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Agronomy QUESTION #2097
Question 1
Phosphorus cycling in agricultural soils differs fundamentally from nitrogen cycling because:
  • Phosphorus has a significant atmospheric reservoir like nitrogen
  • Phosphorus has no atmospheric reservoir — it cycles exclusively through soil-plant-organic matter pathways, and its availability is controlled by soil pH-dependent precipitation/dissolution reactions and biological mineralization of organic P✔️
  • Phosphorus is more mobile in soils than nitrogen and prone to leaching in all soil types
  • Phosphorus transformations are driven by the same microbial guilds (Nitrosomonas, Nitrobacter) that drive nitrogen cycling
Correct Answer Explanation
Unlike N (atmospheric reservoir, gaseous transformations), P cycles in a closed soil-plant system with no atmospheric component. Soil P exists as: soil solution P (plant-available, <1 ppm), labile P (loosely sorbed), non-labile P (strongly sorbed/precipitated), and organic P. Availability is highly pH-dependent: maximum at pH 6.0–6.5; precipitated as Ca-phosphates at high pH and Fe/Al-phosphates at low pH.