content, and adequate total phosphorus. Agronomist A recommends high-rate P fertilizer applications. Agronomist B recommends liming the soil to pH 6.5 before applying moderate P rates. Evaluate which approach better addresses the underlying problem and why.

Agronomist B's approach is superior because it addresses P fixation by reducing iron oxide activity

What happens to base saturation when acidic cations increase in the soil?

it decreases

Which of the following best describes the primary form of phosphorus available for plant uptake in soils?

Dihydrogen phosphate (H₂PO₄⁻)

How does potassium contribute to photosynthesis in plants?

It regulates stomatal opening, ensuring CO₂ uptake for photosynthesis.