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ABSTRACT

P deficiency is considered as an important limitation of crop productivity. The lowland rice areas of Southern Vietnam as Fluvisols, Acrisols have been emphasized to be investigated how to improve the problem.

The study aims at (i) to identify the potential of phosphorus releasing and adsorption in lowland areas of Fluvisols and Acrisols in Southern Vietnam; (ii) to identify the potential of the competitive anion application i.e. silicate to facilitate phosphorus becoming available to rice (Oryza sativa L.).

The study indicated that the negative correlation between P adsorption and P availability was recognized, and then major factors causing P availability in case of lowland rice culture in Southern Vietnam. If anionite measures could determine available P, the competitive anion application would facilitate phosphorus becoming available to rice under condition of lowland in both Mekong Delta and Dong Nam Bo (Southeastern region). The effectiveness by Na₂SiF₆ was evaluated in case of limitation of P adsorption and increasing available P in lowland rice areas.

In acid sulfate soils and alluvial soils, available P varied from 21 to 33 and from 27 to 95 mg P/ kg, respectively. Available P amount negatively correlated to clay percentage, organic matter, amorphous aluminum and iron.

Acrisols exhibited the potential to release P better than Fluvisols.

Available P by Onioami measure did not correlate to P absorption by rice plant. Otherwise, available P by Anionite positively correlated to the P absorption.

In acid sulfate soils, Na₂SiO₃ and Na₂SiF₆ affected Q_max and P_0.2 decreasing 6-21% and 41-73%, respectively, by the binary Langmuir equation. If Na₂SiO₃ and Na₂SiF₆ could be applied at 1, 14 and 42 days after sowing (DAS), Q_max would be constant as compared to check, but P_0.2 would significantly declined 7-30%.

In Acrisols, Na₂SiO₃ was applied at 1, 14 and 42 DAS; Q_max would decline 10-23% as compaed to check; P_0.2 decreased 4-70%.

In Acrisols, Na₂SiF₆ was applied at 1, 14 and 42 DAS; Q_max would decline 11-58% as compaed to check; P_0.2 decreased 12-56%.

If single or combined application of silicate or silicate + P fertilizers could be done, P and N absorbing by rice plant, biomass, and tiller number / hill would significantly increase.

Silicate sodium or silicofluoride applications decreased Fe and Al, increased P/Fe and P/Al at 25 DAS under conditions of acid sulfate soils.

P adsorption capacity could be ranked as acid sulfate soils > alluvial soils > grey soils. In addition, it depended to organic matter, clay (%), amorphous Fe and Al, and soil pH.