Lecture 2 SWP
Lecture 2 SWP
Lecture 2 SWP
Soil provides the mechanical and nutrient support & water storage
necessary for plant growth
Soil Structure
Soil Porosity
Mineral matter consists of small mineral particles of either sand, silt, or clay
Organic matter is made up of decaying plant and animal substances
(distributed in and among the mineral particles)
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.
Soil Texture
• Soil texture is determined by the size of the particles that make up the soil or the
relative distribution sand, silt and clay.
• The traditional method of determining soil particle size is done by separating the
particles into three convenient size ranges.
• 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.
• Generally, only particles smaller than 2 mm in size are categorized as soil particles.
Particles larger than this are categorized as gravel, stones, cobbles, or boulders.
• Silt particles can range in size from 0.05 mm down to 0.002 mm. The physical
appearance of silt is much like sand, but the characteristics are more like clay.
• Clay particles are less than 0.002 mm in size. Clay is an important soil fraction because
it has the most influence on such soil behaviour as water-holding capacity.
• Clay and silt particles cannot be seen with the naked eye. (Figure 2)
Figure 2: Textural Classification
Soil texture is determined by the mass ratios, or the percent by weight, of the three
soil fractions. The soil textural triangle, Figure 3 shows the different textural classes
and the percentage by weight of each soil fraction. For example, a soil containing 30
percent sand, 30 percent clay, and 40 percent silt by weight is classified as a clay
loam.
Figure 3: Textural Triangle
Soil Structure
• Aggregated soils types are generally the most desirable for plant
growth.
GRANULAR BLOCKY PRISMATIC MASSIVE
Density of solids ( Mean Particle Density)-
𝜌 𝑀𝑠
𝑠=
𝑉𝑠
Dry Bulk Density -
𝜌 𝑀𝑠 𝑀𝑠
𝑏= =
𝑉𝑡 𝑉𝑠 +𝑉𝑎 +𝑉𝑤
𝜌 𝑀𝑡 𝑀𝑠 +𝑀𝑤
𝑡= =
𝑉𝑡 𝑉𝑠 +𝑉𝑎 +𝑉𝑤
Porosity
𝑉𝑓 𝑉𝑎 + 𝑉𝑤
𝑓= =
𝑉𝑡 𝑉𝑠 + 𝑉𝑎 + 𝑉𝑤
𝑴𝒔
𝑽𝒇 𝑽𝒂 +𝑽𝒘 𝑽𝒕 −𝑽𝒔 𝑽𝒔 𝝆𝒔
𝒇= = = = 1- = 1-𝑴𝒔
𝑽𝒕 𝑽𝒔 +𝑽𝒂 +𝑽𝒘 𝑽𝒕 𝑽𝒕 𝝆𝒃
𝝆𝒃
= 1-
𝝆𝒔
Voids Ratio
𝑉𝑎 + 𝑉𝑤 𝑉𝑓
𝑒= =
𝑉𝑠 𝑉𝑡 − 𝑉𝑓
Gravimetric Moisture Content
𝑀𝑤
𝑤=
𝑀𝑠
Volumetric Moisture Content
𝑉𝑤 𝑉𝑤
𝜃= =
𝑉𝑡 𝑉𝑠 + 𝑉𝑓
𝑀𝑤
𝑉𝑤 𝑉𝑤 𝜌𝑤 𝑀𝑠 𝑀𝑤 1 𝜌𝑏
𝜃= = = = = 𝑤
𝑉𝑡 𝑉𝑠 +𝑉𝑓 𝑉𝑡 𝑉𝑡 𝑀𝑠 𝜌𝑤 𝜌𝑤
SOIL AND WATER INTERACTIONS
Water stored in the pores of the soil is said to be in capillary storage. An example
of this phenomenon would be to place one end of a glass capillary tube in a pan of
water. Water in the tube will rise to a certain height, which depends on the
diameter of the capillary tube (Figure 4). This phenomenon can act in any
direction and is the key to water being stored in soil pores as illustrated in Figure
5.
Figure 4. Capillary forces illustrated by
how far water rises to tubes of various diameters
Figure 5. HOW Soil Holds Water
Macro pores > 100 µm
Meso pores 30 – 100 µm
Micro pores < 30 µm 1 mm = 1000 µm
Soil Water Tension
• The ease by which water can be extracted from the soil depends on the soil water
tension, also known as the soil water potential.
• Water being held in pores by the capillary storage is held in the soil at a certain
tension. The same is true for water held with the adsorption phenomenon. As the
soil dries, these tensions become larger.
• The wilting point occurs when the potential of the plant root is balanced by the
soil water potential, thus plants are unable to absorb water beyond this tension.
This commonly occurs at approximately 15 bars. At this soil water tension, the
plant will die.
% Volumetric Moisture Content
• Water between the field capacity and the wilting point is water that is
available to the plant.
• However best plant growth and yield occur when the soil water content
remains in the upper half of the plant available soil water range.
• Plants develop the tension, or potential, to move soil water from the soil
into the roots and distribute the water through the plant by adjusting the
water potential, or tension, within their plant cells.
Soil Type Volumetric Moisture Content at Volumetric Moisture Content at Volumetric Moisture Content at
Saturation 𝜽𝒔 Field Capacity 𝜽𝑭𝑪 Permanent Wilting Point 𝜽𝑷𝑾𝑷
• A plant has different water needs at different stages of growth. While a plant is
young it requires less water than when it is in the reproductive stage. When the
plant approaches maturity, its water need drops.
Plant Root Depth
• A plant’s root depth determines the depth to which soil water can be extracted.
• A young plant has only shallow roots and soil water deeper than rooting depth is of
no use to the plant.
• Plants typically extract about 40 percent of their water needs from the top quarter of
their root zone, then 30 percent from the next quarter, 20 percent from the third
quarter, taking only 10 percent from the deepest quarter.
• Therefore, plants will extract about 70 percent of their water from the top half of their
total root penetration.
• Deeper portions of the root zone can supply a higher percentage of the crop’s water
needs if the upper portion is depleted.
• However, reliance on utilization of deeper water will reduce optimum plant growth.
SUMMARY
1. Soil water holding capacity varies with soil texture. It is high for medium
and fine-textured soils but low for sandy soils.
2. Plant roots can use only available soil water—the stored water between
field capacity and permanent wilting point. However, as a general rule, plant
growth and yields can be reduced if soil water in the root zone remains
below 50 percent of the water holding capacity for a long period of time,
especially during critical stages of growth.
3.Although plant roots may grow to deep depths, most of the water and
nutrients are taken from the upper half of the root zone. Plant stress and
yield loss can occur even with adequate water in the lower half of the
root zone.
Crop (Plant) Growth Stages
The growth of a crop to its maturity is considered in 04 stages,
namely
• Initial stage
• Crop Development stage
• Mid stage
• Late stage