Fundamentals of Irrigation and On-farm Water Management: Volume 1 || Soil: A Media for Plant Growth

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  • Chapter 4

    Soil: A Media for Plant Growth

    The role of soil in the soilplantatmosphere Continuum is unique. Although soil is

    not essential for plant growth and indeed plants can be grown hydroponically (in a

    liquid culture), usually plants are grown in the soil, and soil properties directly

    affect the availability of water and nutrients to plants. Soil water affects plant

    growth directly through its controlling effect on plant water status and indirectly

    through its effect on aeration, temperature, nutrient transport, uptake, and transfor-

    mation. The knowledge of soil-water status and its movement in soil is important

    and has practical implications in agricultural, environmental, and hydrological

    situations. Soil-water movement in the field depends on the hydraulic properties

    of the soil. The understanding of these properties is helpful in good irrigation design

    and management.

    4.1 Basics of Soil

    4.1.1 Definition of Soil

    Soil is a typical heterogeneous multi-phasic porous system, which, in its general

    form, contains three natural phasic components: (1) the solid phase of the soil

    matrix (formed by mineral particles and solid organic materials), (2) the liquid

    phase, which is often represented by water and which could more properly be called

    the soil solution; and (3) the gaseous phase, which contains air and other gases.

    Practically speaking, a soil particle must pass through a 2-mm sieve to be called soil

    particle. Soil particles are either organic or inorganic.

    The soil particles are aggregated by organic matter, mineral oxides, and charged

    clay particles. The gaps between the particles link together into a meandering

    network of pores of various sizes. Through these pore spaces, the soil exchanges

    water and air with the environment. The movement of air and water also allows for

    heat and nutrient to flow.

    M.H. Ali, Fundamentals of Irrigation and On-farm Water Management: Volume 1,DOI 10.1007/978-1-4419-6335-2_4,# Springer ScienceBusiness Media, LLC 2010

    107

  • 4.1.2 Agricultural Soil

    The agricultural soil is one which, with the presence of nutrient and water, can

    facilitate plant growth.

    4.1.3 Components of Soil

    The primary components of soil are as follows:

    (a) Minerals

    (b) Water

    (c) Air

    (d) Organic matter

    To understand soil and its behavior, we must first understand its composition

    (Fig. 4.1) and the interrelation among the components.

    (a) Mineral

    The mineral fraction of soil consists of the actual soil mineral particles. Tradi-

    tionally, soil minerals are classified on the basis of their size. The USDA soil

    textural classification system divides minerals into sand, silt, and clay particles on

    the basis of their sizes, which are as follows:

    Sand particle: 20.05 mm

    Silt particle: 0.050.002 mm

    Clay particle:

  • (c) Air

    The third component of soil is soil-air. It is composed primarily of nitrogen,

    carbon dioxide, and oxygen. When the gravitational water drains away, the air fills

    the large voids or pores between the soil particles. As more water is used, more air is

    drawn into the soil. When rainfall or irrigation events occur, the air is forced out of

    the soil. The oxygen in the soil air is needed to support the growth of plant roots.

    Plant roots respire or breathe much like animals taking oxygen from the air and

    using it to oxidize sugars and carbohydrates to make energy needed for growth and

    the uptake of nutrients. Ideally, a soil should contain 1530% air-filled pore space.

    (d) Organic matter

    The fourth component of soil is organic matter. It includes all materials that are

    primarily composed of carbon, hydrogen, and oxygen. The common or laymens

    definition of organic matter would be living, dead, or partially decayed plant and

    animal substances. This definition includes all the plants (live and dead plants)

    roots, rhizomes, and leaves. The entire soil microbial community is also a part of

    the soil organic matter pool.

    Soil organic matter has many beneficial functions in soil such as retaining water,

    retaining nutrients, reducing the saturated hydraulic conductivity of sandy soils, and

    acting as a binding agent between soil particles to promote the formation of soil

    aggregates. Organic matter has a high cation exchange capacity (CEC), which

    allows it to attract and retain cations in an exchangeable form. The high CEC

    also tends to help buffer the soil pH and prevent much fluctuation in pH. Organic

    matter is effective in absorbing and retaining water and minimizing the potential to

    wash off of the soil with runoff. It serves as a food source for soil microbes and

    helps to maintain an adequate and healthy population of soil microbes. Finally, it

    acts or serves as a source of plant nutrients.

