Biochar and hemp as peat substitutes in organic growing media : effects on nutrient availability and nutrient uptake

Abstract: This study aimed at evaluating the suitability of biochar and hemp as partial or complete replacement of peat in horticultural growing media, with specific emphasis on the effects on nutrient availability and plant nutrient uptake. The study was motivated by the environmental concerns surrounding the use of peat, along with a desire to explore possible uses for biochar and a crop residue from hemp cultivation. In total, 13 growing media treatments were evaluated, comprising of six peat/biochar-treatments, six hemp/biochar treatments (biochar rates 0 – 31.25 % v/v) and one control treatment (a commercial peat-based growing media). A greenhouse pot experiment was set up to assess the effect on plant growth of lettuce, Lactuca sativa L. Prior to cultivation, important physicochemical properties of the media were determined; dry bulk density, water-holding capacity, total porosity, air-filled porosity, pH and electrical conductivity. In order to study the impact on nutrient availability and plant nutrient uptake, growing media samples were analyzed for readily available plant nutrients, both before and after lettuce cultivation. Additionally, the harvested lettuce was analyzed for its nutrient content. Overall, the physicochemical measurements showed more suitable properties in the control treatment and the peat-based growing media, compared to the hempbased growing media. However, all treatments except the control had a slightly to highly alkaline pH, which was unfavorable for plant growth. Regarding lettuce plant performance, both the peat-based growing media and the hemp-based growing media performed much below a satisfactory level. Severely impaired plant growth could be observed in all treatments except the control but particularly in the hemp-based treatments. The negative plant response was associated with an overall N deficiency and/or reduced N availability in the growing media with increasing biochar rates. This was most likely caused by N volatilization, due to the alkaline pH of the growing media.