Field trial were conducted in low-phosphorus (P) organic soils to determine the influence of two Ρ fertility regimes and five lambsquarters population densities on lettuce marketable yields. Phosphorus was either banded (125 Kg P/ha) or broadcast (250 Kg P/ha) prior to lettuce planting. Lambsquarters population densities of 0, 2, 4, 8 or 16 plants per 5.4 m2 were allowed to interfere with "South Bay" lettuce within each Ρ regime. Marketable lettuce yield and head number were collected 8 weeks after lettuce emergence. Results indicate that banding Ρ can reduce the negative impact of lambsquarters on lettuce. In all cases, lettuce yields increased approximately 13% when Ρ was banded. At the same time, when Ρ was banded no differences were observed between 2 lambsquarters per 5.4 m* and the weed-free control. However, when Ρ was broadcast, 2, 4, and 8 lambsquarters per 5.4 m2 were enough to reduce lettuce marketable yield in about 33, 50 and 65%, respectively. No significant differences were observed between 8 and 16 lambsquarters per 5.4 m2 when Ρ was broadcast. Maximum lettuce yield reductions occurred at either 4, 8, or 16 lambsquarters per 5.4 mJ when Ρ was banded, corresponding to approximately 34%.


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