Question

The limiting factor in nitrogen fixation of soil is

• A. soil nature (pH)
• B. light
• C. temperature
• D. air

Answer 1

Answer: (A)

Extreme of soil $pH$ affects nodulation by reducing the colonisation of soil and the legume rhizosphere by rhizobia. Highly acidic soils $(pH<4.0)$ frequently have low levels of phosphorus, calcium and molybdenum and high concentration of toxic elements. While highly alkaline soils $(pH>8.0)$ tend to be high in sodium chloride, bicarbonate and borate, and are often associated with high salinity which reduce nitrogen fixation.