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 life2. regulates soil structure3. mediates chemical reactions4. 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

SALINIZATIONEvaporation 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 QUESTIONSWhat 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. (3,000 - 6,000 ppm salt results in trouble for most cultivated plants.) Salt in soils decreases the osmotic potential of the soil so that plants can't take up water from it.

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.

WHAT WOULD HAPPEN TO AN ONION CELL IN PURE WATER? IN 0.5% SALT CONCENTRATION? IN

5% SALT CONCENTRATION?

TextProblems 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?

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.

LAB QUESTION: Is salinization affected by soil type?

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.?

TextSalinization 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.