EFFECTS OF PHOSPHORUS FERTILITY REGIMENS AND COMMON LAMBSQUARTERS (Chenopodium album) INITIAL REMOVAL TIMES ON LETTUCE (Lactuca sativa)

Field trials were carried out in organic soils naturally deficient in phosphorus (P) fertility to determine the effects of different fertility regimens of this nutrient and common lambsquarters initial removal times on lettuce marketable yields. Phosphorus was either banded (125 Kg P/ha) or broadcast (250 Kg P/ha) prior to lettuce planting. A single lambsquarters population density of 4 plants per 5.4 m2 was allowed to interfere with "South Bay" lettuce within each Ρ regime. Lambsquarters were removed at 2, 4, 6 or 8 weeks after lettuce emergence, along with the weed-free control. When lettuce was grown under weed-free conditions, banded Ρ increased lettuce marketable yields by 27% compared to Ρ broadcast. With banded P, no significant differences were observed between the 2 week removal and the weed-free control. On the other hand, differences were accounted between the weed-free control and the 2 week removal (aproximately 24% reduction). No differences on lettuce marketable yields occurred after 4 and 6 weeks for broadcast and banded P, respectively. Maximum yield reductions of about 47 and 51 % for banded and broadcast Ρ were measured. These findings suggest that banding Ρ in lettuce stands increase the time of lambsquarters interference necessary to reduce lettuce yields.