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Why Study Soil-Plant-Water Relations?


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Title: Why Study Soil-Plant-Water Relations?

Why Study Soil-Plant-Water Relations?
An-Najah National University Faculty of
graduated studies Department of environmental
  • Soil, Water Plant Relationship 400555
  • Date 8-6-2011
  • Dr. Heba Al-Fares

  • A . Population
  • Of the four soil physical factors that affect
    plant growth (mechanical impedance, water,
    aeration, and temperature) (Shaw, 1952 Kirkham,
    1973), water is the most important.
  • Drought causes 40.8 of crop losses in the United
    States, and
  • excess water causes 16.4
  • insects and diseases amount to 7.2 of the losses
    (Boyer, 1982).

  • Water is lost from both the soil surface
    (evaporation) and the plant surface
    (transpiration), and is seldom optimal for
    maximum crop production in dry land
    (non-irrigated) agriculture.

  • People depend upon plants for food.
  • Because water is the major environmental factor
    limiting plant growth,
  • we need to study soil-plant-water relations to
    provide food for a growing population.

What is our challenge?
The human population growth curve
B. The Two-Square-Yard Rule
  • The population is limited by the productivity of
    the land.
  • There is a space limitation which is a space of
    two square yards per person.
  • The suns energy that falls on two square yards
    is the minimum required to provide enough energy
    for a human beings daily ration.

Movement of water through the soil-plant-atmospher
e continuum
  • The movement of water through the SPAC is divided
    into three parts
  • 1) water movement in the soil and to the plant
  • 2) Water movement through the plant, from the
    root to the stem to the leaf and
  • 3) water movement from the plant into the

What do plant growth curves look like?
  • A. The Importance of Measuring Plant Growth and
    Exponential Growth
  • Equations describing plant-growth curves
    demonstrate how we can quantify, and thus
    predict, plant growth.
  • Because water is the most important soil physical
    factor affecting plant growth, it is important to
    quantify plant growth to determine effects of
    water stress.
  • We first consider the growth of the bacterium

  • If the start of our observations, at the time 0
    min, there is 1 cell. When 20 min have elapsed
    there are 2 cells. When 40 min have elapsed there
    are 2 2 22 cells.
  • if N denotes the number of cells present at the
    moment when t minutes have elapsed, then the
    relation we seek is given by the equation
  • N 2t/20.
  • Linnaeus showed that an annual plant would have
    a million offspring in twenty years, if only two
    seeds grew up to maturity in a year.
  • X 220
  • where X is the number of offspring from the plant
    in twenty years.

B. Sigmoid Growth Curve
  • The S-shaped, or sigmoid, curve is typical of the
    growth pattern of individual organs, or a whole
    plant, and of populations of plants

The Role of Water in Plant Life
  • Water comprises more than 80 of the living and
    growing cells of most plants.
  • All actively growing plants have continuous
    liquid phase from soil to leaf.
  • Growing plants need large amounts of water
  • Water lose through leaves transpiration- in
    dry climates, weight of water lost may be 100s or
    1000s of time dry weight of plants. 
  •  Water loss through stomata. If these partially
    close to shut down some transpiration, it
    inhibits CO2 intake and slows photosynthesis.
  •  Plant suction in the day might be so high that
    little growth takes place. Crop may make large
    portion of its growth at night.

The Role of Water in Plant Life
  •  Number of units of water/unit of D.M. produced
    is called transpiration ratio. 
  • Inverse of this ratio called water use

The Role of Water in Plant Life
  • Soil the unconsolidated cover of the earth,
    made up of mineral and organic components, water
    and air and capable of supporting plant growth.
    Most important function GROW PLANTS
  • As a medium for plant growth, soil performs four
  • Anchors roots
  • Supplies water
  • Provides air
  • Furnishes minerals for plant nutrition
  • The pore space between the solids is taken up by
    water and air.
  • Air takes up part of the pore space not occupied
    by water. As the water increases, the air
    content decreases.

The Role of Water in Plant Life
  • Functions of Water in the Plant
  • Plants differ from animals because they are
    nutritionally self-sufficient, or autotrophic.
  • Water serves as a hydrogen donor and thereby as a
    building block for carbohydrates, which are
    synthesized by plants making use of sunlight.
  • Exchange of gases uptake of CO2 and release of
    water vapour to the atmosphere (transpiration).
  • Plants live permanently in one place, so they
    have to remove water from the soil water
    reservoir in their immediate vicinity.

