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Soil water plant relationships

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Chapter - 4
Soil water plant relationships
Instructional objectives
On completion of this lesson, the student shall learn:
1. The soil and water system that is needed for plant growth
2. Classification of soils with regards to agriculture
3. Classification of water held within soil pores
4. Soil water constants and their significance
5. Watering interval for crops
1.0 Introduction
The basic soil, water, and plant relationships are important to agricultural producers, but
especially to irrigation users that desire to use best management practices such as irrigation
scheduling. Irrigation scheduling determines when and how much water needs to be added to a
crop’s root zone to promote optimum yields.
Both soil and water are essential for plant growth. The soil provides a structural base to the
plants and allows the root system (the foundation of the plant) to spread and get a strong hold.
The pores of the soil within the root zone hold moisture which clings to the soil particles by
surface tension in the driest state or may fill up the pores partially or fully saturating with it
useful nutrients dissolved in water, essential for the growth of the plants. The roots of most
plants also require oxygen for respiration. Hence, full saturation of the soil pores leads to
restricted root growth for these plants. (There are exceptions, though, like the rice plant, in which
the supply of oxygen to the roots is made from the leaves through aerenchyma cells which are
continuous from the leaves to the roots).
2.0 Soil’s Physical Characteristics
The various physical characteristics of soil affect the interaction between soil, water, and air.
2.1 Composition. A unit of soil is a combination of solid material, composed of mineral and
organic matter, and open space, called pores. By volume, most soils are roughly 50 percent
solids and 50 percent pore space.

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The mineral matter makes up about 45 to 47 percent of the total soil volume. This mineral matter
consists of small particles of sand, silt, or clay.
Organic matter is made up of decaying plant and animal substances and is distributed in and
among the mineral particles. Organic matter can account for up to about 5 percent of the overall
soil makeup by volume, but many agricultural soils have less than 1 percent organic matter.
The pores, spaces that occur between the mineral particles, are important because they store air
and water in the soil.
Figure 1 shows the approximate relationship between the substances in the soil composition,
with the pore space shown split between air and water. The amount of water and air present in
the pore spaces varies over time in an inverse relationship. This means that for more water to be
contained in the soil, there has to be less air. The amount of water and air in soil pore space is
essential to crop production.
Fig. 1 Typical soil composition by volume
2.2 Soil texture: The size of the particles that make up the soil determine soil texture.
The traditional method of determining soil particle size consists of separating the particles into
three convenient size ranges:
sand,
silt, and
clay.
Generally, only particles smaller than 2 mm (1/12 inch) in size are categorized as soil particles.
Particles larger than this are categorized as gravel, stones, cobbles, or boulders.
Sand particles range in size from 2 mm to 0.05 mm. There are subcategories assigned to this
range that include coarse, medium, and fine sand.

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Chapter - 4 Soil water plant relationships Instructional objectives On completion of this lesson, the student shall learn: 1. The soil and water system that is needed for plant growth 2. Classification of soils with regards to agriculture 3. Classification of water held within soil pores 4. Soil – water constants and their significance 5. Watering interval for crops 1.0 Introduction The basic soil, water, and plant relationships are important to agri­cultural producers, but especially to irrigation users that desire to use best management practices such as irrigation scheduling. Irrigation scheduling determines when and how much water needs to be added to a crop’s root zone to promote optimum yields. Both soil and water are essential for plant growth. The soil provides a structural base to the plants and allows the root system (the foundation of the plant) to spread and get a strong hold. The pores of the soil within the root zone hold moisture which clings to the soil particles by surface tension in the driest state or may fill up the pores partially or fully saturating with it useful nutrients dissolved in water, essential for the growth of the plants. The roots of most plants also require oxygen for respiration. Hence, full saturation of the soil pores leads to restricted root growth for these plants. (There are exceptions, though, like the rice plant, in which the supply of oxygen to the roots is made from the leaves through aerenchyma cells which are c ...
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