TRANSPIRATION

If the plant transpires less than this potential amount, the yield will be proportionately less.  Any type of stress will cause the transpiration (and therefore the yield) to be reduced.  Insufficient soil moisture and high soil water salinity are common causes of plant stress. All irrigation water contains salts.  Wheat and other crops have selective membranes in the roots which are able to exclude the salt from the water which is used for transpiration.  The salt which is put into the soil with irrigation water will stay in the soil if it is not washed (leached) out. When a soil dries out, two things happen.  First, the water that remains is held tighter by the soil.  Second, the salt in the soil is now concentrated with less water.  This salt has a large attraction to water.  Both factors combine to create a water stress on the crop which reduces the yields.

Even if the soil is moist, a high soil salt concentration can hold the water so tightly that it is unavailable for plants.  A soil extract EC (electrical conductivity) of about 25 ho/c will cause a soil that has just been irrigated to appear completely dry to a plant.  Such dangerous salt concentrations can build up within 3 or 4 years of irrigation with salty water if adequate leaching is not done continuously.

EVAPORATION

Evaporation is water vapor loss into the air without passing through a plant.  Evaporation occurs from spray droplets between the sprinklers and the ground, from wet plant surfaces and from wet soil surfaces. Although some amount of evaporation will cool the air and reduce the transpiration demand, the benefits of this trade-off are frequently over-estimated.  Evaporation losses greater that 10-15% are completely non-beneficial in arid conditions.  Even the first 10-15% lost is only partially effective in reducing transpiration requirements.

The largest and most under-estimated evaporation loss occurs from wet soil surfaces and plant surfaces.  After each irrigation, evaporation dries the top several centimeters of the soil.  When pivots are rotated at high speeds in very hot arid conditions, almost all of the applied water can be lost to evaporation. Slow pivot rotation in desert conditions is extremely important.  The percentage of irrigation water lost to surface evaporation can be reduced from almost 100% to 20% or less on bare soil.  The percentage of surface evaporation is less when the surface is covered by plants, but can still average 50% with fast rotations.

Water often ponds on the soil surface with slow rotations.  This can be caused by both the types of salts in the water (sodium and bicarbonates) and the sprinkler design.  There is not one simple solution that will solve this problem.  However, a complete program of minimizing evaporation losses, treating the soil and water and modifying sprinkler packages is necessary for maximizing the effectiveness of irrigations.  Unless a comprehensive management approach is used, a good soil could be destroyed within a few seasons of irrigation.

Evaporation Summary: Evaporation losses are mostly non-beneficial.  These losses can be reduced by improved irrigation practices.  Evaporation losses in arid to semi-arid areas can be greatly reduced by improved water and soil conditions, especially during hot weather.

THE SOIL RESERVOIR

   The soil is a:

  a.  Water reservoir.

  b.  Nutrient source and reservoir.

  c.  Base for mechanical support of plants.

In irrigated desert conditions with coarse to medium (sand to loam) textures soils, a healthy wheat crop should have a rooting depth of 1.2 - 1.6 meters.  This contrasts with growing conditions under center pivot irrigation in other circumstances where a root zone depth of .3 - .5 meters can be sufficient.  If a wheat crop under center pivots in arid to semi-arid areas only has a .3 - .5 meter rooting depth, poor yields can be anticipated due to water stress and salt buildup.  In fact, cultivation of wheat with such a small root zone would generally reduce wheat yields within a few years of irrigation with salty water.       

If the complete soil moisture reservoir is not utilized (e.g., if only the top .5 meters are kept moist), the following negative things can happen:

a.  Salts accumulate in the root zone and stress the crop.

b.  There is no insurance (i.e., no large soil moisture reservoir) against equipment

breakdowns.

c.  The plants cannot make use of all of the natural nutrient reservoir.

d.  During the crop growth stage, when the irrigation system may not have flow capacity to meet the irrigation demand, the small soil moisture reservoir dries up rapidly and crop growth stops.

e.  Late in the irrigation season, if the soil surface seals up and water cannot penetrate into the soil, the crop will wilt and die if no stored moisture is available deep in the root zone.