  • Water is an important constituent of all plants.
  • Root, stem and leaf of herbaceous plants consist
    of 7095 water.
  • In contrast, water comprises only 50 of ligneous
    tissues, and finally dormant seeds contain only
    515 water.
  • water has a unique physiological importance in
    the life of plants for CO2 assimilation, for
    biochemical transformations and for the
    transmission of impulses and signals.

  • As a chemical agent it takes part in many
    chemical reactions, for instance in assimilation
    and respiration. It is a solvent for salts and
    molecules, and mediates chemical reactions.
  • Water is the medium of transport for nutrient
    elements and organic molecules from the soil to
    the root and the means of transport of salts and
    assimilates within the plant.
  • Stimulation and motion of organelles and cell
    structures, cell division and elongation are
    examples of processes controlled by hormones and
    growth substances, and water is the carrier of
    these messengers, enabling the regulatory system
    of the plant.

  • Water confers shape and solidity to plant
  • The hydrostatic pressure in cells is dependent on
    their water content, and permits cell enlargement
    against pressure from outside, which originates
    either from the tension of the surrounding tissue
    or from the surrounding soil.
  • The large heat capacity of water greatly dampens
    the daily fluctuations in temperature that a
    plant leaf might undergo, due to the
    considerable amount of energy required to raise
    the temperature of water.
  • The vapour that transpires from leaves causing
    cooling due to evaporation

Availability of Soil Water to Plants    
  •  Water moves into the plant whenever suction in
    the water in the plant is greater than that in
    the water in the soil. Most plants withdraw water
    from soils until soil moisture reaches about 15
  • Fine textured soils hold more water than sands at
    field capacity 
  • Fine textured soils are less droughty 

Depending on soil texture, which is determined by
the particle-size distribution, soils will vary
in water content at field capacity and at the
permanent wilting point. Both characteristic
values enclose the plant-available water content.
Silt loam soil contains the maximum of available
water. The water at the permanent wilting point
is not available to plants. The fineness of
texture increases with the silt and clay content,
presented as approximate percentages.
Water Requirements of Crop Plants
  •  The rate at which water if available would be
    removed from the soil and plant surface is
    potential evapotranspiration (PET). 
  • The ratio of evapotranspiration (ET) to (PET)
    gives figure called relative evapotranspiration
    or crop coefficient (Kco).
  • Energy is required to evaporate water from soil
    and to cause plants to transpire.   
  • Crops utilize only 1 to 2 of energy
    received. Utilization of energy may become the
    next limiting factor when moisture is adequate
    and good cropping practices are followed.
  •  Many crops have critical stages of growth when a
    water deficient will cause unusually large
    reduction in yield.  

Adaptation Strategies of Plants to Overcome Water
  • According to the presence and supply of water,
    ecologists divide terrestrial plants into
  • hygrophytes,
  • mesophytes and
  • xerophytes.

  • Hygrophytes are plants that thrive in generally
    humid habitats, where there is no shortage to the
    water supply throughout the growing season.
  • In temperate zones, in addition to these plants
    with a humid biotype, there are many shade-loving
    herbaceous forest species that also belong in
    this category.

  • Xerophytes are adapted to water shortage, which
    may occur regularly and may persist over long
    periods of time.
  • Anatomical and physiological specialization has
    taken place to meet the requirements of these
    plants so that they can survive extended periods
    of drought.
  • To this group belong succulent plants that
    establish an internal water reservoir for use
    during drought, thereby postponing desiccation.
  • Another group of xerophytic plants are able to
    endure considerable water loss from their tissues
    without losing their ability to survive.

  • Mesophytes fit in between these two extremes.
  • Many plants from temperate climates belong to
    this group, but the cultivated plants from those
    regions are also included.
  • The latter cannot endure an extreme form of arid
    climate without being irrigated.
  • However, for short periods of water shortage they
    are well prepared.
  • When water supply falls short, they can reduce
    their transpiration rate dramatically and modify
    other processes.

  • How do plants react to water shortage?
  • , and
  • what kind of strategies have they developed with
    respect to drought resistance?

