Effect of Phosphorus Level on Extractable Micronutrients and Their Distribution Among Soil Fractions
Reduced tillage can cause high P levels at the soil surface which may influence soil properties so as to redistribute micronutrient metals among soil fractions and alter their plant availability. The objective was to determine the effect of P levels on the distribution of Mn, Cu, Fe, and Zn among soil fractions and on their extractability by commonly used soil extractants. Eight topsoils were treated with 0, 30 and 60 mg P kg−1 soil and none or one level of a blend of Mn, Cu, and Zn. After 22 wks, the soils were sequentially extracted to separate the exchangeable, organic, Mn oxide, amorphous Fe oxide, crystalline Fe oxide, and residual fractions. Mehlich-1, Mehlich-3, and DTPA (diethylenetriamine-pentaacetic acid) extractable metals and Mehlich-3 P were also determined. Increasing P moved Mn from the less-soluble fractions (crystalline Fe oxide and residual) to the intermediately-soluble fractions (Mn oxide and amorphous Fe oxide) and had little effect on the plant-available fractions (exchangeable and organic). Copper moved to the exchangeable from the residual fraction for the fine-textured soils. Zinc moved from the Mn oxide and crystalline Fe oxide fractions to the exchangeable fraction for all soils. Added Cu and Zn were evident in the more plant-available fractions for most soils. Increased P increased extractable Mn and Fe for Mehlich-1 and −3 and DTPA. Phosphorus treatments increased soil pH probably because of reactions with Fe and Al oxides producing hydroxyl ions. However, initial reactions may have lowered soil pH causing metals to be more soluble.
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