A three-year old plastic greenhouse with sandy loam soil was selected for an experiment on effects of fertilization on soil nutrients and salts in the soil under two consecutive croppings of tomato (spring and winter tomato) and the mechanism of soil salinization was elucidated quantitatively. In the autumn of 2000, nine treatments (N0M0, N0M1, N0M2, N1M0, N1M1, N1M2, N2M0, N2M1 and N2M2, where N stands for urea, and N0: N 0 kg/hm², N1: N 255 kg/hm² and N2: N 600 kg/hm²;and M for sheep manure and M0: 0 kg/hm², M1: 26250 kg/hm² and M2: 52500 kg/hm²) were arranged at random each having three replicates, and in the following winter six treatments N0M0, N0M1, N0M2, N1M0, N1M1and N1M2 were laid out. During the experiment the field was covered with plastic sheet to keep warmth and water regime. The results showed that over-supply of urea or sheep manure led to fast accumulation of nutrients and salts. The increase in the content of water soluble NH₄⁺, which resulted from transformation of the urea applied, led to increase in water soluble K⁺、Ca²⁺ and Mg²⁺ through exchange of ammonium cations. Sheep manure with high contents of water soluble K⁺、Ca²⁺ and Mg²⁺ supplied the soil with high quantity of K⁺、Ca²⁺ and Mg²⁺.