SOIL FERTILITY MANAGEMENT AS A FACTOR IN WEED-CONTROL - THE EFFECT OF CRIMSON CLOVER RESIDUE, SYNTHETIC NITROGEN-FERTILIZER, AND THEIR INTERACTION ON EMERGENCE AND EARLY GROWTH OF LAMBSQUARTERS AND SWEET CORN

Previous experiments have shown that crimson clover (Trifolium incarnatum I,.) used as a green manure may supply weed control benefits as well as nitrogen (N) to a subsequent crop of corn (Zea mays I,.). In contrast to use of synthetic N fertilizer, use of fresh, incorporated crimson clover residue as an N source has been found to suppress lambsquarters (Chenopodium album L.) aboveground drymatter accumulation but to only temporarily reduce that of sweet corn. One possible cause of the clover's suppressive effect is the initial low availability of N that may occur after residue incorporation in the soil. A factorial treatment combination of +/- crimson clover residue and four rates of N fertilizer was used in two field experiments to further document the clover's influence on early plant growth and development and to test the hypothesis that low initial N availability is responsible for the clover's previously observed suppressive effects. The presence of crimson clover residue was found to reduce total emergence of lambsquarters by 27%, while application of N fertilizer increased lambsquarters emergence by almost 75%. Lambsquarters emergence was also delayed by the residue treatment. Addition of N did not alleviate the clover's suppressive effect on total emergence or emergence rate of lambsquarters. Sweet corn emergence and emergence rate differed by less than 5% in 0 N/+residue and 0 N/-residue treatments. Applications of N to residue plots suppressed rather than enanced sweet corn emergence. Lambsquarters aboveground biomass accumulation was 46% lower in the residue than nonresidue treatments at 23 days after planting (DAP) and remained 26% lower at 53 DAP. Addition of N did not alleviate the suppressive effect of the clover residue on lambsquarters aboveground drymatter accumulation. Sweet corn aboveground biomass accumulation was not affected by the presence of the clover residue. The results of the experiments indicate that the suppressive effect of crimson clover residue on lambsquarters emergence and growth is not attributable to initial low availability of N. However, given the stimulatory effect of N fertilizer on lambsquarters development, use of crimson clover as an N source would appear to provide weed control benefits both as a direct suppressant of weed emergence and growth and as a substitute for fertilizer N.