Salinity in Agriculture*

* Why do we Irrigate?
* What Happens when you Irrigate?
* Where does all the Salt come from?
* What Problems does Salinity cause?
* Why is Research on Salinity so Important?

* Why do we Irrigate?

Irrigation is an ancient and important agricultural practice. Crop yields are higher under irrigation and less dependent on the effects of weather. While only 15% of the world's cultivated land is irrigated, it accounts for 35-40% of the global food harvest. Projected population growth rates for the next 30 years will require an increase in food production equal to 20% in developed countries and 60% in developing countries to maintain present levels of food consumption. Expansion of irrigated agriculture was in large part responsible for the "green revolution " in food production and will continue to play an essential role in providing the needed increases in food and fiber production, especially in developing countries.

* What Happens when you Irrigate?

Irrigation inevitably leads to the salinization of soils and waters. In the United States yield reductions due to salinity occur on an estimated 30% of all irrigated land. World wide, crop production is limited by the effects of salinity on about 50% of the irrigated land area. In many countries irrigated agriculture has caused environmental disturbances such as waterlogging, salinization, and depletion and pollution of water supplies. Concern is mounting about the sustainability of irrigated agriculture.

* Where does all the Salt come from?

Application of irrigation water results in the addition of soluble salts such as sodium, calcium, magnesium, potassium, sulfate, and chloride dissolved from geologic materials with which the waters have been in contact. Evaporation and transpiration (plant uptake) of irrigation water eventually cause excessive amounts of salts to accumulate in soils unless adequate leaching and drainage are provided. Excessive soil salinity reduces yields by lowering plant stand and growth rate. Also, excess sodium under conditions of low salinity and especially high pH can promote slaking of aggregates, swelling and dispersion of soil clays, degrading soil structure and impeding water and root penetration. Some trace constituents, such as boron, are directly toxic to plants.

* What Major Problems does Salinity cause?

Over the course of history, thriving civilizations declined in part due to their inability to sustain food production on lands that had been salinized. It is estimated that 10 million hectares are now being lost every year as a result of salinity and/or waterlogging. Many of these problems are caused by excessive use of water for irrigation due to inefficient irrigation distribution systems, poor on-farm management practices, and inappropriate management of drainage water. Inefficient on-farm irrigation practices cause local salinity problems. Local problems increase as a result of poor on-farm drainage. Excessive irrigation increases salt loading in water tables and downstream aquifers which causes regional salinization. Lack of local and regional drainage systems results in lands being put out of agricultural production.

* Why is Research on Salinity still so Important?

In the future, global food needs will continue to increase while the soil and water resources available for new crop production will be limited and of diminished quality. The need to protect soil resources as well as to conserve water will continue to increase. Water must be utilized more efficiently and its quality protected. World agriculture must expand its base of production and increase production on lands currently under cultivation. Appropriate management practices to control salinity must be implemented on irrigated fields, in irrigation projects, and for geohydrologic systems. In order to meet the ever increasing demands for food and utilizing ever decreasing and more marginal soil and water resources, the nation and much of the world community will continue to look to the U. S. Salinity Laboratory for expertise and leadership in salinity and water quality research and applications to solve these problems.

General Salinity and Water Questions*

How do you measure the salinity of water?

Salts in water can be measured by relatively simple methods.

One is by Electrical Conductivity (the reciprocal of electrical resistance) using an appropriate conductivity meter for the measurement. Such instruments are used by farmers and can be purchased relatively cheaply compared to most scientific instruments. For a historic background on this method there is a century-old paper by Whitney and Means, 1897 (USDA, Div. of Soils Bul. 7, 15 pp.) and our own USDA Agric Handbook 60, 1954,

Another method involves weighing water in a weighing container, and evaporating the water, then re-weighing and determining weight/volume (by difference). This is a little tricky because a sensitive balance must be used, and high temperatures my volatilize some salts.

When is water too salty to drink?

The Environmental Protection Agency sets the standards for safe drinking water that involves total salts and specific chemical component limitations.

How do I find out about the importance of water in California?

In California, there are many publications and countless newspaper articles regarding water needs and associated problems. A good summary is a special issue of "California Agriculture", 1984, Vol 38 (10). For current problems and concerns, another resource is a periodical magazine published by the Water Education Foundation, called "Western Water". Your local library can help you obtain copies of all mentioned references.

Fertilizer and Crop Requirements*

Question:
Is there any practical model available to estimate fertilizer and water requirements of crops in arid regions under irrigated farming?
In Saudi Arabia, both the soil and water are highly saline and all the crops are irrigated via a central pivot system, mainly wheat, barley, alfalfa and forage grasses. Vegetable crops are either grown in greenhouses or in the field using drip (subsurface or aboveground) or sprinkler systems.
Needed is a basic model to determine fertilizer requirements of crops for a target yield using data on soil fertility, temperature, relative humidity, SAR, water quality etc.

Reply:
Information on irrigation water requirements for various crop species (including grains, grasses, alfalfa, and wheat) is included in a USDA Soil Conservation Service, Part 623 National Engineering Handbbok, Chapter 2, Irrigation Water Requirements, printed Sept 1993. This publication also includes some basic information on salt tolerance and how to compute water needs.
We have no specific information on where you can get crop fertilization recommendations, but Extension Service should be a good place to start.

