The density of the soil
The denser the soil, the more difficult it is for soil particles to slide past each other. A higher density, therefore, gives more resistance and a better bearing capacity. Natural density can vary greatly. For example, peat soil with a high organic matter content has a much lower density than poor sandy soil. A mechanically compacted soil also has a higher density. Another form of compaction is drying (shrinking). The latter causes cracks with high permeability to form on some soils, which improves drainage if the superficial water can be drained off quickly through the subsoil.
Organic matter content
Peat soils are characterized by a high organic matter content. Due to the low density of organic matter, peat soils are less firm than sandy soils. Load-bearing capacity problems are also encountered in sandy areas where the organic matter content increases on poorly drained parcels. The deterioration of peat soil by adding sand to the topsoil is a widely used method to improve the bearing capacity of the topsoil. The sand is usually removed from the subsoil through deep plowing, mixing milling, digging, or with a sand machine. Hundreds of hectares have also been sanded by using sand that has been dug up elsewhere. These are all measures to lower the organic matter content of the topsoil and thus increase the carrying capacity.
Soil with a high clay content (heavy clay) is often used as grassland. A lot of peat soil is also covered with a clay layer. Clay shrinks with little drying, which means that the load-bearing capacity increases quickly. In dry conditions, clay even becomes very hard and cracks can appear. These cracks ensure smooth drainage of water to the substrate. If there are drains in the subsoil and/or if the subsoil will also remain dry longer and therefore have a good bearing capacity. Long-term moistening causes the cracks to swell up again. An improvement of the drainage on clay soil therefore mainly improves the bearing capacity in the autumn. Over the course of winter and spring, puddles easily form on clay soils that have been heavily leveled. This also applies to peat soils with a heavy clay cover. Especially when there aren’t trenches and problems arise with enclosed layers due to, for example sagging. A high lutum content in the top layer also increases the risk of smearing. This causes structure deterioration when driving or grazing. Clay grassland, the top centimeters of which is smeared, has poor permeability, which makes it easy to puddle. The risk increases as more valuable topsoil are plowed under during leveling and black clay comes to the surface. The fact that the new sod is very hollow in the first period after sowing also promotes easy spreading.
In addition to organic matter and clay content, the groundwater level has a significant influence on the carrying capacity. It is mainly the moisture tension that causes this. The higher the moisture tension, the better the bearing capacity. The moisture tension depends on the moisture content of the soil. The wetter the soil, the lower the moisture tension, and the drier the soil, the higher the moisture tension. This relation is significant for our dewatering standards. By improving the drainage, the bearing capacity also improves. This is the starting point for ditch level lowering and drainage. Lowering the ditch water levels on some well-drained soils at a ditch distance that isn’t too big (30 to 40 meters) is sufficient to improve drainage. In addition, drainage is often required on other types of soil and/or at large ditch distance.
With poor drainage, bad grasses such as Ostrich grass (spiny love grass) and red fescue are at their best. They have the advantage of being very good turf formers and they are also quite winter resistant. The sod formation increases the load-bearing capacity of the sod. However, these grasses have the disadvantage that they have a bad taste and low dry-matter yield. They are also difficult to tread. Good-quality grasses prefer dry growing conditions. Good grasses can also survive better than bad grasses with good drainage. As a result, grassland that has been sown will therefore remain in place for longer with good drainage. This also increases the dry matter yield, as does the palatability and the resistance to treading. Good grasses are usually clump-forming, this means that the sod density decreases, and the load-bearing capacity of the sod are lower when there are many good types of grass in the grassland.
Good drainage makes the soil drier and increases the bearing capacity. The decrease in the bearing capacity as a result of the presence of good grasses and the increase in the bearing capacity due to good drainage almost cancel each other out. The result of good drainage ensures a grass stock with better grasses, better palatability, and resistance to treading, but above all, it also ensures a higher dry matter production. When improving the drainage condition of a plot, the good grasses, when present, will slowly take over at the expense of the bad grasses. During the period which can last from two to five years, grassland production is often poor. This period is also called the “dumb period”. To prevent this period as much as possible, the grassland is best sown in the short term after an improvement in the drainage.