ARS | Publication request: Relationships Between Immobilized Phosphorus Uptake in Two Grain Legumes and Soil Bioactive P Pools in Fertilized and Manure-Amended Soil

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 12, 2008
Publication Date: November 30, 2008
Citation: Rao, S.C., Dao, T.H. 2008. Relationships Between Immobilized Phosphorus Uptake in Two Grain Legumes and Soil Bioactive P Pools in Fertilized and Manure-Amended Soil. Agronomy Journal. 100:xx-xx.

Interpretive Summary: Animal manure is a valuable nutrient resource on the farm. It is generally accepted that phosphorus (P) in livestock manure is an effective nutrient source for crop production. In regions with a high density of confined animal production operations, manure has become less desirable of an asset. Tying up soluble phosphorus with manure additives rich in aluminum or iron (Fe) have received a lot of interest to adsorb and lower soluble P concentrations in manure or manure-amended soils. As concerns of reduction in availability of immobilized P to growing plants prevail, this study was conducted to evaluate the ability of pigeon pea and soybean to utilize manure P immobilized by Fe. These grain legume crops can access poorly available forms of P by chemically altering the rhizosphere through exudation of organic compounds that can dissolve bound iron phosphates. Changes in soil bioactive P pools and P uptake during the growing season were determined at three growth stages of the legumes using a novel mild ligand-based enzymatic soil fractionation procedure. The addition of P as manure P was beneficial to plant dry matter yield and P uptake. The Fe amendment did not affect plant dry matter production at all rates. At the whole plant level, the suppression of P solubility and therefore plant uptake was effective at the highest rate of Fe addition, i.e., 1 to 3 ratio, even with the legumes¿ ability to dissolve iron phosphates. Temporal changes in bioactive P fractions showed that plant P was taken up from the inorganic P pools. Non-cropped soil corroborated that organic enzyme-labile P pool replenished these water-soluble and insoluble mineral fractions. Although the use of routine soil test estimates was correlated to the soil bioactive P pool, its results were not correlated with plant and selected plant components¿ P, at least not as well as the latter pool to assess availability of immobilized P.

Technical Abstract: The practice of mixing additives with animal manure to reduce source P solubility have raised concerns of reduced availability in treated manure to growing crops. An outdoor pot experiment was conducted to characterize the mineralization of cattle manure P and uptake of soil P as modified by iron amendments in pigeon pea (Cajanus cajan L. Millsp.) and soybean [Glycine max (L.) Merr.]. Triple superphosphate or manure amended with Fe at 1:1 or 1:3 molar ratio of manure P:Fe was applied to Dale silt loam (Pachic Haplustolls) at the rate of 21 mg kg-1. Whole plants were harvested at three stages of development, i.e., flowering, pod-fill, and physiological maturity for determining P uptake and correlation to changes in soil P pools during the growing season. The addition of P as manure P was beneficial to plant dry matter yield and P uptake. The Fe amendment did not affect plant dry matter production at all rates. At the whole plant scale, the suppression of P solubility and therefore plant uptake was effective at the highest rate of Fe addition, i.e., 1 to 3 molar ratio, even with the legumes¿ ability to secrete siderophores. Temporal changes in selected bioactive P fractions, EEPi and complexed P, indicated that plant P was taken up from the inorganic (WEP + EEPi) P pools. Non-cropped soil corroborated that organic enzyme-labile P pool replenished these water-soluble and insoluble (WEP + EEPi) fractions. Although the use of Mehlich 3 soil test estimates was correlated to the EEPi pool, its results were not correlated with plant and selected plant components¿ P, at least as well as the latter pool to assess plant availability of immobilized P.