Biochar can change how water moves through different types of soil. The researchers used NMR and filter paper to look at soil particles and water retention in soils with different amounts of biochar. They found that the effect of biochar on soil water retention and particle size varied depending on the type of soil and the amount of biochar added. In fine-grained soils, adding 8% biochar made the pores smaller, while in medium-grained soils, 6% biochar had the biggest impact. Coarse-grained soils benefited most from 2% biochar. Adding biochar improved water retention in fine and medium-grained soils at low suction levels, but had little effect on coarse-grained soils. The best biochar-to-soil ratio depends on the sizes of the biochar and soil particles.