Effects of several kinds of charcoal applied to soil on citrus growth and vesiculararbuscular mycorrhizal (VAM) development were investigated. Satsuma mandarin (Citrus unshiu Marc.) trees on trifoliate orange (Poncirus trifoliata Raf.) rootstocks were transplanted to root boxes using the soil mixed with charcoal derived from rice husk, citrus juice sediment or western spruce bark. The trees were inoculated with the spores of Glomus fasciculatum (Thaxter) Gerdemann and Trappe emend. Walker and Koske. Elongation of the roots in the charcoal treatments was more vigorous than that in the charcoal-free control. The fresh weigths of the root, shoot and the whole tree increased in response to charcoal application. The intensity of VAM infection in any charcoal treatment was higher than that in the control. In particular, the percentage of the infection in the rice husk charcoal plot was 41.5 and P concentration in the leaf exceeded that of the control.
In a Citrus iyo orchard, the percentage of VAM infection was 52% in the rice husk charcoal plot, the highest among plots. The intensity in the Bahia grass (Paspalum notatum Flugge.) plot was next, followed by the third highest rate found in the abandoned plot which had not been cultivated in recent years. The lowest percentage of VAM infection was in a clean-culture plot. A microscopic observation also revealed that in a charcoal-treated plot there were many sites where VAM fungi infected the root.