MICROBIAL POPULATIONS AND NITROGEN IN SOIL GROWING CONSECUTIVE CEREAL CROPS INFECTED WITH TAKE‐ALL

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Abstract
Summary: Annually, for the past 12 years, a consecutive cereal‐cropping sequence was begun. During 1969–72 soil was collected from plots with similar fertilizer treatments, but with different sequences of crops. In 1971 and 1972 wheat seedling bioassays showed that take‐all disease became miximal after two or three crops, decreased to the fifth or sixth crop and then remained fairly constant in the‘take‐all decline’state.The sequence with maximum disease had most NH4+ ‐N and least NO3‐ ‐N in the rhizosphere soil in spring 1972, but there were no similar relationships in bulk soil in either of 2 years. Soil and rhizoplane populations of ammonifying and denitrifying bacteria were notably smaller in autumn 1970 and 1971 in short cereal sequences (2‐5 yr.) than in the longer sequences. After γ‐irradiating or autoclaving irradiated soils, all sequences contained much NH4+ ‐N and although disease developing from added inoculum differed among soils, it did not follow the take‐all decline pattern. Diffusate from sterilized soil favoured growth of the pathogen (Gaeumannomyces graminis var. tritici) least when it originated from the sequence that supported maximum disease.