Adding biochar to agricultural soil has been suggested as an approach to enhance soil carbon sequestration. Biochar has also been used as a soil amendment to reduce nutrient leaching, reduce soil acidity and improve water holding capacity. Walnut shells and woody material are waste products of orchards that are cheap, carbon-rich and good feedstock for production of biochar. The effectiveness of biochar as an amendment varies considerably as a function of its feedstock, temperature during pyrolysis, the biochar dose to soil, and mechanical composition. Biochar was produced from pyrolysis of walnut shell at 900 °C and soft wood at 600 to 700 °C. We measured the effect of these different parameters in two types of agricultural soil in Jilin and Beijing, China, a silt clay loam and a sandy loam, on the soils’ particle size distribution and water retention characteristics. Biochars with two different doses were applied to each soil type. Soil field capacity and permanent wilting point were measured using a pressure plate extractor for each combination of biochar and soil type. The results show that the effect of biochar amendment on soil water retention characteristics depend primarily on soil particle size distribution and surface characteristics of biochar. High surface area biochar can help raise the water holding capacity of sandy soil.
Water retention characteristics of soil bio-amendments used as growing media in pot culture