The Effect of Soil Phosphorus Levels on Mycorrhizal Infection of Field-grown Onion Plants and on Mycorrhizal Reproduction1

Abstract

Seeds of onion (Allium cepa , L.) were sown on 2 muck soils that were high and low in available phosphorus and which contained an indigenous population of mycorrhizal spores (Glomus sp.) . Treatments were 4 levels of P (0, 30, 97, and 193 kg/ha) and inoculum of the mycorrhizal fungus Glomus etunicatus Becker & Gerdemann. In the soil that was low in available P (3 kg/ha) bulb weight increased with added P. Root infection by the mycorrhizal fungus and mycorrhizal spore numbers in the soil were negatively correlated with added P. Bulb weight and mycorrhizal spore number at harvest increased when mycorrhizal inoculum was added to the soil. In the soil that was high in available P (97 kg/ha) bulb weight, root infection, and spore numbers were not influenced by added P or added mycorrhizal inoculum. Root infection data from both soils suggested a threshold level of soil P below which mycorrhizal infection was high and above which infection was low. The levels of P commonly added to muck soils may negate any usefulness of mycorrhizae but addition of P might be reduced if mycorrhizal spore numbers were increased through inputs of mycorrhizal inoculum or cultural practices.