External phosphorus requirement of mycorrhizal and non-mycorrhizal barley and soybean plants

Abstract

An experiment was conducted under greenhouse conditions to evaluate the effects of vesicular arbuscular mycorrhizal (VAM) fungi on the external P requirements of barley and soybeans. The plants were grown in pots containing a P-deficient soil. A range of 10 P levels was obtained by adding 0, 20, 30, 40, 50, 60, 70, 110, 160, or 310 mg P kg-1 as NaH2PO4 2H2O. Half of the pots were inoculated with the VAM fungus Glomus intraradices. The P concentration in the soil solution was determined using an adsorption isotherm and plotted against the relative yield. Barley did not respond to mycorrhizal inoculation and we concluded that P nutrition was not the limiting factor on the growth of this lowmycotrophic plant. In contrast, mycorrhizal inoculation stimulated the growth of soybeans. The external P requirements were 0.110 μg ml-1 for mycorrhizal and 0.148 μg ml-1 for non-mycorrhizal soybeans to obtain 80% of the maximum yield. In terms of P fertilization this corresponds to a saving of 222 kg P2O5 ha-1. The mycorrhizal dependency of the soybean was highly correlated with the P concentration in the soil solution and it is proposed that both values should be displayed together.