EFFECTS OF PHOSPHORUS FERTILITY ON COMPETITION BETWEEN LETTUCE (LACTUCA-SATIVA) AND SPINY AMARANTH (AMARANTHUS-SPINOSUS)

Field studies were conducted to determine the influence of phosphorus (P) fertility and method of application (banded versus broadcast) on t he competitive interaction of lettuce and spiny amaranth. Weed density significantly affected lettuce head weight and spiny amaranth shoot b iomass after 5 wk of interference. Weed density and duration of interf erence had little or no effect on P content of lettuce tissue. Duratio n of interference did affect P concentration of spiny amaranth; howeve r, weed density did not. Spiny amaranth competition reduced lettuce yi eld, but P was not the limiting factor. Duration of interference and m ethod of P application interactively affected lettuce head weight; how ever, only duration of interference affected spiny amaranth biomass. S even wk of interference caused a decrease in lettuce head weight of 20 , 8, and 24% when P was broadcast, banded, or not applied, respectivel y. Banding of P reduced the negative impact of spiny amaranth of lettu ce. Although method of P application influenced the interaction betwee n lettuce and spiny amaranth, interspecific competition between the tw o species probably was not due to competition for P but some other fac tor. Nomenclature: Lettuce, Lactuca sativa L., 'Southbay,' spiny amara nth, Amaranthus spinosus L. #3 AMASP.