Chapter Physical properties of soil


This chapter includes the following topics:

with additional information on the following:

• Soil profiles

• Plantrequirements

• Topsoil and subsoil

• Compositionofsoils

• Characteristics of sand, siltand clay

• Formationofsoils

• Soil texture

• Naturalsoilprofiles

• Soil structure

• Soils of the British Isles

• Cultivations

• Managementofmainsoiltypes

• Bed systems

Soil Texture Guide
Figure 17.1 Formation of soil begins with the weathering rocks which includes the powerful force of the sea. 295

Plant requirements

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.

Composition of soils

Mineral soils form in layers of rock fragments over the Earth's surface. They are made up of mineral matter comprising sand, silt and clay particles. There is also a small quantity of organic matter which is the part derived from living organisms. This framework of solid material retains water and gases in the gaps or pore space. The water contains dissolved materials including plant nutrients and oxygen and is known as the soil solution. The soil atmosphere normally comprises nitrogen, rather less oxygen and rather more carbon dioxide than in normal air, and traces of other gases. Finally, a soil capable of sustaining plants is alive with micro-organisms (organisms, such as

Mineral Composition Soil
Figure 17.2 Composition of a typical cultivated soil. The solid fraction of the soil is made up of mineral (50-60 per cent) and organic (1-5 per cent) matter. This leaves a total pore space of 35-50 per cent that is filled by air and water, the proportions of which vary constantly.

bacteria, fungi and nematodes, too small to be seen with the naked eye). Larger organisms such as earthworms and insects are also normally present (see p321).

The composition of a typical mineral soil is given in Figure 17.2 . which also illustrates the variation that can occur. The content of the pore space varies continually as the soil dries out and is rewetted. The spaces can be altered by the compaction or 'opening up' of the soil which in turn has a significant effect on the proportions of air and water being held.

Over a longer period the organic matter level can vary. The composition of the soil can be influenced by many factors and under cultivation these have to be managed to provide a suitable root environment. Organic soils have considerably higher organic matter content and are dealt with in Chapter 18.

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