Abstract: Effects of application of urea and （NH₄）₂SO₄ on nitrification in and acidification of red soils different in initial pH(5.14,4.77 and 4.34)were studied through incubation experments.It was found that application of urea accelerated nitrification in and thereby acidification of the three soils,and that at the end of the incubation,soil pH decreased by 2.14,1.23 and 0.56 units,separately,in the three soils relative to their respective maximum pHs observed during the incubation.Apllication of （NH₄）₂SO₄ also accelerated nitrification in the soil hign in initial pH(>5.0),but it had no significant effect on acidification of the soil.However,nitrification in and acidification of the two soils low in initial pH(<5.0)were inhibited by the application of （NH₄）₂SO₄.Soil acidification rates were found to be linearly correlated with nitrification rates in the three soils.These findings suggest that nitrification of NH₄⁺-N in the soil is one of the main factors responsible for the acidification of the red soil of crop fields,but urea and （NH₄）₂SO₄ showed different effects on nitrification in and acidification of these red soils.

Key words: red soil, acidity, nitrogen fertilizer, nitrification rate, acidification rate