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  Section: Principles of Horticulture » Physical properties of soil
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Plant requirements

Physical properties of soil
  Plant requirements
  Composition of soil
  Formation of soils
  Natural soil profiles
  Soils of the British Isles
  Topsoil and subsoil
  Soil components
  Soil texture
  Soil structure
  Management of main soil types
Figure 17.1 Formation of soil begins with the weathering rocks which includes the powerful force of the sea.
Figure 17.1 Formation of soil
begins with the weathering
rocks which includes the
powerful force of the sea.

The growing tip of the root wriggles through the growing medium following the line of least resistance. Roots are able to enter cracks that are or can be readily opened up to about 0.2 mm in diameter, which is about the thickness of a pencil line. Compacted soils severely restrict root exploration, but once into these narrow channels the root is able to overcome great resistance to increase its diameter. Anything which reduces root exploration and activity can limit plant growth. When this happens action must be taken to remove the obstruction to root growth or to supply adequate air, water and nutrients through the restricted root volume.

The root normally provides the anchorage needed to secure the plant in the soil. Plants, notably trees with a full leaf canopy, become vulnerable if their roots are in loose material, in soil made fluid by high water content or are restricted, e.g. shallow roots over rock strata close to the surface. Until their roots have penetrated extensively into the surrounding soil, transplants are very susceptible to wind rocking: water uptake remains limited as roots become detached from the soil and delicate root growth is broken off. The plant may be left less upright.

In order to grow and take up water and nutrients the root must have an energy supply. A constant supply of energy is only possible so long as oxygen is brought to the site of uptake (see respiration). Consequently the soil spaces around the root must contain air as well as water. There must be good gaseous exchange between the atmosphere around the root and the soil surface. This may sometimes be achieved by the selection of plants that have modifications of their structure that enables this to occur throughout the plant tissues (see adaptations), but it is normally a result of maintaining a suitable soil structure. A lack of oxygen or a build-up of carbon dioxide will reduce the root’s activity. Furthermore, in these conditions anaerobic bacteria will proliferate, many produce toxins such as ethylene. In warm summer conditions roots can be killed back after one or two days in waterlogged soils.


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