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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.

Soilless root media have little capacity to retain PO₄ or K, and this contributes to leaching of these nutrients during greenhouse crop production. The objective of this research was to evaluate the suitability of precharged alumina as a sole source of PO₄ and K during greenhouse production of potted chrysanthemum [Dendranthema ×grandiflora Kitam. (syn. Chrysanthemum ×morifolium Ramat.)]. Phosphate and K adsorption and desorption curves were created at 25 °C for two particle sizes (0.5 to 0.9 and 1.8 to 3.2 mm) of alumina (Al₂O₃; acid-washed and unwashed), and a medium of 7 peat: 3 perlite (v/v) using solutions of KH₂ PO₄ (P at 0 to 20,000 mg^.L^-1). Based on these curves, 1.8 to 3.2 mm, unwashed alumina was selected for use in the studies. Precharged alumina was tested in two greenhouse studies at 10% and 30% (v/v) of a peat-perlite medium used to produce `Sunny Mandalay' chrysanthemum. Phosphate, K, and pH were determined on unaltered root medium solutions collected throughout the 10-week cropping cycle, and foliar analyses were conducted on tissue collected at the middle and end of the cycle. Potassium release was adequate to meet chrysanthemum demand for 4 weeks, but inadequate for the remainder of the production cycle. Precharged alumina retained and released PO₄ at sustained concentrations (P at <2 mg·L^-1) over the course of a 10-week cropping cycle. Growth of plants receiving PO₄ from precharged alumina was not significantly different from the controls receiving liquid fertilizer (P at 46.5 mg·L^-1) at each watering when precharged alumina comprised 30% of the medium, and only slightly less when precharged alumina comprised 10% of the medium. A phosphorus budget showed that while 36% (103 mg) of the applied PO₄-P was lost in the leachate of the controls, only 0.1% (2 mg) was lost from plants produced with alumina-P. This research demonstrates that in a soilless medium with physical properties similar to standard commercial mixes, low but adequate PO₄ concentrations can be achieved and sustained using current production practices.

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.

Although many factors that influence substrate pH have been quantified, the effect from fertilizers continues to be elusive. A multifactorial experiment was conducted to test macronutrient effects using a rarely used statistical method known as the central composite design. Five nutrient factors, including nitrogen (N) carrier ratio (NH₄ ⁺ vs. NO₃ ^–) and concentrations of phosphorus (P) (as H₂PO₄ ^–), potassium (K), combined calcium (Ca) and magnesium (Mg), and sulfur (S), were varied at five levels each encompassing the proportionate range of these nutrients in commercial greenhouse fertilizers. Although a typical factorial experiment would have resulted in 5⁵ = 3125 treatments, the central composite design reduced the number to 30 fertilizer treatments. An experiment was conducted twice in which ‘Evolution White’ mealy-cup sage (Salvia farinacea Benth.) was grown in 14-cm-diameter pots (1.29 L) in a 3 peat:1 perlite (v/v) substrate amended with non-residual powdered calcium carbonate to raise the substrate pH to ≈5.6 to 5.8. Harvests occurred after 3 and 6 weeks of growth. A statistical model described substrate pH over time with significant effects including four main effects of N carrier ratio, P, K, and combined Ca and Mg; three squared terms of N carrier ratio, P, and K; and seven interaction effects. The resulting model was used to calculate substrate pH levels between 25 and 45 days after planting, and it showed that N carrier had the greatest impact on substrate pH.