Plant Growth Responses to Vesicular‐arbuscular Mycorrhiza

S ummary The benefits of mycorrhizal infection were compared in four irradiated and unsterilized soils containing little soluble phosphorus. Mycorrhizal infection was produced in onion seedlings by pre‐planting in a heavily infected soil and transplanting into the test soil, or by sowing in situ on a cushion of Endogone inoculum. In all soils, irradiated or not, mycorrhizal seedlings grew very much better than non‐mycorrhizal ones. In two of the soils natural infectivity was very low and most uninoculated seedlings remained non‐mycorrhizal even after 16 weeks. In the other two soils most plants became mycorrhizal in the unsterilized soils. In one of them the indigenous mycorrhizal fungi aided phosphorus uptake of the host plant, in the other they did not. Infections produced by the indigenous fungi in the two soils were anatomically different, though both were typical VA mycorrhiza. It is concluded that inoculation may have practical significance in some low phosphate soils containing either few indigenous mycorrhizal fungi or ineffective strains.