In order to improve the yield of rice cultivated on acid sulfate soil, 2 field experiments and 1 pot experiment were conducted continuously for 5 and 2 crops, respectively (1997–2000), in Tri Ton district, An Giang province, Vietnam. Soil for the pot experiment was taken from the 2 field experiments to study the residual effect on phosphorus availability.

Both organic and inorganic phosphorus had a possitive effect on the rice yield. Compared with the treatments being fertilised at the same doses of P, a significantly higher yield was obtained in the treatment of mixed inorganic P fertiliser and manure. This effect was found only in the first crop. From the second crop onward, rice yields were not different among treatments (mixed fertilisers, inorganic P fertiliser, and manure only). Manure-only treatment resulted in rice yield equal to the treatment with 60 kg P₂O₅ in the form of superphosphate. Supplying phosphorus in both organic and inorganic forms over several crops resulted in an accumulation of phosphorus in soil, which became available for rice growth in the following crop season. Adding P fertiliser modified the P fraction in acid sulfate soil mainly to the form of Fe-P.