Soil Science & Fertilizer Chemistry Flashcards

Which soil textural class has the highest water-holding capacity per unit volume?

Answer: Clay

Clay soils have the greatest surface area per gram, giving them the highest water-holding capacity.

The process by which ammonium ions are converted to nitrate in the soil is called:

Answer: Nitrification

Nitrification is the two-step microbial oxidation of NH4+ to NO2- and then to NO3- by Nitrosomonas and Nitrobacter bacteria.

Which fertilizer material has the highest potential acidity when applied to soil?

Answer: Ammonium sulfate

Ammonium sulfate has the highest potential acidity (110 lb CaCO3 equivalent per 100 lb) because it supplies two acid-forming NH4+ ions plus the acidifying sulfate anion.

What is the primary function of molybdenum (Mo) in plant nutrition?

Answer: Nitrate reductase activation

Molybdenum is a cofactor for nitrate reductase, the enzyme that reduces NO3- to NH4+ within plant tissue.

A soil with a CEC of 20 meq/100g and 60% base saturation contains how many meq of base cations per 100g?

Answer: 12 meq

Base saturation percentage × CEC = 0.60 × 20 meq/100g = 12 meq/100g of base cations.

Which nitrogen fertilizer is most susceptible to ammonia volatilization when surface-applied to high-pH soils?

Answer: Urea

Urea undergoes rapid hydrolysis to ammonium carbonate at the soil surface, which at high pH promotes ammonia gas loss.

Soil organic matter contributes to CEC primarily because of:

Answer: Negative charges from dissociated carboxyl and phenolic groups

Organic matter carries pH-dependent negative charges from deprotonated carboxyl (-COOH) and phenolic (-OH) functional groups that attract and hold cations.