They are also called heavy soils. This group includes soils in which the percentage of clay is at least 35%, although in many cases this percentage is more than 45%. They are soils with a high capacity to retain moisture and nutrients and are difficult to cultivate. Drainage and aeration problems are often encountered and root development in the soil is hampered. They are difficult to treat when there is a lot of moisture, because they are very water permeable and muddy quickly. This makes their surface impervious to air so that the soil quality is degraded and plant growth is hindered.