Green Compost as a Peat Substitute Under Organic and Organo-Mineral Fertilization: Effects on Growth and Nutrient Status of Photinia and Star Jasmine

Green compost is a promising peat substitute in container nursery production, but plant responses are often species-specific. This study evaluated three growing media containing 0, 36, and 72% (v/v) green compost, replacing none, half, or all of the peat fraction, combined with three fertilization protocols (unfertilized, organic, and organo-mineral), on growth, biomass, biomass allocation, and leaf macronutrients of Photinia × fraseri ‘Red Robin’ and Trachelospermum jasminoides. Fertilization was the main factor affecting growth in both species, with the organo-mineral protocol giving the highest values, whereas substrate composition did not affect aboveground growth. In Photinia, the highest compost proportion reduced root fresh weight and increased leaf K, Ca, and Mg concentrations, mainly under organic fertilization, without improving growth; unfertilized plants allocated more biomass to roots only in the peat-based substrate. In Star Jasmine, complete peat replacement did not reduce plant dry weight. However, root fresh weight decreased, and unfertilized plants in the peat-free substrate did not differ from fertilized ones. Green compost can therefore replace up to half of the peat in Photinia and all of it in Star Jasmine, provided that adequate fertilization is supplied. These results, from one growing cycle and compost batch without a mineral-only control, need confirmation.