Effects of sorbed orthophosphate on zinc status in three soils of eastern Canada

SUMMARY P‐Zn interactions can affect fertilizer use and produce Zn deficiencies with certain crops. Phosphorus‐Zn sorption‐desorption reactions were studied in topsoil and subsoil samples from three Quebec soils. Soils were equilibrated with P solutions, then with Zn solutions, and finally with solutions containing no P or Zn. The first equilibration evaluated P sorption (P s ), the second evaluated Zn sorption (Zn s ) after P sorption (P s ), and the third evaluated Zn desorption (Zn D ) as related to added P. Subsequently, Zn fractions were extracted sequentially with KNO 3 (Zn kno 3 ), NaOH (Zn NaOH ) solutions and concentrated HN0 3 + H 2 0 2 (Zn HNO ,). One mmole sorbed P resulted in increases of 0.5 to 1.0 meq (mean = 0.72) increases in cation exchange capacity (CEC). Increased Zn s with added P was equivalent to 4 to 5% of the increase in CEC induced by P s in the Uplands (sand) and St. Bernard (loam) soils, and 0.4 to 0.9% in the Dalhousie (clay) soils, while one meq increase in CEC resulted in 1.5‐3.5% decrease in Zn D . There existed positive correlations between P s and extractable soil Fe materials. Phosphate sorption enhanced associations between Zn s , Zn D or Zn fractions and soil organic or crystalline Fe contents, confirming that P addition increased specific sorption of Zn on Fe components. Other mechanisms including precipitation, P‐induced negative charge and ‘bridge’ effects are also discussed.