Improving phosphorus availability in two upland soils of Vietnam using Tithonia diversifolia H. |
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Authors: | Phan Thi Cong Roel Merckx |
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Institution: | (1) Institute for Agricultural Sciences of South Vietnam, 121 Nguyen Binh Khiem Street, District 1, Hochiminh City, Vietnam;(2) Laboratory of Soil and Water Management, Department of Land Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium |
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Abstract: | Phosphorus was added to two acidic upland soils (a Cambisol and a Ferralsol) at two rates (9 mg P kg−1 and 145 mg P kg−1) either in an inorganic P form (KH2PO4) or as a green manure (Tithonia diversifolia H. at 2.5 g kg−1 and 40 g kg−1). The effect of P source on the chemical availability of P was assessed in an incubation experiment by measuring resin extractable
P, soluble molybdate reactive (DMR-P) and unreactive P (DMU-P). Soil pH and extractable Al were monitored during the incubation
period of 49 days. Green manure addition caused an immediate and sustained increase in soil pH and an immediate and sustained
decrease in extractable Al. Labile P (resin P + DMR-P + DMU-P) was increased more by P added as a green manure than when added
in inorganic form in one soil (Ferralsol), while it decreased or did not differ in the other one (Cambisol). In both soils,
the concentrations of soluble DMU-P were frequently higher where Tithonia had been added. The effects of green manure amendment on physical factors governing the phosphorus supply through diffusive
transport were also investigated. Aggregate size distribution was substantially changed by green manure amendment due to a
shift in the percentage of microaggregates (<250 μm in diameter) to larger sizes. Changes in soil aggregation as a consequence
of green manure amendment led to a reduction in specific surface area (SSA) of the whole soil. Coupled with the large increase
in effective cation exchange capacity caused by green manure amendment in both soils, and the decrease in SSA, there was an
increase in the net negative surface charge density in both soils. In summary, at a large addition rate – and in addition
to the well-known effect derived from the extra supply in P, green manure amendment may improve the chemical availability
and diffusive supply of P through the following mechanisms: (i) an increase in soil pH increasing the solubility of phosphate
sources; (ii) a decrease in extractable Al reducing the fixation of added P; (iii) increased macro-aggregation and reduced
specific surface area and porosity leading to fewer sorption sites for P and hence enhanced diffusion rates; and (iv) increased
negative charges and reduced positive charges at the soil surface resulting in a net increase in repulsive force for P. The
induced changes in most measured soil properties were smaller in the Ferralsol than in the Cambisol.
This revised version was published online in June 2005 with a corrected article title. |
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Keywords: | extractable aluminium green manure amendment pH phosphorus porosity soil aggregation specific surface area Tithonia diversifolia |
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