Why Study Soil-Plant-Water Relations?

1
Why Study Soil-Plant-Water Relations?
An-Najah National University Faculty of
graduated studies Department of environmental
sciences

• Soil, Water Plant Relationship 400555
• Date 8-6-2011
• Dr. Heba Al-Fares

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Introduction

• 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).

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• 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.

4

• 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.

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What is our challenge?
The human population growth curve
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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.

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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
  root
• 2) Water movement through the plant, from the
  root to the stem to the leaf and
• 3) water movement from the plant into the
  atmosphere

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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
  E.coli.

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• 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.

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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

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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.

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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
  efficiency 

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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
  functions
• 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.

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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.

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• 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.

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• 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.

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• Water confers shape and solidity to plant
  tissues.
• 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

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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
  bars. 
• Fine textured soils hold more water than sands at
  field capacity 
• Fine textured soils are less droughty 

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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.
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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.  

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Adaptation Strategies of Plants to Overcome Water
Shortage

• According to the presence and supply of water,
  ecologists divide terrestrial plants into
• hygrophytes,
• mesophytes and
• xerophytes.

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Hygrophytes

• 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.

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Xerophytes

• 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.

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Mesophytes

• 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.

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

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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
  dormancy.

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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

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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
  species.

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• 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
  occur.
• 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
  photosynthesis

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• There are also water savers among C3 and C4
  plants
• 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
  leaves

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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

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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

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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
  sugarbeet

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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

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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
  area).
• A small LAI is the second cause of reduced
  productivity.
• 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

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Relationship between net primary production of
terrestrial forests and annual precipitation as a
rough index of the level of available water.
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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).
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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).)
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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
  develop.
• 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.

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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
  by
• Mechanical forces
• Chemical reactions

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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
  type.
• Parent material largely determines chemical
  characteristics of the derived soils.
• ? The interaction of organisms parent material
  with climate produce a soil with characteristic
  features.

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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.

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Soil genesis

• Also, ice formed by frost can separate the soil
  matrix into aggregates of characteristic size and
  form.
• 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
  profile.

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Physical weathering

• Physical weathering splits rocks and minerals,
  but their chemical composition is not basically
  changed.
• 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
  valleys.
• Here the fine debris is deposited in alluvial
  fans, burying the former bottom of the valley,
  and leveling the topographical features.

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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
  acids.
• In this way even the very insoluble silicates are
  finally broken down.
• Increasing the temperature accelerates the
  kinetics of destruction.

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• 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.

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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.

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Soil Water 

• One of the most important factors affecting crop
  production.
• 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.

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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)

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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

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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.
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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

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WATER CONTENT

• 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

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Water content at different soils
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• 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
  suction 
• When soils at wilting point - 15 bars of suction

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Water Holding Capacities of Soils

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

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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.

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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.
•  

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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. 

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• 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
  second.    
• 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

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Drainage

•    
• 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

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Soil Fauna and Vegetation Cover

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

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Soil Fauna and Vegetation Cover

• Soil animals of different sizes work through the
  ground,
• 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.

64
Soil Fauna and Vegetation Cover

• The development of the soil and of the plant
  association go along with (and depend on) each
  other.
• 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.

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• 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

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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
  figure.
• 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.