Soil Science & Fertilizer Chemistry Flashcards

Which form of phosphorus is most available for plant uptake in the soil solution?

Answer: H2PO4-

H2PO4- (dihydrogen phosphate) is the dominant plant-available form in the slightly acid to neutral pH range (6.0–7.0) where P availability peaks.

Sulfur deficiency symptoms in plants most closely resemble deficiency of which other nutrient?

Answer: Nitrogen

Both sulfur and nitrogen deficiencies cause yellowing (chlorosis) of leaves, but sulfur deficiency appears first on younger leaves while nitrogen deficiency starts on older leaves.

The bulk density of a compacted soil is 1.6 g/cm³. Assuming particle density of 2.65 g/cm³, what is the total porosity?

Answer: About 40%

Porosity = (1 - Db/Dp) × 100 = (1 - 1.6/2.65) × 100 ≈ 39.6%, approximately 40%.

Which soil order is characterized by a spodic horizon containing illuviated organic matter, aluminum, and iron?

Answer: Spodosols

Spodosols are defined by their spodic horizon (Bs or Bhs), where leached humus, Al, and Fe accumulate in a distinct subsurface layer.

When diammonium phosphate (DAP, 18-46-0) dissolves near a seed, the pH of the fertilizer band temporarily:

Answer: Rises to approximately 8

DAP dissolution releases NH3 gas initially, raising the localized pH to about 7.5–8.5 before nitrification proceeds.

Which soil property is MOST directly measured by a standard lime requirement test?

Answer: Buffering capacity against pH change

Lime requirement tests (such as the Shoemaker-McLean-Pratt buffer test) measure the soil's buffering capacity to determine how much lime is needed to raise pH to a target level.

In waterlogged (anaerobic) soils, nitrate nitrogen is most likely to be lost through:

Answer: Denitrification to N2 gas

Denitrification by anaerobic bacteria converts NO3- to N2O and N2 gases, which escape to the atmosphere in waterlogged, oxygen-depleted soils.