Abstract: Nitrogen,phosphor and potassium are essential elements for vegetable fertilization.Pot and field experiments were conducted using root length to correlate with cumulative NPK absorption to establish a mathematical model by regression analysis.The results showed that,within a growth period,the curves of root length increase and NPK absorption of water spinach,lettuce,radish and Chinese cabbage were all in an "S" shape.Two root growth models for the water spinach and 30 NPK cumulative absorption models for the pot and field experiments all reached a statistical significance level.Based on the models,the following indices could be obtained: root length increase constant(l0),nutrient absorption constant(k),maximum NPK absorption rate(Inmax),and time(tmax) and maximum NPK absorption amount(Umax) and time(tc).Results showed that the fertilization had significant positive effect on k,Inmax and Umax.But tmax and tc were determined largely by the vegetable’s nutrition characteristics and little by the fertilization.The fertilization was most effective at Inmax,and therefore,Inmax should be the base for top dressing NPK applications.Umax appeared during late growth stage.And,fertilization during the mid-to late-growth period ought to provide the needed nutrients for the vegetables.