Biochar considerably increases the easily available water and nutrient content in low-organic soils amended with compost and manure.

The soil hydraulic properties of two low-organic soils (Fluvisol; Regosol) were investigated following their amendment with biochar alone or in combination with manure, compost and co-composted biochar. Self-irrigating boxes containing the soil and amendment combinations were purposed with a battery of soil moisture sensors as well as soil porewater sampling devices. Static sampling determined bulk density, porosity and derived soil water retention curves. The aim of this study was to identify the most advantageous amendment combinations to enhance soil water retention whilst simultaneously avoiding excessive nutrient leaching arising, primarily, from manure application. Biochar significantly decreased bulk density and increased total porosity when compared to compost in the Fluvisol, whereas manure affected the greatest changes in the Regosol. All of the tested amendments adjusted the shape or extent of the soil water retention curves, but biochar addition resulted in the greatest increase (⁓50%) in easily available water content (for plants) in both soils, when compared to the control. Saturated hydraulic conductivity was, however, not changed by any of the amendments which reflects a lack of influence on infiltration. An enhancement in nutrient retention occurred in some of the soil amendment configurations, such as for co-composted biochar at 2% dosage and 5% manure-biochar mixture, as revealed by porewater analysis. In summary, the application of biochar with and without additional compost and manure can enhance soil water retention in low-organic soils whilst maintaining or enhancing nutrient retention. Such finding supports the application of mixed organic amendments to low-organic (and therefore drought-prone) arable soils.

Application of co-composted biochar significantly improved plant-growth relevant physical/chemical properties of a metal contaminated soil.

A woody-biochar was added to waste biomass during a composting process. The resulting compost-char was amended to a metal contaminated soil and two plant species, L. perenne and E. sativa, were grown in a pot experiment to determine 1) plant survival and stress factors, 2) uptake of metals to plants and, 3) chemical characteristics of sampled soils and pore waters. Compost supplemented with biochar after the composting process were also tested, as well as a commercially available compost, for comparison. Co-composting with biochar hastened the composting process, resulting in a composite material of reduced odour, increased maturity, circum-neutral pH and increased moisture retention than compost (increase by 3% of easily removable water content). When amended to the soil, CaCl2 extractable and pore water metals s were reduced by all compost treatments with little influence of biochar addition at any tested dose. Plant growth success was promoted furthest by the addition of co-composted biochar to the test soil, especially in the case of E. sativa. For both tested plant species significant reductions in plant metal concentrations (e.g. 8-times for Zn) were achieved, against the control soil, by compost, regardless of biochar addition. The results of this study demonstrate that the addition of biochar into the composting process can hasten the stability of the resulting compost-char, with more favourable characteristics as a soil amendment/improver than compost alone. This appears achievable whilst also maintaining the provision of available nutrients to soils and the reduction of metal mobility, and improved conditions for plant establishment.