Drought escape
  • Those plants that are adapted to drought escape
    will germinate from dormant seeds only when there
    is abundant rainfall.
  • Afterwards they can manage with a limited supply
    of water because they can terminate vegetative
    growth and become reproductive after a very short
    life cycle of just a few weeks, even ending with
    mature seed.
  • Subsequent dry periods are escaped through seed

Drought escape
  • Among cultivated plants, the short-lived two
    rowed barley is a drought escaper.
  • Groundnut and cowpea are classed in this group
    along with the C4 plants from the different
    species of millet.
  • All of these crops reach maturity, although
    annual precipitation may not exceed 250300 mm

drought avoidance.
  • Plants at adapted todrought avoidance may avoid
    or at least retard desiccation of their tissues
    by increasing water uptake, reducing water loss,
    or by enhancing the internal storage of water.
  • Like the first group these plants maintain a
    water balance that is largely in equilibrium.
  • They belong to the hydrostable or homoiohydric

  • Water savers
  • Many of these plants are succulents and can save
    a large volume of water within parenchymatous
    tissue when the very short periods of rainfall
  • Quite a number of species in the family Cactaceae
    belong to this group.
  • Cacti, as well as plants of the families
    Crassulaceae and others are representatives of a
    group that demonstrate CAM.
  • These CAM plants effect a unique physiological
    adaptation to water shortage.
  • During the night, however, they will be opened
    for CO2 assimilation and accumulation in the form
    of organic acids, which during the daytime supply
    CO2 again for producing carbohydrates by

  • There are also water savers among C3 and C4
  • In many cases the plants possess distinct
    anatomical features such as stomates that are
    deeply sunk into the epidermis, thick and
    leathery or fleshy leaves, small leaves, leaves
    with waxy coatings over the cuticle and leaves
    with a felt-like cover of fine hairs.
  • Some of the water savers restrict water loss
    during dry periods by rolling or folding their

Water spenders
  • These plants raised water through deep rooting
    system during the night from deep layers to more
    shallow ones, where the water was released from
    the roots into the surrounding soil.
  • This hydraulic lift enables plants to make use
    of a larger water supply during the day for
    transpiration and for CO2 assimilation.
  • Water spenders include esparcet.
  • This is a perennial deep-rooted forage legume,
    adapted to calcareous soils and native to
    Mediterranean regions

Drought tolerance
  • Plants relying on this strategy are able to
    tolerate a certain level of tissue desiccation.
  • During phases of desiccation they limit their
    vital functions quite considerably.
  • The plants are said to be hydrolabile

Osmotic adjustment
  • The capability of solute accumulation is termed
    osmotic adjustment.
  • When desiccation develops slowly over time, many
    plants are able to accumulate inorganic ions or
    organic compounds, such as sugars, alcohols and
    amino acids, in their tissues.
  • The solutes are concentrated in the cytoplasm and
    vacuoles, but the water content of the cells is
    maintained at a more or less stable level.
  • By osmotic adjustment plants guard against a loss
    of turgidity.
  • This adjustment will allow the plant to survive
    periods of drought more vigorously and for longer
    periods of time, and can allow the extraction of
    water from soil
  • Sorghum is considered as a crop species
    characterized by a strongly developed drought
    tolerance compared with other crops.
  • soybean are capable of osmotic adjustment, and
    the same is true of other grain legumes and

Water and Net Primary Production
  • There is a well defined relationship between
    water use and the amount of dry matter produced.
  • the net primary production, i.e. gross primary
    production minus respiration

Water, temperature and radiation
  • Factors influence plant growth and can regulate
    net primary production through
  • Net assimilation rate (NAR, rate of growth per
    unit of leaf area).
  • A small biomass will result in a small leaf area
    index (LAI, total green area of one side of a
    leaf as a ratio of one unit of soil surface
  • A small LAI is the second cause of reduced
  • The Crop Growth Rate (CGR) is the rate of growth
    per unit of soil surface area.
  • CGR NAR LAI (1.1)
  • Equation 1.1 establishes that the productivity of
    a crop stand is dependent on the photosynthetic
    net productivity of the single leaf and of the
    size of the total leaf canopy

Relationship between net primary production of
terrestrial forests and annual precipitation as a
rough index of the level of available water.
Seed yield of groundnut as related to water use.
The water use includes the transpiration of the
crop and the evaporation from the soil. Lysimeter
studies in Georgia and Florida, cultivar is
Florunner (after Boote and Ketring, 1990).
Influence of temperature on the rates of gross
photosynthesis, respiration and net
photosynthesis (A) as well as on growth rate (B).
The three cardinal points for temperature are
the minimum, optimum and maximum values (Tmin,
Topt and Tmax). (Schematic after Pisek et al.,
1973 for (A) and Fitter and Hay, 1981 for (B).)
The Role of Water in Soil
  • Soil Genesis and Soil Functions
  • Water is of primary importance for soil genesis.
  • Without the action of water, soils would not
  • Soils originate from parent rock.
  • The first step towards soil formation is the
    weathering of these rocks.
  • Water contributes to the processes of weathering
    through physical and chemical actions.

