Effects of supplemental nitrogen rate and source on biomass production by three weed species in fallow vegetable land

In 1983, a study was conducted to evaluate three weed species for their biomass yield potential on a fallowed tomato land with various nitrogen (N) rates and sources. Dogfennel (Eupatorium capillifolium (Lam.)Small), sida (Sida rhombifolia L.), and pigweed (Amaranthus hybridus L.), were grown with three N rates: residual, 1x and 2x (1x = 56 kg ha^?1 N) and with two N sources: NH₄NO₃ and isobutylidene diurea (IBDU). Sida produced the greatest fresh and dry biomass yields and percentage dry matter, 43·3, 23·3 Mg ha^?1 and 54·1%, respectively. Yields of the three crops increased with increasing N rates and were higher with NH₄NO₃ than with IBDU. Overall, sida at the 2x N rate with NH₄NO₃ had the highest fresh, 73·8 Mg ha^?1, and dry biomass, 37·9 Mg ha^?1, yield. Lowest residual soil total soluble salt concentration, 170 ppm, was found after pigweed, in plots which did not receive additional N. Additional N application increased residual soil total soluble salts after each of the three weeds. Thus, additional N, in NH₄NO₃ form, was necessary for high yields in a fallowed vegetable land, but the additional fertilizer also increased soil total soluble salts, regardless of the plant species.