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SOIL WATER
IRRIGATION - artificial provision of
water to support agriculture
70% of ALL
FRESHWATER
used by humans.
WATERLOGGING - over irrigation can
raise the water table, & suffocate plant
roots, soil microbes die.
WATER
1. sustains life
2. regulates soil structure
3. mediates chemical reactions
4. regulates chemical content by
properties such as cohesion,
adhesion, freezing expansion
How does all this relate
to soil?
Three main types of forces which contribute to the
energy state of soil water include:
•Gravitational water has a positive energy and
can flow out of the soil through the large pores.
•Osmotic potential is due to the attraction that
salts (ions) have for water through the
phenomenon of osmosis. The pressure applied by
a solution to prevent the inward flow of water
across a membrane. This energy is negative
relative to free water.
•Matric the potential energy of water attracted to
soil solids. (Adhesion & cohesion) It too is
negative.
Explain effect if the water side is in a plant
root and the solution side is in the soil.
Reverse sides
Distilled water = 0
Higher
concentrated
solutions = more
negative
LAB QUESTION: What is
the relationship between soil
type and matric potential?
LAB QUESTION RESTATED: What
type of soil has the greatest (least)
adhesion with water? Explain
SALINIZATION
Evaporation in arid areas draws
water up through the soil along
with dissolved salts. Irrigation
causes repeated evaporation,
bringing more salts up. READ CASE STUDY:
HISTORY OF MONO
LAKE
Prevention: plant crops that do not
require irrigation, use water with low
salt concentration, irrigate efficiently
LAB QUESTIONS
What is the salt toxicity level for plants?
What is the salt toxicity level for seed
germination?
Salinization refers to a build up of salts in soil,
eventually to toxic levels for plants.
Salt in
(3,000 - 6,000 ppm salt results in trouble for most cultivated plants.)
soils decreases the osmotic potential of the soil so that
plants can't take up water from it.
WHAT WOULD HAPPEN TO AN ONION
CELL IN PURE WATER? IN 0.5% SALT
CONCENTRATION? IN 5% SALT
CONCENTRATION?
When soils are salty, the soil has greater
concentrations of solute than does the root, so
plants can't get water from soil. (Remember osmosis
-- water "tries" to accomplish dilution -- it moves from
areas with lower concentration of dissolved
substances to areas with higher concentration?) The
salts can also be directly toxic, but plant troubles
usually result primarily from inability to take up water
from salty soils.
Text
Problems with salinization are most
commonly associated with excessive
water application, rather than with too
little.
Where does the salt come from?
All irrigation water contains dissolved
salts derived as it passed over and
through the land, and rain water also
contains some salts. These salts are
generally in very low concentration in the
water itself.
However, evaporation of water from the
dry surface of the soil leaves the salts
behind. Perhaps you have seen the
whitish salt crust that can result on the
surface of soils?
LAB QUESTION: Is salinization
affected by soil type?
Salinization is especially likely to become
a problem on poorly drained soils when
the groundwater is within 3 m or less of
the surface (depending on the soil type).
In such cases, water rises to the surface
by capillary action, rather than percolating
down through the entire soil profile, and
then evaporates from the soil surface.
EXPLAIN WHY
Salinization isn't really a problem
where farmers irrigate extensively in
areas that, during winter, have high
precipitation.
Where might this occur in the U.S.?
Text
Salinization is a worldwide problem,
particularly acute in semi-arid areas which
use lots of irrigation water, are poorly
drained, and never get well flushed. These
conditions are found in parts of the Mideast,
in China's North Plain, in Soviet Central Asia,
in the San Joaquin Valley of CA, and in the
Colorado River Basin; all areas where the
soil profile never (or rarely) gets well flushed.
Globally, something on the order of 20% of the
world's irrigated acreage is estimated to be
affected by salinization
Salinization obviously reduces crop productivity.
In the US, salinization may be lowering crop
yields on as much as 25-30% of the nation's
irrigated lands. In Mexico, salinization is
estimated to be reducing grain yields by about 1
million tons per year, or enough to feed nearly
million people. In extreme cases, land is actually
being abandoned because it is too salty to farm
profitably.
The "treatment" for salinization is to flush the soil
with lots of water. However, this results in
salinization of the river and groundwater where
the flush water goes.
At times in summer, the Red River in TX and OK
is saltier than seawater from its load of leached
salts. In addition, the flushing is very hard on the
soil structure.
In extreme cases, when the salt crust is too
thick, it can't be flushed, as water just runs off
the salty surface.
EXAMPLE OF SALINIZATION
ISSUES
The Colorado River was too
salty for the Mexican farmers to
use for irrigation. In
accordance to the 1944 United
States-Mexico Treaty for
Utilization of Water, which
guarantees Mexico an annual
quantity from certain water
resources, the U.S. had to
either desalinate water or
release water from Lake Mead
to Mexico
EXAMPLE OF SALINIZATION
ISSUES
The U.S. Bureau of Reclamation
thinks it might even be cheaper and
smarter to pay farmers in the Yuma
area not to grow crops. The
Colorado River water they now use
could then go to Mexico, reducing
the need for the expensive
desalination.