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Soil Science Society of America Journal Abstract - Soil Chemistry

Exchangeable Calcium/Magnesium Ratio Affects Phosphorus Behavior in Calcareous Soils


This article in SSSAJ

  1. Vol. 77 No. 6, p. 2004-2013
    Received: Mar 26, 2012
    Published: October 18, 2013

    * Corresponding author(s): wole.akinremi@ad.umanitoba.ca
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  1. Mihiri C.W. Manimel Wadua,
  2. Vladimir K. Michaelisb,
  3. Scott Kroekerc and
  4. Olalekan O. Akinremi *d
  1. a Dep. of Soil Science Univ. of Manitoba Winnipeg, MB, Canada R3T 2N2
    b Dep. of Chemistry Univ. of Manitoba Winnipeg, MB, Canada R3T 2N2 currently at Dep. of Chemistry and Francis Bitter Magnet Lab. Massachusetts Institute of Technology Cambridge, MA 02139
    c Dep. of Chemistry Univ. of Manitoba Winnipeg, MB, Canada R3T 2N2
    d Dep. of Soil Science Univ. of Manitoba Winnipeg, MB, Canada R3T 2N2


Calcium and magnesium are two dominant cations in calcareous soils that undergo precipitation reactions with applied P fertilizers. Unlike Ca, the reactions of Mg with P have not been thoroughly studied. The objectives of this study were to determine the effect of the exchangeable Ca/Mg ratio on the solubility and mobility of P in model calcareous soils and to identify the P species using 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR). A cation exchange resin was saturated to achieve five different saturation ratios: approximately 100% Ca, 60:40 Ca/Mg, 50:50 Ca/Mg, 30:70 Ca/Mg, and 100% Mg. The resin was mixed with quartz sand and CaCO3 to form model calcareous soils. Fertilizer monoammonium phosphate (MAP) was surface applied on soil columns. According to the analysis of 2-mm segments, water-soluble P increased with decreasing Ca saturation while the acid-soluble P decreased with decreasing Ca saturation. The decreasing Ca saturation (or increasing Mg saturation) enhanced deeper penetration of added P. Dicalcium phosphate dihydrate (DCPD) was the dominant P species formed in soils with 100 to 50% Ca saturation. Magnesium phosphate trihydrate (MPTH) was identified as a prominent P phase with a poorly ordered structure in soils with 70 to 100% Mg saturation. The DCPD and MPTH were identified as minor P phases in 30:70 and 60:40 Ca/Mg soils, respectively. Both chemical and NMR results confirmed that Mg, unlike Ca, was not a strong cation for P precipitation. The role of exchangeable Mg in reducing the formation of Ca-P precipitates could have a positive effect on the P availability in soils with low Ca/Mg saturation.

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