Dr. Donald Suarez (dsuarez@ussl.ars.usda.gov) has been leading a project to develop a generalized crop model for FAO that deals with salinity and crop yield. I am not aware of models for barley, forages, and alfalfa that take in to account and integrate all of the factors in which you are interested.
Dr. Derrel Martin (dmartin@ unlinfo.unl.edu) and others at the University of Nebraska have been working on water and fertilizer requirements under center pivot irrigation. You may want to direct your questions to Derrel

Plant Cell and Root Growth, Water, and Sodium Chloride*

Questions asked about Plant Cells and Roots, Water, and Sodium Chloride

* 1. How does salinity affect root growth? Does it increase or decrease root growth?
* 2. Does salinity affect cell division or cell enlargement?
* 3. Do plants actually take up salt while the water diffuses in?
* 4. Does water diffuse out of plants?

How does salinity affect root growth? Does it increase or decrease root growth?
Unfortunately, we don't know all the answers! Salt in the root zone decreases root growth. In some plants we call halophytes (literally salt plants) a little bit of salt seems to improve overall growth, in both roots and shoots (if you measure total biomass, i.e. the weight of the plant material). Examples of this are seen in barley and atriplex. Why this is the case is not known, but it is speculated that these plants require Sodium ion (Na +) or Chloride ion (Cl -) for growth.
In most cases, however, salinity decreases both root and shoot growth in plants, especially in glycophytes. Glycophytes are plants adversely affected by salts, or literally sweet plants, as opposed to salt plants. Shoot growth is usually decreased more than root growth and as a result the root/shoot ratio changes (the total weight of the roots and divided it by the total weight of the shoot)

References:

• Braun, Y., Hassidim, M., Lerner, H.R., and Reinhold, L. 1986. Studies on H +-translocating ATPases in young plants of varying resistance to salinity. PLANT PHYSIOLOGY. vol. 81,pp.1057-1061.
• Munns, R. and Termatt 1986. Whole-plant responses to salinity. AUSTRALIAN JOURNAL OF PLANT PHYSIOLOGY, vol. 13, pp. 143-160.
• Gilbert, G. A., Wilson, C., Madore, M. A. 1997. Root-zone salinity alters raffinose oligosaccaride metabolism and transport in Coleus. 1997. PLANT PHYSIOLOGY. vol 115, pp. 1267-1276.

Does salinity affect cell division or cell enlargement?

Both. In Halophytes , growth is stimulated by low amounts of salt (equivalent to about 3000 ppm). If you had a lot of time on your hands and bothered to count each and every cell, you would find more cells. Also, you would find that the cells are, on average, larger. Mostly though, plants increase in size by cell enlargement.
When Glycophytes are affected by salinity, cells in the roots are smaller and there are fewer of them. Under severe stress, there just isn't a whle lot of root there -fewer cells and smaller cell. Same with the shoot.

Do plants actually take up salt while the water diffuses in?
Ions get into the roots via several mechanisms. Ions that are at lower concentrations outside the plant that inside are taken up by a processes called active transport which requires energy and is mediated by a protein. Ions that exist at higher concentrations outside the plant than inside can diffuse in, but again, a protein is probably invovled. These proteins are called transporters, pores or channels depending on their exact nature and how they operate. Boths roots and shoots of plants grown in saline environments will have higher salts levels.
Some plants exclude toxic ions like Na + and Cl -. By exclude, it is really meant that they limit the influx of ions. This is accomplished by limiting ion uptake at the level of the roots or by compartmentalizing ions in areas of the plant, even in cells that are away from important metabolic sites and actively growing tissues. In some cases, it appears that salts are sequestered in older leaves that are eventually shed (abcised). Some halophytes have specialized leaf cells called salt glands that excrete salt.

Does water diffuse out of plants?
Water does indeed diffuse out of the leaves by the process called transpiration. Because water molecules cohere to each other via chemical bonds, called hydrogen bonds, water molecules at the top of the plants are connected to water molecules in the soil much like the cars of a train. When water transpires (a diffusion process) from the leaves, other water molecules are brought closer to the root surface. This waterway is actually called the transpiration stream. Ions move in the transpiration stream much like a non-powered boat floats along a river stream. Thus, the transpiration stream brings ions from the soil water, first to the root where they must cross the plasma membrane barrier, and eventually to the leaf. At the leaf the water molecules can escape back into the atmosphere through another specialized leaf cell called a stomate. Ions, however, will be left behind.

Criteria used by to distinguish between plants with different Salt Tolerances.

Question:

Identify the criteria used to distinguish between plants that have High, Medium, and Low Salt Tolerance

Answer:

* According to * pages 65-67 (html) of USDA Handbook 60 , the salt tolerance lists are arranged according to major crop divisions; and in each division, crops are listed in three groups. Within each group, crops are listed in the order of decreasing salt tolerance, but a difference of 2 to 3 places in the column may not be significant. * ECe values given in the top of the column represent the salinity level at which a 50-percent decrease ion yield may be expected compared to yields on nonsaline soils under comparable growing conditions. Links: * Handbook 60, Chapter 4, Pages 65-67only, (html) * Handbook 60, Chapter 4, (2.83mb), FTP Download (pdf) Handbook 60, Introduction & Menu of Web Version, (html)

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