The Role of Water in Soil
  • The development of soil can be thought of as
    occurring in two phases
  • Soil Genesis the weathering of rock substrates
  • Mechanical forces
  • Chemical reactions

The Role of Water in Soil
  • 2) Soil Formation Hans Jenny (1941)
    characterized soil formation as a function of
    five independent variables climate, organisms,
    topography, parent material, time.
  • Organism include such elements as the soil
    microbial community, litter inputs, vegetation
  • Parent material largely determines chemical
    characteristics of the derived soils.
  • ? The interaction of organisms parent material
    with climate produce a soil with characteristic

Soil genesis
  • Soil genesis is accompanied by the formation of
    soil structure, which is essentially dependent on
    soil water.
  • Water causes the clay minerals to swell and
    shrink, and the soil matrix becomes subdivided by
    planes of weakness or by visible fissures.

Soil genesis
  • Also, ice formed by frost can separate the soil
    matrix into aggregates of characteristic size and
  • Without water there would be no transport
    processes in the soil. Water in the soil is
    seldom in a state of equilibrium.
  • Usually it is in constant motion within the soil
    profile. The reason is that the energy state of
    water, its potential, is generally not the same
    but varies between different locations within the

Physical weathering
  • Physical weathering splits rocks and minerals,
    but their chemical composition is not basically
  • Water is the main agent and works through frost
    action, but in arid regions differential thermal
    expansion of minerals can also split rocks.
  • The fragments formed are transported by surface
    water from higher elevations downhill into the
  • Here the fine debris is deposited in alluvial
    fans, burying the former bottom of the valley,
    and leveling the topographical features.

Chemical weathering
  • Chemical weathering breaks down minerals by
    hydration, hydrolysis and dissolution.
  • The disruptive force of water is greatly
    augmented by protons or hydronium ions (H3O)
    that are derived from organic and inorganic
  • In this way even the very insoluble silicates are
    finally broken down.
  • Increasing the temperature accelerates the
    kinetics of destruction.

  • Water moves cations, silicic acid, and iron and
    aluminium compounds solutes as well as colloidal
    solids deeper into the soil body or even beyond
    the soil into deeper strata.
  • Disintegration, displacement, precipitation and
    leaching are essential parts of the pedogenic
    processes, supported by water.

Soil water
  • Soil water is the most limiting factor for crop
    production in the world.
  • Only 45 of the earth's arable land receives
    adequate moisture for crop growth
  • Soil water carries nutrients to a growing crop
    and has a significant effect on aeration and
    temperature of the soil.

Soil Water 
  • One of the most important factors affecting crop
  • Water must be available to replenish that lost by
    evaporation and transpiration.
  • Soil water carries nutrients in solution to the
    growing crop.
  • Has significant effect on aeration and
    temperature conditions of the soil.
  • Seldom is the water content of soil at optimum
    value for maximum crop production.

How is Soil Water Classified?
  • 1) Hygroscopic Water is held so strongly by the
    soil particles (adhesion), that it is not
    available to the plants.
  • 2) Capillary Water is held by cohesive forces
    greater than gravity and is available to plants.
  • 3) Gravitational Water is that water which cannot
    be held against gravity.
  • as water is pulled down through the soil,
    nutrients are "leached" out of the soil (nitrogen)

Soil water
  • There are certain limits for soil water.
  • Field capacity is when the soil pores are so full
    of water that the next drop will leach downward
    out of the rooting zone.
  • The opposite extreme is wilting point, the level
    at which plant roots can no longer take in water
    and turgor is lost (wilting).
  • The goal of a soil, water, plant continuum is to
    maintain the soil water between these extremes,
    allowing nutrient movement, aeration, and
    supplying water in excess of evaporation and
    transpiration (evapotranspiration).
  • Measuring plant available water and adjusting
    water levels with irrigation is another way
    mankind has tried to modify the environment to
    maximize food and fiber production

Depending on soil texture, which is determined by
the particle-size distribution, soils will vary
in water content at field capacity and at the
permanent wilting point. Both characteristic
values enclose the plant-available water content.
Silt loam soil contains the maximum of available
water. The water at the permanent wilting point
is not available to plants. The fineness of
texture increases with the silt and clay content,
presented as approximate percentages.
Moisture holding capacity is an essential feature
of soils
  • Soil can become saturated if all pores filled
  • All water is hold by soil particulars, at field
    capacity (FC)
  • Capillary water is usually present
  • Extracted by plants
  • Wilting point (WP)
  • Plant no long extract water
  • All affected by soil texture
  • Sand
  • Lower capacity
  • Clays
  • Higher capacity

