Relationships between Immobilized Phosphorus Uptake in Two Grain Legumes and Soil Bioactive Phosphorus Pools in Fertilized and Manure‐Amended Soil

Mixing P‐immobilizing additives with manure has raised concerns of irreversible reduction in P availability to growing crops. A potted plant growth experiment was conducted to characterize cattle manure P mineralization as modified by iron amendments and uptake by pigeon pea ( Cajanus cajan L. Millsp.) and soybean [ Glycine max (L.) Merr.]. Triple superphosphate, untreated, or manure amended with Fe at 1:1 or 1:3 molar ratio of manure P:Fe, was applied to Dale silt loam (fine‐silty, mixed, superactive, thermic Pachic Haplustolls) at the rate of 20 mg kg −1 . Whole plants were harvested at three stages of development (vegetative, flowering, and physiological maturity) to determine and correlate P uptake to changes in soil bioactive P pools over the growing season. Dry matter production was unaffected by Fe at both application rates. Phosphorus solubility and plant uptake were reduced at the 1:3 P:Fe molar ratio rate, in spite of the legumes' reported ability to secrete siderophores. Within‐season changes in bioactive P fractions indicated that P was taken up from the inorganic (water‐extractable P [WEP] + inorganic ligand‐exchangeable P [EEP i ]) pools. Comparing soil P pools in cropped and noncropped soils, the decrease in the organic phosphohydrolase‐labile P (PHP) pool corroborated the fact that the PHP pool replenished the WEP + EEP i fractions. Although Mehlich 3 P and EEP i were related, the Mehlich 3 P pool showed no significant relationship with whole plant or any plant part P. The latter did not perform as well as the EEP i pool in assessing plant availability and enzymatically mediated turnover of immobilized P.