The end result of not keeping the complete soil reservoir moist (plus leaching some extra water through) is: Crop yields are reduced and over time the land could become barren.

Soil Reservoir Summary: Under arid to semi-arid soil and climatic conditions, the soil should be kept moist to a minimum of 1.5 meters in depth.  Increased infiltration rates will increase water application efficiency and refilling of the soil moisture reservoir.

SALINITY

SALINITY INTRODUCTION

All irrigation water contains salt.  The TOTAL salt concentration in irrigation water is measured in two ways.  Reported as PPM (Part Per Million), it is based upon the weight of salt in a water sample.  One thousand PPM means that 1,000 grams of salt are contained in every million grams of water. Total salinity reported as EC (Electrical Conductivity) indicates the number of charges (not weight) in a water sample.  Common EC units are millimhos per centimeter (mmho/cm).  One mmho/cm is approximately equivalent to 650 PPM.

The following are typical indicators of high salt concentration in the soil of a wheat field:

a.  Poor germination.

b.  Patches in a field with no wheat growth.

c.  White crusts on the soil surface.

d.  Wheat grows well at the beginning of the season and suddenly turns yellow and stops growing.

e.  Very dark green leaves.

f.  Stunted plant growth.

g.  Low grain yields.

Salt from the irrigation water will stay in the soil unless leached out.  Surface runoff does not appreciably reduce soil salinity levels.  The leaching water must actually infiltrate into the soil and pass all the way down through the root zone to leach out salts. Salts can accumulate in the soil up to certain levels before negatively affecting crop yields.  After the soil salinity passes that threshold, production tonnages drop off sharply.  The only way to reduce the total salt concentration or even maintain it below the threshold level is to adequately leach the salts from the soil.

The Sodium component of irrigation water salinity causes the soil surface to seal up, preventing sufficient water to infiltrate for leaching and meeting crop transpiration needs.  This contributes to high salt level buildup in the soil. Bicarbonates (HCO) in the irrigation water combine with Calcium (Ca), causing a reaction in the soil which increases sodium concentrations.  Often sodium levels in the irrigation water are not high enough to cause a problem alone, but the presence of Bicarbonates will make the sodium reach hazardous levels.  Bicarbonates can be removed from the irrigation water before it hits the soil by acidifying the irrigation water (e.g. with an SO2 Generator).

It is a common misconception that only high concentrations of salt affect crop production.  Irrigation water may have a very low total salt concentration, but if the few salts which are present contain relatively high amounts of sodium, bicarbonate, or toxic elements, crop production will be substantially reduced or eliminated.  The reduction of crop yield due to these salt constituents is often higher than if the water had a high EC.

Salinity Introduction Summary: Both total salinity and the concentrations of some specific ions (e.g., Sodium, Bicarbonate, and Boron) are important in determining the suitability of water for irrigation unless acidified.

TOTAL SALINITY

Wheat yields are only indirectly affected by the irrigation water EC itself.  Yields are directly affected by the Soil Water Quality.  Soil water salinity depends upon:

a.  The salinity of the irrigation water.

b.  The extra percentage of water applied for leaching.  Irrigation water with low total salinity will eventually cause high soil salinity if extra water is not  continually leached down through the root zone.

c.  The moisture content of the soil (i.e., how dry the soil gets before an irrigation).  If the soil is dry, the crop plants see the same amount of salt in less water (i.e., the salt is more concentrated).

