Other physical factors
Pores in the soil that are connected with the outside air produce a supply of oxygen (O2) and a discharge of carbon dioxide (CO2). Therefore pores are important to plant growth, mineralisation and soil life. Soil with less water and more air heats up quicker in the spring and that helps to start up mineralisation. Pores in the soil also give roots room to grow. Some plant roots, such as sunflowers, flax and clover, require pores of 0.3 to 0.5 mm. Other crops, such as onion or leek, have thicker roots and require larger pores. Roots can grow into small pores (0.2 mm) and enlarge them, causing pressures of 0.5 up to 2.4 MPa. The thickening of the roots, often with rings, shows that the roots experienced soil resistance. Soil life contributes to the formation of the pores. Worm tunnels often contain strings of roots.
There is no life without water. The same applies to the soil - a range of soil organisms simply don't survive a dried-out soil. Yet if there is too much water, the air is pushed out and life is impossible for organisms that need air (oxygen), such as plant roots and micro-organisms.
If there is no frost and the soil is not completely dried out, water evaporates from the soil. Besides evaporation by the plant (transpiration) there is evaporation through the soil itself (evaporation). Water uses heat to evaporate, which makes water the heat regulator of the soil. In summer evaporating water will prevent the temperatures of the soil from rising too high. However in the spring, water ensures that a wet soil heats up very slowly.
Water also affects the structure of the soil. When a sandy soil is fully dried out, the sand particles no longer stick together and the soil will blow away. Heavy clay becomes rock hard when it dries out. If the soil is saturated with water, it exceeds its liquid limit and comes a pappy, slick mass. If the moisture content is right, the soil particles stick together but create an airy and stable structure.
The bearing capacity of a soil is often determined by the density of the soil, the moisture levels and the level of compacting of the soil. If a soil contains too much water, the bearing capacity reduces. Good bearing capacity may be at odds with a good soil structure - a soil that contains large amounts of organic matter and pores has a lower density and therefore the bearing capacity reduces. If the top soil shows signs of distortion or ruts after driving on or being used as pasture, you have little bearing capacity.
The penetration resistance is the measure for the density of the soil. The density is determined by natural processes, but also by cultivation measures such as working the land with heavy machinery. The latter can produce compacting, which in turn affects soil life, the oxygen and moisture supply and root growth. A penetration resistance between 1.5 and 3.0 MPa reduces root growth, but with a penetration resistance of > 3.0 MPa root growth is seriously impeded.