WHEAT-STRAW PLACEMENT EFFECTS ON TOTAL PHENOLIC-COMPOUNDS IN SOIL ANDCORN SEEDLING GROWTH

The effects of placement of wheat (Triticum aestivum L.) straw on the release of phenolic compounds potentially phytotoxic to corn (Zea mays L.) seedling growth were examined in plastic pots under controlled co nditions in growth cabinets. The treatments were soil only (control), soil with straw only in the top 2.5 cm, and soil with mixed straw. Eac h of these treatments was either sown or not sown with corn. Corn radi cle length and seedling biomass were significantly reduced, relative t o other treatments, when wheat straw was present only in the top layer of soil. Water extracts from soil with surface straw had higher amoun ts of total phenolic compounds than the other treatments at each of th e sampling times. The concentration of phenolic compounds increased wi th time, with average amounts at 14 and 28 d being 53 and 77%, respect ively, of the concentration at 42 d. The presence of corn reduced the amount of phenolic compounds at 14 and 42 d compared with when corn wa s absent (P = 0.01). Water extracts prepared from soil after 42 d from all the treatments were used to investigate germination of com seeds. Germination of corn seeds and radicle extension were inhibited by wat er extracts from all the above soils at 42 d compared with the control (deionized water). Coleoptile length was inhibited when concentration of phenolic compounds exceeded 10 ppm. Soil NO3-N differences among t he treatments were insignificant at the various sampling times. This i ndicated that the observed corn growth differences were mainly due to the presence of phenolic compounds.