Replacement series studies were conducted under controlled conditions to determine the effect of phosphorus (P) rates and population densities on the competitiveness of smooth pigweed and common purslane with lettuce. Densities were 2, 4, and 8 plants per 113 cm2, whereas P rates were 0, 0.4, and 0.8 g PL−1 soil. A P-deficient Histosol (0.3 mg water-extractable P L−1 soil) was used. High P fertility enhanced the competitive ability of lettuce in smooth pigweed-lettuce mixtures. Smooth pigweed was not responsive to P rates. However, luxurious P consumption by smooth pigweed occurred, reducing the amount of the nutrient available 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. Both weed species were more competitive than lettuce. Competition for P appears to be the main mechanism of common purslane interference on lettuce grown in low-P organic soils. Alternative fertilization strategies (i.e., banded applications) may reduce the effect of smooth pigweed on lettuce.

Greenhouse and field experiments were conducted to determine the effects of nitrogen (N) supply and purple nutsedge population densities on the yield of radish. In the greenhouse studies, additive series with purple nutsedge densities of 0, 50, 100, 200 or 350 plants m−2 were established. Nitrogen rates of 0, 110, 220, or 330 kg ha−1 were provided to the potting medium. A significant density by N interaction was found for radish fresh weight. Within a given nutsedge density, radish yield decreased as N rate increased. In field studies, additive series of 0, 50, 100, 150, or 200 nutsedge plants m−2 were established the same day radish was sown. Nitrogen rates were 100 or 200 kg ha−1. Marketable radish yield losses and nutsedge shoot dry weight and height were determined 30 d after seeding the crop. Nutsedge densities and N rates interactively influenced radish root yield. Radish yield loss reached 100% at nutsedge densities of 75 and 125 plants m−2 at 200 and 100 kg N ha−1, respectively. Purple nutsedge produced larger shoot biomass as N increased from 100 to 200 kg ha−1. Results of both greenhouse and field studies showed that as N increased, the negative effect of the weed on the crop was enhanced.

Manipulations of the soil environment can affect the growth and competitive ability of annual weeds because of the large influence that soil conditions exert on seedlings early in the growing season. Our objective was to identify soil nitrogen (N) management systems with weed suppression potential. We hypothesized that competition from wild mustard against sweet corn would be weaker when N was supplied by organic sources (organic) or a split application of NH4NO3 fertilizer applied at planting and 4 wk thereafter (split) than when NH4NO3 fertilizer was applied in a single dose at planting (early). This hypothesis was tested in a 2-yr field experiment conducted in central Maine. Wild mustard's maximal relative growth rate (RGR) was 12% lower (P < 0.05) in 1997 and 1998, and the amount of time needed to achieve maximal RGR was delayed by 0.8 d (P < 0.05) in 1997 in the organic compared to the early treatment. The competitive effect of wild mustard on sweet corn yield was lower in the organic treatment than in the early and split treatments in 1 of 2 yr. In 1997, competition from wild mustard reduced marketable ear yields of sweet corn by 30%, but the magnitude of yield reduction did not differ between the three N addition treatments. In contrast, in 1998, sweet corn yield in the organic treatment was not reduced by weed competition, whereas yield loss in the early and split treatments was 20 and 35%, respectively. The mechanisms underlying selective suppression of weeds, but not crops, by organic N sources require further attention.