Seasonal variation in allelopathic effects of corn residue on corn and cress seedlings
Several reports have revealed that corn (Zea mays L.) yields are reduced in a continuous corn cropping. This decrease in yield had been associated frequently with phytotoxins released from crop residues during the process of decomposition and from living plants. The objectives of this study were (1) to investigate the seasonal variation in allelopathic effects of corn residues left after harvest on corn seedling growth under varying tillage practices (no-till, disc, and plow) and (2) to investigate how these allelopathic substances were released from the corn plant or corn residues;Soil samples from fields previously planted to corn and with corn residues left after harvest were bioassayed using corn seedling growth in the growth chamber. Also, soil samples from a previously planted corn field and with corn plants currently growing were collected every month from April to September, 1982, for corn bioassay. The soil samples collected monthly were from plots with different tillage practices. The results showed that soil with corn residues produced shorter plants and smaller root, shoot, and biomass weights than the control (fallow soil). The most pronounced effect was on root weight. The April, August, and September soil samples were inhibitory to corn growth, whereas June and July soil samples had stimulatory effects. The April soil sample from the no-till treatment was the most inhibitory and produced plants which were yellow. Since the inhibitory effect of August and September soil samples probably could not have come from chemicals released from decomposing corn residues, another study was conducted. The results revealed that allelopathic substances were being released by living corn plants through root exudation or rain-leached substances;Using XAD-4 resin to trap allelopathic substances, the monthly soil samples were each attached to a circulating mechanism with XAD-4 resin column. The extracts from the column were bioassayed using cress seed germination. Cress seed germination index, germination rate, onset of 1% germination, and maximum germination percentage were derived using the SAS nonlinear regression program and the Richards' function. The results showed that germination index and germination rate were inhibited by the samples from the months of April, May, August, and September but were stimulated by June and July soil samples.