INFLUENCE OF PHOSPHORUS FERTILITY ON INTRASPECIFIC AND INTERSPECIFIC INTERFERENCE BETWEEN LETTUCE (LACTUCA-SATIVA) AND SPINY AMARANTH (AMARANTHUS-SPINOSUS)

Greenhouse studies were conducted to determine the effect of phosphoru s (P) fertility on intra- and interspecific competition between lettuc e and spiny amaranth for 4 wk after emergence. Total lettuce shoot bio mass per pot and weight per plant increased 39 and 44% in response to increased P fertility, respectively. P fertility had no impact on grow th of spiny amaranth. Total shoot biomass of spiny amaranth increased with increasing density from four to eight plants, however, lettuce di d not. Total shoot biomass of both species increased as density increa sed from 4 to 16 plants. Spiny amaranth, but not lettuce, weight per p lant decreased in response to intraspecific competition. Reciprocal yi eld analysis showed that spiny amaranth produced 2.4 times more biomas s than lettuce when competing intraspecifically and four times more bi omass under interspecific competition. Lettuce weight per plant was no t affected. Relative yield analysis indicated that spiny amaranth was more competitive than lettuce regardless of P fertility. However, incr eased P fertility increased competitiveness of lettuce. Relative crowd ing coefficients indicated that spiny amaranth at the lowe density wit h low P fertility was 3.3 times more competitive than lettuce. Additio n of P caused lettuce and spiny amaranth to be equally competitive at the lowest density; however, at the highest density, spiny amaranth wa s 4 times more competitive than lettuce regardless of additional P. No menclature: Lettuce, Lactuca sativa L.; spiny amaranth, Amaranthus spi nosus L. #3 AMASP.