Optimizing Solution P Concentration in a Peat-Based Medium for Producing Mycorrhizal Seedlings in Containers

An investigation was undertaken to test the hypothesis that amending peat to increase its P bu€ er capacity and optimizing the P concentration of the amended medium for mycorrhizal activity will enhance its usefulness for raising mycorrhizal seedlings of tree species. The approaches entailed mixing a small quantity of soil of high P adsorbing capacity with peat and constructing a P sorption isotherm for the medium in order to establish solution P concentration near-optimal to optimal for mycorrhization of seedlings. A P sorption isotherm based on incubating the medium with graded amounts of P at 50% of available water-holding capacity was developed. Target solution P concentrations established using the approach enabled us to identify the optimal solution P concentration for mycorrhizal development on roots of our indicator plant L eucaena leucocephala grown in the medium. Arbuscular mycorrhizal colonization, host growth, and P status of L . leucocephala pinnules measured at target solution P concentrations ranging from 0.12 to 1.0 mg L ± 1 revealed that AM fungal activity and symbiotic e€ ectiveness was maximum at solution P concentration of 0.2 mg P L ± . Medium solution P concentrations in excess of 0.2 mg L ± 1 tended to depress AM fungal colonization, but colonization level did not decline below 43% .