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Seeds of red pepper and tomato were sowed and cultivated in a soil blended with powdery poly(l-lactide) (PLLA), and poly(butylene succinate) (PBS). PBS depressed the growth of the two plants significantly even at a concentration as low as 5%, whereas PLLA up to 35% affected negligibly or even boosted the growth of the two plants. pH and number of microbial cells in the soil after 80 days of cultivation were almost the same independently whether the soil was blended with the two polymers or not. In contrast, the molecular weight of PBS decreased much faster than that of PLLA. Because succinic acid and 1,4-butane diol, from which PBS was synthesized, exhibited toxicity to both plant and animal cells to retard the germination rate of young radish seeds and to deform the morphology of HeLa cells significantly [1], the monomers and the oligomers produced from the PBS degradation should have a detrimental influence on the growth of the two plants.