Rock Phosphate as a Source of Phosphorus for Vesicular-arbuscular Mycorrhizal Development and Growth of Citrus in a Soilless Medium

Abstract

Abstract Vesicular-arbuscular mycorrhizal (VAM) development of Glomus intraradices Schenck & Smith and growth response of Carrizo citrange [Poncirus trifoliata (L.) Raf. × Citrus sinensis (L.) Osbeck] and sour orange (C. aurantium L.) seedlings were examined in peat-perlite medium fertilized with 4 levels of superphosphate (SP) and rock phosphate (RP). Loss of P from SP-amended medium was exponential, whereas release of P was linear from RP-amended medium. With SP, root colonization by G. intraradices and growth response of both citrus rootstocks were reduced compared to RP-amended medium. VAM colonization and growth response were less with SP-amended medium than RP, probably because of the initially high available P from SP which inhibited VAM development, and because P later declined to levels insufficient to support maximum growth. When RP was used as a controlled-release source of P, VAM colonization was comparable to that observed in mineral soils. Growth of VAM plants exceeded that of uninoculated plants fertilized with increased levels of soluble P because of P-induced Cu deficiency in absence of VAM. An additional advantage of RP was long-term availability of P, compared to SP which leached within weeks after application to peat-perlite. No further P fertilization may be necessary if RP and VAM inoculum are incorporated into soilless media before planting, whereas repeated application of soluble P would be required for slow-growing woody plants like citrus.