  • Water content is a measurement of the amount of
    water in the soil either by weight or volume and
    is defined as the water lost from the soil upon
    drying to constant mass at 105C

Water content at different soils
  • Energy Concept of Soil Moisture 
  • Expression in terms of energy makes it more easy
    to compare availability of the moisture in soils
    of different textures.    
  • Most commonly accepted unit at present is bars of
    suction. Suction is negative pressure, the higher
    the numerical value the lower the energy status
    of the water. 
  • Soils at field capacity - 0.1-0.3 bars of
  • When soils at wilting point - 15 bars of suction

Water Holding Capacities of Soils
  • The amount of water a soil can retain is
    influenced by
  • soil texture
  • soil structure
  • organic matter.

Water Holding Capacities of Soils
  • Soil Texture
  • The smaller the soil particles, the greater the
    soils water holding capacity. Clay has more
    water holding capacity than sand.
  • Small soil particles (clay) have more small pores
    or capillary spaces, so they have a higher water
    holding capacity. Large soil particles (sand)
    have fewer capillary spaces, therefore less
    ability to hold water.

Water Holding Capacities of Soils
  • Soil Structure
  •  is the way soil particles (sand, silt, clay)
    arrange, and combine. Many soil structure classes
    are differentiated on the basis of their
    aggregate size, shape, arrangements
  • A soil structure has a direct correlation to the
    amount of water it can retain.

Water Holding Capacities of Soils
  • Organic Matter
  • Organic matter aids in cementing particles of
    clay, silt, and sand together into aggregates
    which increases the water holding capacity.
  • Decomposition of organic matter also adds
    vital nutrients to the soil. 

  • Water Movement in Soils   
  • Water will move in a soil from one point to
    another if the water at the first point has
    higher energy status than the water at the
  • Water entering a dry soil is held at higher
    suction in the zone below the wetting front and
    water moves down. The rapidity of movement
    depends on the size of the energy difference and
    soil characteristics.
  • If water is applied to the surface by rain or
    irrigation much faster than it can enter soil and
    be transmitted downward, the excess water
    accumulates on the surface. If the slope is
    great, erosion will likely result (unless surface
    stable or protected by plant residues).
  • The kinetic energy of rain can break down
    aggregates at the soil surface

  • Detrimental effects of excess water are 
  • Water moving laterally across the surface 
  • Water loss and erosion
  • A layer with limited water permeability 
  • Can be treated with subsoiler

Soil Fauna and Vegetation Cover
  • The soil is a porous body built up from inorganic
    and organic solid particles with pore spaces in
  • The development of immature soils is accompanied
  • humification,
  • weathering of minerals,
  • release of nutrients,
  • a vigorous development of plant cover,
  • stronger rooting and
  • soil colonization and an intensified nutrient
  • Such soils become more porous.

Soil Fauna and Vegetation Cover
  • Soil animals of different sizes work through the
  • mixing inorganic and organic particles, creating
    connecting pores, and
  • stabilizing aggregates within soil horizons and
    near the surface.
  • In the course of this development the soils
    quality as a habitat for plants improves.

Soil Fauna and Vegetation Cover
  • The development of the soil and of the plant
    association go along with (and depend on) each
  • The soil is effectively protected against erosion
    by a dense vegetative cover, by roots and by a
    litter layer.
  • Stabilized soil surfaces allow rain water to
    enter the soil at high rates, a very important
    characteristic during intense rain storms.

  • The favorable state of soil conditions under
    vegetation cover will not last forever.
  • Climatic change, leaching of calcium carbonate,
    progress in weathering, acidification, or human
    impacts can contribute to soil degradation and
    loss of its properties as a habitat

Controlling and Measuring Soil Moisture  
  • Maximum crop production would be attained if soil
    moisture suction could be held at a value low
    enough that the energy exerted by the plant would
    be minimal. 
  • Instruments of many types marketed for measuring
    soil moisture. 
  • Tensiometer 
  • More useful in sands
  •  Removing a soil sample from appropriate
    depth/drying/weighing will give you a reasonable
  • Some soils change color as they go from wet to
    dry. You "feel" it.   Observe both crop and soil
    closely for signs of moisture stress. Leaf
    rolling in corn
  • Measure rainfall/estimate ET on open pan.
    Computer using meteorological data to make
    recommendation to farmer.
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