Effect of Fertilizer-N and Herbicides on the Growth and N Content of Soybeans and Cowpeas

Little is known about herbicide-N fertilizer interaction with respect to the growth and N content of soybeans (Glycine max (L.) Merr.) and cowpeas (Vigna unguiculata). This study was undertaken to evaluate the effect of added N and trifluralin (α, α, α-trifluoro-2, 6-dinitro-N, N-dipropyl-ρ-toluidine), nitralin [4-(methylsulfonyl)-2, 6-dinitro-N, Ndipropylaniline] and diphenamid (N, Ndimethyl-2, 2-diphenylacetamide) on the growth and N uptake by these two crops. Soybeans and cowpeas were grown in greenhouse for 74 days in soil treated with various levels of N (0, 25, and 50 ppm) and herbicide (0, 1, 2, and 3 ppm). Leaf chlorophyll, dry matter yields, N concentration and N uptake were determined. All N-treated soybean and cowpea plants had greater chlorophyll content, produced more growth and contained higher N than untreated check. An increase in growth and N uptake suggests that symbiotic N fixation probably failed to supply all of N required for optimum growth. Application of 1 ppm trifluralin slightly increased growth and N content of cowpea and enhanced chlorophyll content, N concentration and root dry weights of soybean. However, higher concentrations exhibited strong phytotoxic effects. Soybeans treated with 3 ppm of herbicide developed stunted and chlorotic leaves and had fewer nodules than untreated seedlings. Soil amended with nitralin and diphenamid reduced leaf chlorophyll, growth and N uptake of soybeans and cowpeas. Moreover, nitralin exhibited a stronger inhibitory effects in roots than tops and was slightly less phytotoxic than trifluralin. Diphenamid-treated plants developed chlorotic leaf margins, followed by necrosis at the leaf edge. N addition to trifluralin treatment only affected N content of soybeans significantly, whereas there was increased chlorophyll content, N concentration and top dry weights of cowpea. On the other hand, N applied to nitralin and/or diphenamid-treated soybeans and cowpeas resulted in an increase in dry weight and N content of tops, probably due to better nutrient and water supply to plants.