Soil Fertility and Analysis Flashcards

A soil test reports 25 ppm of available phosphorus using the Mehlich-3 extraction method. This value is best described as being in which fertility category for most Midwestern crops?

Answer: Medium to high

Mehlich-3 P values around 25 ppm generally fall in the medium to high range adequate for most crops, so additional P applications may provide little economic response.

Cation exchange capacity (CEC) is primarily determined by which two soil components?

Answer: Clay and organic matter

Clay minerals and organic matter carry negative surface charges that attract and hold cations, making them the primary determinants of CEC.

Which of the following nitrogen forms is most susceptible to leaching losses in sandy, well-drained soils?

Answer: Nitrate (NO3-)

Nitrate carries a negative charge and moves freely with soil water, making it highly susceptible to leaching in coarse-textured soils.

A producer applies 200 lb/ac of 0-46-0 (triple superphosphate). How many pounds of P2O5 are applied per acre?

Answer: 92

The second number in the fertilizer grade represents % P2O5, so 200 lb × 0.46 = 92 lb P2O5/ac.

Soil pH of 5.5 is likely to cause reduced availability of which nutrient due to increased fixation by iron and aluminum?

Answer: Phosphorus

At pH below 6.0, iron and aluminum ions react with phosphate ions to form insoluble compounds, significantly reducing P availability.

The base saturation percentage for calcium in a productive agricultural soil should ideally range from:

Answer: 60–80%

Optimal calcium base saturation for most crops is 60–80% of CEC, ensuring adequate Ca supply without displacing other cations.

Which buffered soil test extractant is commonly used in the southeastern United States to assess available potassium and phosphorus?

Answer: Mehlich-3

Mehlich-3 is a multi-element extractant widely adopted in the southeastern US because it effectively extracts P, K, Ca, Mg, and micronutrients in a single solution.