The structure of soil refers to the arrangement of individual soil particles into larger units called peds or aggregates. The shape of these aggregates in soil horizon determines its structural type. The four principal types of soil structure are platy. blocky columnar/prismatic,and granular/crumb. Soil structure is very important to the growth of plants and the movement of water and air in the soil. For instance, crumb structures are good for seed and root penetration. Blocky and play structure will hinder root growth, but columnar structures allow good drainage.