LEACHING REQUIREMENT

Salt will build up in the root zone of an irrigated soil unless extra water is applied to leach it down and through the root zone. When this is complete, the moisture content at the bottom of the root zone should be near field capacity. Field capacity refers to the amount of water that the soil can hold against gravity. Each soil layer must be exceed field capacity before any water will move down past it. If insufficient water is applied to move the salts down and out of the root zone, it is only a matter of time before fields become completely locked up with salts, and incapable of growing crops.  Many cases can be found in the world where the top of the root zone has become too salty for production within two or three years because little or no extra water has been applied for leaching. Research and years of experience have shown that mature plants will not be harmed by high total salinity until the saturated soil pasted extract (EC extract) exceeds 6.0 mmho/cm (on average, but still dependent on the crop).

It is also important to keep in mind that the top of the soil will reach a low salinity concentration earlier than the deeper part of the root zone. Hence, a crop could be planted and would germinate well with less leaching; however, later growth would be reduced as the roots tried to expand into the deeper, saltier soil zones. 

Leaching Requirement Summary:  Leaching is a necessary part of irrigation. Without it, the soil will eventually become sterile.

PRE-IRRIGATION

 A second problem related to total salinity is poor germination and emergence.  Most crops are sensitive to soil water salinity during early growth stages, so special salinity control practices are essential before planting. A mandatory irrigation practice is the thorough leaching of salts from the soil surface before planting.  At the same time, any salts which might have accumulated elsewhere in the root zone must be leached.  Many conventional irrigation designs do not provide enough flow capacity to match the evapo-transpiration requirements at the end of the crop growing season.  As a result, salt levels in the soil build up at the end of the growing season because there is no extra water for leaching.  In this case, pre-irrigation (irrigation before planting) leaching should be to reduce this salt buildup.

Irrigation systems should be operated prior to planting for sufficient time to reduce the extract EC in the complete root zone to at least 6 mmhp/cm.  Soil samples must be taken as deep as 1.5 meters to determine the effectiveness of a leaching program.  The leaching requirements given in Table 1 are for a maintenance program.

SODIUM AND BICARBONATE

INTRODUCTION TO SODIUM PROBLEMS

Clay particles in the soil contain negative charges.  Calcium (CA), Magnesium (Mg), Sodium (Na), Potassium (K) and Ammonium (NH ) have positive charges and can be absorbed (attached) to the clay particles in soil.  The number of charges in a soil is called Cation Exchange Capacity (CEC), and is usually reported in units of milli-equivalents per 100 grams of soil. A sandy soil will have lower CEC than a clay soil.  This means sand has a smaller nutrient reservoir, and must be fertilized frequently in small amounts.  Such fertilization (especially with inexpensive forms of Nitrogen) is easily done through irrigation systems where water has been treated with H2 SO3 .

Irrigation with water containing relatively high amounts of sodium will cause a large percentage of the CEC in the soil to be occupied by sodium.  The percentage of the CEC filled with sodium is called the exchangeable sodium percentage (ESP).  A soil with an ESP greater than 15 is classed as a saline-alkali soil. High ESP soils produce lower crop yields because of the following:

a. Insufficient water infiltrates into the soil. As a result, soil particles disperse (deflocculate) and form a crust on the soil surface.  Permeability is a soil surface problem, and it occurs with soil of all textures.

b.  Nutrients such as Phosphorus, Zinc, Copper, Manganese, and Iron become Atied up@ at soil ph=s greater than 8.0 and are thus unavailable for plant use.  As an example, as much as three times the amount of Phosphate fertilizer needed by the crop may be applied each year on a saline-alkali or alkali soil, yet the crop will still suffer Phosphorus deficiencies.

c.  Calcium deficiencies.  Calcium is a plant nutrient in addition to being important for soil structure.  It must be available in the soil and water in concentration at least as great as 2 milli-equivalents per liter to prevent nutrient deficiencies.

Throughout the world, the problem of soil surface crusting after only a few seasons is particularly serious on sandy textured soils such as loamy sands and sandy loams.

WHY SODIUM IS DIFFICULT TO REMOVE FROM THE SOIL

Most of the sodium is absorbed onto the soil particles.  It cannot be removed simply by leaching, as can be done with high total salt concentrations (high EC=s).  Salt is in solution and is not attached to anything.

