Table 4. Effects of consecutive monoculture on total phenolic acids in soil.

	Unplanted soil	One-year monoculture soil	Two-year monoculture soil
Total phenolic acids in soil (μmol/g)	7.49±0.13c	8.64±0.15b	9.45±0.42a
Length of lettuce root (cm)	2.64±0.23a	2.49±0.27a	2.07±0.26b
Lettuce root growth inhibition rate	0%	5.7%	21.5%

Note: Data are means±SE, and different letters show significant differences at the 5% level according to its p-value between total phenolic acid contents by Tucky's test (P≤0.05, n = 3) and lengths of lettuce root by Tucky's test (P≤0.05, n = 50).