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A soil material high in metal oxides collected from the Bw horizon of a Hemcross soil in the state of Oregon was charged with phosphate, added to a soilless root medium, and evaluated for its potential to supply phosphate at a low, stable concentration during 14 weeks of tomato cropping (three successive crops). Three rates of phosphate were charged on the soil material, 0, 2.2, and 6.5 m P/g soil material and the soil material was incorporated into a 3 peatmoss: 1 perlite (v:v) medium at 5 % (40 g) and 10 % (80 g) of the volume of a 13.6-cm pot (1.0 L of medium). Uncharged soil material incorporated into soilless root medium at 5% and 10% reduced soil solution phosphate to deficient levels for 2 and 7 weeks, respectively. Phosphate was adequately supplied for 7, 10, 12, and more than 14 weeks in the 2.2P-5%, 2.2P-10%, 6.5P-5%, and 6.5P-10% treatment, respectively, as determined by symptoms of P deficiency. Phosphate and K levels in soil solution were highest at the beginning of crop 1 and tended to decline thereafter. Incorporation of soil material into soilless root medium improved pH stability whether it was charged with phosphate or not. The loss of the phosphate-charged soil material was negligible, 0.3% for the 6.5P-5% treatment and 1.2% for the 6.5P-10% treatment. The minimum critical concentration of soil solution phosphate for tomato in a 3 peatmoss: 1 perlite (v:v) medium as determined by the pour-through extraction procedure was found to be 0.3 mg·L^–1 or slightly less.

Soilless root media retain very little phosphate. This characteristic necessitates continual application of phosphate, which leads to excessive application and leaching. The phosphate desorption characteristics of synthetic hematite (a-Fe₂O₃), goethite (a-FeOOH), allophane (Si₃Al₄O₁₂ ^*nH₂O), and a commercial alumina (Al₂O₃), previously determined for their maximum adsorption capacities, were evaluated to determine their potential for providing a low, constant soil solution phosphate supply with low phosphate leaching from soilless root media. The desorption isotherms of the clay minerals were obtained by introducing 10 mM KCl solution at 0.2 ml/min flow rate into a stirred flow reaction chamber loaded with clay adsorbed with phosphate at maximum adsorption capacity. The suspension in the reaction chamber was held at pH 6.4 during desorption. Effluent solutions were collected for phosphorus analysis until the equilibrium concentration of phosphorus in solution reached 0.05 mg•L^-1. Adsorbed phosphorus at 0.05 mg•L^-1 equilibrium concentration in solution was in the order allophane (19 mg•g^-1) > alumina™ goethite (8 mg•g^-1) > hematite (1.3 mg•g^-1). The equilibrium concentration of phosphorus in solution over time showed that allophane releases phosphate for a longer time than the other clay minerals at a desirable soil solution concentration for plants, less than 5 mg•L^-1. Among the clay minerals tested, allophane showed the most favorable potential to supply phosphate to plants in soilless root media.

Agricultural limestone is classified based on particle-size distribution, a key factor influencing neutralization capacity. This property is an effective basis for liming recommendations for agronomic purposes which allow for gradual rise in soil pH and residual neutralization for three years. Inconsistencies are prevalent when agricultural limestone is used for horticultural applications which require rapid attainment of target pH and residual neutralization for only four months. Variations in pH among batches of substrate produced with the same limestone rate and pH drift from the same initial pH during crop production infer that factors other than particle diameter also influence limestone neutralization capacity. In this study the relationship between specific surface and diameter of limestone particles was examined. Limestones obtained from twenty North American quarries were wet-sieved into eight particle diameter fractions from 600 to <38 μm (passing 30 through 400-mesh screens). Specific surface (m²/g) of particles was measured in three replications for each fraction following the BET theory that dinitrogen gas (N₂) condenses in a continuous mono-molecular layer on all particle surfaces. At each particle diameter fraction, specific surface varied significantly (five-fold differences) among quarries. Large specific surface may indicate many reactive interfaces, hence high neutralization capacity. In containerized production, typical to horticulture, preponderance of root over substrate mass and short crop duration dictate narrower characterization of limestone than is currently used. Specific surface may describe limestone neutralization capacity more finely than does particle diameter.