UNDERSTANDING AND SOLVING HIGH SODIUM PROBLEMS

Soil crusting problems due to sodium are greatest at low soil salinity levels.  It is a common mistake to begin a heavy leaching program to remove high soil salt levels without first checking the sodium levels in the water. If water with high sodium content is used for leaching, the soil surface can suddenly seal up once the EC is lowered.  It may take years to reclaim such a sealed up soil.

Sodium must be replaced on the cation exchange sites, and cannot simply be leached away.  The replacement must be by Calcium (Ca) or Magnesium (Mg) cations.  At high concentrations in the soil water, the Calcium and Magnesium will Abump@ the Sodium off the soil exchange sites.  The opposite happens if the Sodium concentration in the irrigation water is relatively high. Bicarbonate ions in the irrigation water will combine with Calcium to make Lime.  Lime is insoluble, and is essentially a rock of Calcium Carbonate.

Removal of the Bicarbonate ions from the irrigation water will prevent the formation of Lime and allow Calcium to occupy more exchange sites, displacing the unwanted Sodium.

The solution process for Sodium problems is:

1.                  Continuously acidify the water (e.g., with sulfurous or sulfuric acid) to remove Bicarbonates if they are present in relatively large amounts.

2.                  Replace Sodium in the soil with Calcium, either by adding Calcium to the soil or by dissolving Calcium forms present in the soil.

3.                  Leach the Sodium Sulfate out of the soil along with other salts.

COMMON ALKALINITY (SODIUM) PROBLEMS

Situation 1: The soil already has high levels of Sodium.  There is very little Calcium of any form in the soil.

Solution: Calcium must be added to the soil in a form that will allow it to replace the Sodium from the exchange sites.  When added to the soil and then irrigated, Gypsum (CaSO4 ) will slowly dissolve into Calcium (Ca) and Sulfate (SO4 ) ions.  The Calcium replaces Sodium on the soil exchange sites.  The Sodium Sulfate (NaSO4 ) salt is leachable, and will be removed along with other salts as the soil is leached by irrigation.                 

Gypsum must be applied as a fine dust to the soil surface and thoroughly mixed into the top 30 centimeters of soil.  At least 85% of the Gypsum must be fine enough to pass through a 100-mesh sieve.  Application rates of Gypsum for initial reclamation are given in Table 3.

Situation 2: The soil already has high levels of Sodium.  Calcium is in the soil as Lime (CaCO3 ).  Note: The presence of lime can be detected by putting a few drops of acid on a soil sample.  If it fizzles, lime is present.

Solution: Although there is sufficient Calcium, it is completely unavailable to the exchange sites.  Lime does not dissolve at a typical soil pH of 7.2-8.4. Lime can be dissolved by applying elemental Sulphur to the soil.  Bacteria in the soil convert the Sulfur to Sulfuric Acid, which dissolves the lime.  Elemental sulfur must be applied as a fine dust to the soil surface and be thoroughly mixed into the top 25 centimeters.  It should be the same fineness as Gypsum.  Because Sulfur will burn if exposed to a spark, it must be handled and spread with the proper equipment.  It is widely used in the Southwestern U.S.

Some Sulfur products are available which are dust free and will easily disperse when irrigated.  They are easy to handle, but often contain about 10% swelling clay.  The swelling clay may actually increase water penetration problems. As with Gypsum, the amount of elemental Sulfur needed depends upon the soil ESP and CEC and irrigation water EC and adjusted SAR (defined later).  Table 3 gives approximate Sulfur recommendations.

If the irrigation water is relatively high in bicarbonates, they should be removed with continuous water acidification during and after leaching to prevent the problem from recurring.

