Solubility characteristics of residual phosphate in a fertilized and limed ultisol
Except at very low P rates, residual soil P (i.e., that measured by most extraction methods) continues to increase with annual additions of P fertilizer. Phosphorus retention mechanisms, however, are still much debated and difficult to prove or disprove. In an effort to determine whether the solubility of phosphate minerals controlled soil-solution P concentration in the field under some conditions, samples of a Benndale sandy loam were collected from plots of a field experiment with different pH and P levels and analyzed in the laboratory. Several soil pH levels were first established (5.0-8.0); this was followed by annual additions for 7 yr of concentrated superphosphate at five rates varying from 0 to 392 kg P ha/sup -1/ yr/sup -1/ and cropping annually. Soil-solution composition was determined in samples taken 1, 3, and 5 yr after discontinuing P fertilization. Except for the no P treatment, solution P increased with increasing pH up to pH 5.8, then it decreased as pH increased, suggesting the accumulation of a basic phosphate mineral such as hydroxyapatite at about pH 5.8 and above. However, this was not matched by the ion activity product (IAP) for Ca/sub 5/OH(PO/sub 4/)/sub 3/ or any other P mineral. Thus, IAP's could not be used to predict that solid-phase minerals were or were not controlling solution P. Sequential equilibration of selected soil samples with 0.01 M MgCl/sub 2/, however, provided data that strongly supports the hypothesis that hydroxyapatite was present in a high P, high pH soil but not in a low P, high pH or a high P, low pH soil.
- Research Organization:
- Auburn Univ., AL
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 6065514
- Journal Information:
- Soil Sci. Soc. Am. J.; (United States), Vol. 51:4
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
PHOSPHATE MINERALS
SOLUBILITY
PHOSPHORUS
SOIL CHEMISTRY
ACID NEUTRALIZING CAPACITY
CALCIUM PHOSPHATES
EQUILIBRIUM
FERTILIZERS
HARVESTING
LOAM
MAGNESIUM CHLORIDES
PH VALUE
QUANTITY RATIO
ALKALINE EARTH METAL COMPOUNDS
CALCIUM COMPOUNDS
CHEMISTRY
CHLORIDES
CHLORINE COMPOUNDS
ELEMENTS
HALIDES
HALOGEN COMPOUNDS
MAGNESIUM COMPOUNDS
MINERALS
NONMETALS
OXYGEN COMPOUNDS
PHOSPHATES
PHOSPHORUS COMPOUNDS
SOILS
WATER CHEMISTRY
553000* - Agriculture & Food Technology