ABE 1 MODULE 4 QUIZ Flashcards
the relative proportion of variously sized groups of mineral particles in a specific soil or horizon
Soil Texture
the arrangement and organization of soil particles into natural units of aggregation
Soil Structure
Influences the rate at which water and air enter and move through the soil; also affects the
root penetration and nutrient supply in the soil
Soil Structure
Soil-Water System
the ratio of the void volume to the total soil volume (unitless);
determines the amount of pore spaces within the soil;
POROSITY (n or p)
Moisture content on ________
-the ratio of the mass of water to the dry mass of the soil
dry mass basis
Moisture content on ________ -the ratio of the volume of water to the total soil volume
Volume basis
the ratio of the dry mass of the soil to the total soil volume
bulk density (ρb)
the ratio of the dry mass of the soil to the volume of soil particles
particle density (ρs)
ratio of the particle density of the soil with the density of water; it is the ratio of the dry mass of the soil particles to
the mass of water having the same volume as these particles
Real specific gravity (Rs)
ratio of the bulk density of the soil with the density of water; it is the ratio of the dry mass of water with volume equal to the total soil volume
Apparent specific gravity (As)
the equivalent depth of water in the soil at a given condition
present in the soil (dw)
to increase the moisture content from an initial value (mci) to a final value (mcf)
needed (dwn)
volume of water to be applied to increase the soil moisture content from an initial to a final value (units: liters, cm3, m3)
VOLUME OF IRRIGATION WATER (Viw)
water on the soil grains that is not capable of significant movement by the action of gravity or capillary forces
Hygroscopic water
water that exists in the pore spaces of the soil and is retained against the force of gravity in a soil that permits unobstructed drainage
Capillary water
water that will readily move out
of the soil if favorable drainage is provided
Gravitational water
held too tightly by capillary forces and is generally not accessible to plant roots
Unavailable water
drains quickly from the root zone under normal drainage conditions
Gravitational water
difference between gravitational and unavailable water
Available water
causes the dissimilar particles and/or surfaces to cling to one another
adhesive force
causes the similar or identical particles/ surfaces to cling to one another
cohesive force
due to the effect of gravity
gravitational force
caused by salt or ion concentration differences or gradients
osmotic forces
the amount of water the soil profile will hold when all its pore spaces are filled up with water
SATURATION POINT
amount of water required to fill all the pore spaces of a given soil
SATURATION CAPACITY or maximum water holding capacity
–the moisture content of the soil when gravitational water has been removed (after heavy rain or irrigation by flooding)
-the amount of water the soil profile will hold against drainage by gravity at a specified time (1-3 days) after a thorough wetting by rain or irrigation
FIELD CAPACITY (FC)
-the soil moisture content when
plants permanently wilt (plant can no longer extract sufficient water for its growth)
-the soil moisture tension at this point vary from 7 to 40 atmospheres but generally considered to be 15 atmospheres
-as an approximation, permanent wilting percentage can be estimated by dividing the FC by a factor varying from 2.0 to 2.4, depending upon the amount of silt in the soil
PERMANENT WILTING POINT
(PWP) or wilting coefficient
difference in moisture content of the soil between field capacity and permanent wilting point
AVAILABLE MOISTURE (AM)
=that portion of the available moisture that is most easily extracted by plants; approximately 75% of the available moisture
=0.75 AM or ¾ AM
READILY AVAILABLE MOISTURE (RAM)
- amount of water that is allowed to be depleted usually expressed in percent
ALLOWABLE MOISTURE DEPLETION (AMD)
-Water moves from high to low pressure (or potential)
-driving force is difference in its pressure with distance
-water will also move by gravitational forces
-In soils, water moves by pressure (or water potential) and gravitational forces
-Water flow is described by DARCY’s law
WATER MOVEMENT
is the measure of potential
energy in water and as well as the difference between the potential in a given water sample and pure water
Water potential
In the soil…
water moves continuously in the direction of ________ potential energy or from higher water content to lower water content.
