EFFECTS OF PH ON ARBUSCULAR MYCORRHIZA .1. FIELD OBSERVATIONS ON THE LONG-TERM LIMING EXPERIMENTS AT ROTHAMSTED AND WOBURN

The percentage colonization of roots by arbuscular mycorrhizal fungi was measured in the field in two crops of spring oats (Avena sativa L.) and one of maincrop potatoes (Solanum tuberosum L.), successively grown over a period of 3 yr on the long-term liming experiments at Rothamsted and Woburn. In both crops, all years, both phosphorus treatments and at both sites, percentage colonization was little affected by soil pH from over a range approximately 4.5 to 7.5. There was little effect of pH on crop yields. Up to nine species of coarse endophyte could be identified from spores (> 50 mum diameter) in plots of pH 5.5 and above. There were markedly fewer spores at (approximately) pH 5.5 and none in the most acid plots.