The Effect of Soluble and Slowly Soluble Phosphorus Supply on the Growth of Three Citrus Rootstocks in a Composted Pine Bark Substrate

Abstract

Pine bark and peat-based substrates have been shown to have low-phosphorus (P) fixation capacity and high leach-potential, similar to that occurring in high-organic soils lacking in inorganic colloids. A long-term greenhouse experiment was conducted where three rootstock species of varying growth rate, Citrus jambhiri Lush.(RL), Citrus reshni Hort. ex Tan. (CM), and Poncirus trifoliata L. × Citrus sinensis L. (Osbeck) (CC), were grown in 3-L containers in composted pine bark, amended with three forms of P. Two slowly soluble forms (Calmafos and MagAmp) and soluble single superphosphate were incorporated at 0 (control), 200, 400, and 800 g P/m3, in a completely randomized block design (n = six plants). A split fertigation treatment of P at 50 mg·L–1 vs. No P was superimposed on the design (n = 3). Despite significant (P > 0.01) differences in P availability in the substrate after 380 days, particularly between liquid P (μ = 65 mg·L–1) vs. no liquid P (μ = 15 mg·L–1), differences in leaf analysis of seedlings after 235 days showed little significance (2.2 vs. 2.7 mg·g–1). To avoid excessive leaching of P from pine bark substrates, it therefore appears that slow-release forms of P are adequate to maintain relatively high growth rates of citrus stock without supplemental P fertigation.