Influence of Phosphorus Fertility on Intra- and Interspecific Interference between Lettuce (Lactuca sativa) and Spiny Amaranth (Amaranthus spinosus)

Greenhouse studies were conducted to determine the effect of phosphorus (P) fertility on intra- and interspecific competition between lettuce and spiny amaranth for 4 wk after emergence. Total lettuce shoot biomass per pot and weight per plant increased 39 and 44% in response to increased P fertility, respectively. P fertility had no impact on growth of spiny amaranth. Total shoot biomass of spiny amaranth increased with increasing density from four to eight plants, however, lettuce did not. Total shoot biomass of both species increased as density increased from 4 to 16 plants. Spiny amaranth, but not lettuce, weight per plant decreased in response to intraspecific competition. Reciprocal yield analysis showed that spiny amaranth produced 2.4 times more biomass than lettuce when competing intraspecifically and four times more biomass under interspecific competition. Lettuce weight per plant was not affected. Relative yield analysis indicated that spiny amaranth was more competitive than lettuce regardless of P fertility. However, increased P fertility increased competitiveness of lettuce. Relative crowding coefficients indicated that spiny amaranth at the low density with low P fertility was 33 times more competitive than lettuce. Addition of P caused lettuce and spiny amaranth to be equally competitive at the lowest density; however, at the highest density, spiny amaranth was 4 tunes more competitive than lettuce regardless of additional P.