Soil sickness in Iraq: Possible role of allelopathy and soil pathogens

Soil sickness or soil fatigue refers to the progressive loss of soil quality due to repeated culture of single crop and in some cases other crops. The monoculture practice promotes the build up of toxic substances and/or pathogens in the soil. Citrus replant problems and corn and rice declining are common problems in Iraq. We did series of experiments to find the factors responsible for these problems. Soil fungi and nematode were found major agents responsible for citrus replant problems followed by allelopathic compounds of plant and microbial origins, which build up in the soil and act as predisposal agents for the infection of citrus roots by soil pathogens. The corn decline in monoculture system, was due to allelopathic compounds released from corn residues into the soil and remain stable for a relatively long period of time during the growing season of second corn crop and hence, inhibited the growth of corn plants. Growth of rice was found to decrease markedly following wheat in crop rotation. It was found that the allelopathic compounds released during decomposition of wheat residues in soil, adversely affect the growth of rice, directly or indirectly through reduction of biological nitrogen fixation by blue green algae.