    From the components of soil, it is observed that the soil consists of three types

    (phasics) of materials: solid, liquid, and gaseous. The solid part consists of soil

    particle (mineral) and organic matter, the liquid part consists of water and dissolved

    substances, and the gaseous part consists of air and/or water vapor. The soil is thus a

    complex system. Its solid matrix consists of particles differing in chemical and

    mineralogical composition as well as in size, shape, and orientation. The mutual

    arrangement or organization of these particles in the soil determines the character-

    istics of the pore spaces in which water and air are transmitted or retained.

    4.1.4 Importance of Soil for Irrigation

    Soil acts a media of water storage. The loss of water by plants to the atmosphere is an

    inevitable consequence of the necessity of a physical pathway for gaseous exchange

    through which CO2 can be acquired from the atmosphere. Since few plants are

    capable of storing a significant fraction of the water that must be transpired daily,

    4.1 Basics of Soil 109

  • there must be a continuous replenishment of the plant water through the root system.

    The soil pores in the vicinity of the plant roots provide the reservoir by which this

    water is stored and through which it is transmitted to the roots.

    Soils differ in their physical and hydraulic properties that determine in part the

    suitability for plant growth. This chapter deals with the principles of soil properties

    that are related to water absorption, retention, and transmission by soils; and

    methods for their determination/estimation, which must be understood if optimum

    value is to be realized for the management of such a system for irrigation and water

    management.

    4.2 Physical Properties of Soil

    For optimum plant growth, it is essential that the soil provides a favorable physical

    environment for root development that provides the plants needs forwater, nutrients,

    and anchorage. Other essential conditions include aeration, optimum soil tempera-

    ture, seed emergence, root penetration facility (low mechanical impedance), etc.

    4.2.1 Qualitative Description of Soil Physical Properties

    Similar to other physical bodies, soils have unique properties that define them. The

    physical properties of a soil are those characteristics that can be seen with the eye or

    felt between the thumb and fingers. They are the results of soil parent materials

    being acted upon by climatic factors and affected by topography or life forms (kind

    and amount such as forest, grass, or soil animals) over a period of time. A change in

    any one of these usually results in a difference in the type of soil formed. While

    diagnosing whether a soil is healthy or not, or to differentiate one type of soil from

    another, the following physical properties can be found:

    Color

    Texture

    Tilth

    Depth

    Slope

    Drainage or drainability

    Water holding capacity

    Bulk density

    Structure

    Porosity

    The physical properties and chemical composition largely determine the suit-

    ability of a soil for its planned use and the management requirements to keep it

    productive.

    110 4 Soil: A Media for Plant Growth

  • 4.2.1.1 Color

    The color of a soil can give idea to its health, origin, and long-term changes. It can

    also indicate the color of the parent material. Surface soil colors vary from almost

    white, through shades of brown and gray, to black. Light colors indicate low

    organic matter content and dark colors indicate a high organic material content.

    The more humus, the blacker the soil. Light or pale colors in the surface soil are

    frequently associated with relatively coarse texture, highly leached conditions, and

    high annual temperature. Distinct red and yellow colors usually indicate older,

    more weathered soils. Subsoil color can be a valuable indicator of how well the soil

    drains. The red to brown color of subsoils comes from iron coatings under well-

    aerated conditions.

    4.2.1.2 Texture

    Texture refers to the relative amount of differently sized soil particles (sand, silt,

    and clay), or the fineness/coarseness of the mineral particles in the soil. Texture is

    used only to describe the distribution of the inorganic fraction. Humus content has

    nothing to do with texture.

    4.2.1.3 Tilth

    Tilth refers to a soils physical condition. A soil in good tilth is easily worked,

    crumbled, and readily takes in water when dry. Factors that influence a soils tilth

    are texture, organic

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