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 buffer 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 Leucaena 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 effectiveness was maximum at solution P concentration of 0.2 mg P L-1. 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%.