Field trials were carried out in organic soils to determine the effects of different phosphorus (P) fertilization programs and common lambsquarters duration of interference on lettuce. Phosphorus was either banded (125 kg/ha) or broadcast (250 kg/ha) before lettuce planting. A common lambsquarters population density of four plants per 6 m row interfered with ‘South Bay’ lettuce for 2, 4, 6, or 8 wk after lettuce emergence, along with a weed-free control. For banded P, lettuce fresh weight declined linearly (y = 14.82 − 0.97x; r2 = 0.96) as duration of common lambsquarters interference increased. The effect of broadcast P over common lambsquarters duration of interference fit a quadratic equation (y = 10.67 − 1.69x 0.12x2; r2 = 0.98). The difference in the regression model intercepts for both P fertilization programs showed that in the absence of common lambsquarters interference, marketable lettuce fresh weight was higher by banding P than by broadcasting P. Based on predicted values, this difference is approximately 28% ( 4.15 kg per 6 m row) in favor of banded P. Therefore, banding P at 125 kg/ha proved to be beneficial in raising lettuce fresh weight regardless of the duration of interference. For banded P, the model predicts that 10% yield reduction would be observed at 1.53 wk (10.5 d) of common lambsquarters interference. However, with broadcast P, this period declined to 0.67 wk (4.7 d).
Nomenclature: Common lambsquarters, Chenopodium album L. #3 CHEAL; lettuce, Lactuca sativa L.
Additional index words: Competition, cultural weed control, integrated weed management, nutrients.