Production of AM fungus colonized seedlings under organic management: suitability of hydrolyzed fish vs. blood meal as options for N addition

Abstract

The potential benefits of the arbuscular mycorrhizal (AM) symbiosis to crop growth and farm sustainability make utilization of AM fungus inoculum of particular interest to organic farmers. An important consideration in ensuring the colonization of vegetable seedlings by AM fungi during the greenhouse growth phase of production is supplying sufficient nitrogen and other nutrients for plant growth while limiting levels of phosphorus to avoid P-mediated inhibition of colonization. A greenhouse experiment was conducted to compare growth and AM fungus colonization of Allium porrum L. cv. Musselburgh seedlings in two organic potting media amended with high N and low P organic fertilizers vs. a chemical fertilizer solution known to produce well-colonized seedlings. Though amendment of the media with granular blood meal tended to produce the largest seedlings, formation of mycorrhizae was inhibited relative to 3 × per week additions of inorganic control or hydrolyzed fish fertilizers (4.5% vs. 28% and 16% of root length, respectively). A follow-up experiment in a controlled environment chamber demonstrated no detrimental effect of 3 × per week addition of blood meal in aqueous suspension relative to the chemical fertilizer. The incorporation of blood meal into the media had no impact on mycorrhizal development through the first 2 weeks, but inhibited development of new infection units and colonization of roots relative to the other treatments after 5 weeks (7% vs. 38% of root length). Proper choice of organic fertility amendment is essential to produce AM fungus colonized seedlings ready to take advantage of the symbiosis immediately upon outplanting.