PHOSPHORUS EFFECTS ON COMPETITIVE INTERACTIONS OF SMOOTH PIGWEED (AMARANTHUS-HYBRIDUS) AND COMMON PURSLANE (PORTULACA-OLERACEA) WITH LETTUCE (LACTUCA-SATIVA)

Replacement series studies were conducted under controlled conditions to determine the effect of phosphorus (P) rates and population densiti es on the competitiveness of smooth pigweed and common purslane with l ettuce. Densities were 2, 4, and 8 plants per 113 cm(2), whereas P rat es were 0, 0.4, and 0.8 g P L-1 soil. A P-deficient Histosol (0.3 mg w ater-extractable P L-1 soil) was used. High P fertility enhanced the c ompetitive ability of lettuce in smooth pigweed-lettuce mixtures. Smoo th pigweed was not responsive to P rates. However, luxurious P consump tion by smooth pigweed occurred, reducing the amount of the nutrient a vailable for lettuce absorption. In common purslane-lettuce mixtures, the weed was responsive to P rates, increasing its competitive ability , whereas no increase in lettuce competitive ability was observed. Bot h weed species were more competitive than lettuce. Competition for P a ppears to be the main mechanism of common purslane interference on let tuce grown in low-P organic soils. Alternative fertilization strategie s (i.e., banded applications) may reduce the effect of smooth pigweed on lettuce.