Influence of Magnesium-Mica Clay as a Peat-Based Root Medium Amendment On Nutrient and Water Management of Poinsettia ‘Early Glory'

Magnesium-mica (Mg-mica) is a mineral clay that contains 8% Mg, 4% Fe and 2% K. Physical properties of peat-based media with 2, 5, and 10% raw Mg-mica were characterized.  Air-filled porosity decreased from 17% in the 70 peat: 30 perlite control to 13% in mixes of 60 peat: 5 Mg-mica: 35 perlite, and container capacity increased from 57% in the control to 80% for the mixes with 5% Mg-mica.  In a greenhouse experiment, Euphorbia pulcherrima Willd. ex Klotzsch ‘Early Glory’ were grown with treatments of 3 root media (5 uncalcined Mg-mica: 55 peat: 40 perlite, 5 calcined Mg-mica: 55 peat: 40 perlite and 70 peat: 30 perlite control) x 2 fertilizer regimes (20N-4.3P-16.6K without or with Mg).  Planted and fallow pots of the treatment mixes were used to evaluate initial wettability, rewettability, and water retention. Plant growth and tissue nutrient concentrations were determined at mid- and end-crop, and cations in leachate were measured over the course of the production cycle.  Calcining Mg-mica and using fertilizer with Mg did not influence results.  Compared to the control, clay treatments resulted in ~0.5 unit higher pH during the first month of the production cycle, but EC was not influenced.  Mixes with Mg-mica released Mg and Ca into the root medium solution, but neither growth nor cation uptake were influenced in a commercially-meaningful way.  Mixes with Mg-mica retained NH₄-N, but this did not affect plant growth.  The clay amendments favorably impacted physical properties of initial wettability, rewettability, and water holding capacity. The volume of water retained per unit volume of root media with Mg-mica clay (0.42-0.44 cm^3.cm³) was almost twice as great compared to the commercial control (0.24 cm^3.cm³).  Media amended with Mg-mica had greater water retention as indicated by rewet volumes that were 17% to 30% higher than the commercial control.  The Mg-mica amendment improved root medium nutrient retention and released Ca and Mg but did not influence plant growth.  Media amended with Mg-mica resulted in easier wetting and rewetting and had increased water holding capacity; therefore, the clay could be used as a substitute for synthetic wetting agents in peat-based root media, and its use would decrease irrigation frequency during production.