PLANT GROWTH RESPONSES TO VESICULAR‐ARBUSCULAR MYCORRHIZA III. INCREASED UPTAKE OF LABILE P FROM SOIL

S ummary Onion plants were grown in a range of soils labeled with 32 P. It was found that although the mycorrhizal plants had taken up more phosphorus and grown larger, the proportion of 32 P to total P (specific activity) taken up by mycorrhizal and non‐mycorrhizal plants after 10 weeks was not significantly different. It is concluded that the mycorrhizal roots used the same source of labile phosphate but explored a greater volume of soil beyond the zone of phosphate depletion near the root surface. There was no indication that mycorrhizal roots had access to sources of phosphate different from those accessible to non‐mycorrhizal roots. The specific activity of NaHCO 3 ‐extractable phosphorus differed considerably between the eight soils but the specific activity of absorbed phosphorus in the plants always corresponded closely to that of the soil in which they had grown.