Porosity and permeability

Between the grains of any soil or rock there are empty spaces; how much of its volume they take up is its porosity, and you can measure it as the water a dry sample soaks up. Permeability is how easily water passes through: coarse, round sand and gravel have wide, connected gaps and let water through fast, while clay's tiny flat grains hold a lot of water but pass it hardly at all, so clay is almost impermeable. Gentle slopes and plant roots also help rain soak in. Good farm soil mixes sand, clay and humus, holding water without drowning the roots.