Superphosphate is described as having which soil behavior?

• A. Absorbs Quickly
• B. Penetrates the soil very slowly
• C. Expands Vigorously
• D. Flows Rapidly

Answer: (B)

Phosphorus mobility in soil is being tested here. Even though superphosphate dissolves in soil water, phosphate ions readily bind to soil particles and form insoluble compounds with calcium in alkaline soils or with iron and aluminum oxides in acidic soils. This strong sorption keeps most of the phosphate near where it’s applied and slows its movement through the soil, so it penetrates the soil very slowly. Because of this limited mobility, phosphorus fertilizers are typically placed near the root zone to improve availability. Descriptions of rapid absorption, vigorous expansion, or fast flow don’t fit how phosphate behaves in most soils.