Water flows into and through the soil due to the difference in potential or potential gradient
-decreasing
In plants…
-it drives the movement of water through plants
-critical for moving water to
leaves so that photosynthesis can take place
Water potential
defined as the amount of work per unit quantity of pure water that must be done by external forces to transfer reversibly and isothermally an infinitesimal amount of water from the standard state to the soil at the point under consideration.
Total soil water potential
results from capillary and adsorptive forces due to the soil
matrix
capillarity results from the
surface tension of water and its
contact angle with the solid soil particles
Фm = -SMT
a dynamic soil property and will
be at theoretical zero level for a
saturated soil
Matric Potential or Capillary Potential
-arises because of soluble materials (generally salts) in the
soil solution and the presence of a semipermeable membrane
-two recognized membranes in soil-water systems are the cell wall of plant roots and air-water interfaces
Фs = RTC
where:
R - universal gas constant (82 bars cm3/mol K)
T - absolute temperature (K)
C - solute concentration (mol/cm3)
Solute Potential
-determined solely by the
elevation (or vertical distance) of the water relative to a reference point or datum
Positive (above)
Negative (below)
-partly responsible for infiltration and wholly responsible for the removal of excess water in drainage
Gravitational Potential
-vertical distance between the water surface and a specified point
-If soil is saturated, pressure potenial is positive, and also denoted by hydrostatic pressure potential
-If the soil is unsaturated, pressure potenial is negative, and also denoted by matric potential
-in the field, this component is zero above and at the level of water in the piezometer, and always positive below the water level
Pressure Potential
The rate of water movement at any given point is determined primarily by (1) ___________________and (2) __________________
(1) permeability or
hydraulic conductivity
(2) total potential gradient
-Henry Darcy studied (in 1856) the flow of water through a bed of sand to determine the flow in a porous media
-shows the volumetric flow rate or discharge is a function of the flow area, elevation,
fluid pressure and a proportionality constant
_________________ observed that:
1. Q is proportional to the head loss (∆h)
- Q is proportional to the cross sectional area of flow (A)
- Q is inversely proportional to the length of flow path (L)
- Q is influenced by the nature of the flow medium
Darcy’s Law
-Gravimetric method
-Thermo-gravimetric method
(oven drying, alcohol drying, hot air drying, infrared balance)
Direct measurement
-Tensiometric (tensiometer, pressure plate technique)
-Electrical or electromagnetic (gypsum blocks, dielectric methods
-Radiation (neutron scattering,
amma ray attenuation)
-Thermal (heat sensors, soil psychrometer)
-Remote sensing (infrared or electromagnetic properties
of soil)
Indirect method
moisture content is determined by looking at the soil and feeling between fingers the soil sample
FEEL AND APPREARANCE METHOD
-Measures contribution of matric + osmotic potential
to soil-water vapor pressure
-Measures vapor pressure of the thermocouple chamber and assumes equilibrium with the soil in the chamber
THERMOCOUPLES
-good to -98 kPa (-1 bar)
-takes a long time to reach equilibrium in an undisturbed, full- size core
-is used by applying positive air pressure to soil core
PRESSURE CELL (TEMPE CELL)
-Used to determine-available water capacity, water retentivity, hydraulic conductivity and field capacity
PRESSURE PLATE APPARATUS
-measures water pressure relative to atmospheric pressure
- measurement range limited by air entry value of porous cup (0 to -0.8 bars)
TENSIOMETER METHOD
-resistance(R) depends upon amount of H2O in block; Accurate only in dry range (0 to -10 bars)
-output may be MC or AM
-MC = PWP + %AM (FC-PWP)
-An adequate system of soil water measurement provides the irrigator with enough data to manage the system so that the soil water level is controlled over the entire field.
BOUYOUCOS METHOD (electric resistance block)