TABLE 2:

 15                                       900                                         4,700

 20                                     1,800                                        9,500

 30                                     3,600                                       19,000

 40                                     5,400                                       28,500

 50                                     7,200                                       38,000

Both Gypsum and Sulfur treatments give slow results.  Warm weather and irrigation speed up the reclamation of very alkali or saline-alkali soils [but it] often takes several years.  Use of SO2 Generators will reduce both the cost and time of the reclamation, by introducing the sulfur more directly into the soil via the irrigation water.

Solution Summary: Gypsum is used to provide Calcium to alkali soils which do not have lime.  Elemental Sulfur is applied to alkali soils which have lime present.  Gypsum and Sulfur application recommendations are given based on the soil Exchangeable Sodium Percentage.  An SO2 Generator can speed up the process considerably and is essential for reducing Bicarbonates in poor quality irrigation water.

BICARBONATES

Bicarbonates increase the Sodium hazard of irrigation water by combining with Calcium to form insoluble lime in the soil.  The hazards to high soil Sodium contents have been defined as including higher soil salinity due to poor leaching, nutrient toxicities, micro-nutrient deficiencies, and water stress.  All cause lower crop yields. Bicarbonates in the water are particularly harmful because they continuously undo past reclamation efforts at the soil surface.  Soil crusting will form quickly after soil reclamation if the Bicarbonates are not carefully monitored and controlled.

The hazard due to Sodium and Bicarbonates in the irrigation water (for the permeability problem only) can be estimated with the Adjusted Sodium Absorption Ratio (adj. SAR) formula.  General guidelines as established by the U.S. Salinity Laboratory for permeability hazard are found in Table 3.  Although new procedures have been found to better describe the Bicarbonate Hazard (and which generally show the Bicarbonate to be more hazardous than indicated with the adj. SAR formula), at the present time, the adj. SAR is commonly used.

Adj. SAR (Appendix B)                     Permeability Hazard

0 - 6                                         None to Slight

6 - 9                                         Increasing Problems

9 +                                           Serious Problems

To estimate the effect of removing the Bicarbonate ion from the irrigation water, the adj. SAR should be calculated twice; first with the original water quality, and second with all of the Bicarbonates removed.  A similar calculation is given Appendix C.  Typically, if Bicarbonate concentrations are above 140 PPM in world irrigation waters, they will have a very harmful effect on the soil if not removed before irrigation.  However, each water should be analyzed separately.

BICARBONATE REMOVAL FROM IRRIGATION WATER

Bicarbonate (HCO3 ) will remain in the irrigation water at high pH=s.  However, if the water is acidified, the Bicarbonate will combine with Hydrogen (H) ions and form Carbon Dioxide gas (CO2 ) and Water (H2O)

HCO3   +   2H   creates    H2O   =   CO2

All of the Bicarbonates can be removed by lowering the water pH to 5.0.  Of course, such a low pH is not always desirable for numerous reasons.  Therefore, water is only acidified to a pH of 6.2.  This removes about 80% of the Bicarbonates in the irrigation water. Any acid which lowers the water pH will perform this initial task.  However, the most common liquid acids have serious drawbacks which prevent their use:

1.                  Sulfuric acid is very strong and can cause serious injury or death if spilled.  It will easily corrode pivots if not injected at precisely adjusted rates.

2.                  Hydrochloric acid (muriatic acid) is toxic to many plants because of the high Chloride content. It is also dangerous to handle and highly corrosive to pipelines.

Equipment that burns elemental sulfur, such as the Sweetwater SO2 Generator, achieves the desired result.  The reactions are:

--Elemental Sulfur when burned creates SO2 gas.

--SO2 gas then combines with water that creates Sulfurous Acid (H2SO3).

The injection of Sulfurous Acid is very efficient and safe.  Sulfurous Acid, being a weak acid, is used to lower the pH and remove much of the irrigation water Bicarbonate.  The remaining BiSulfate (HSO3 ) then goes through another conversion in the soil, making more acid.  That acid dissolves Lime naturally occurring in the soil or any Lime formed by the small amounts of Bicarbonate which was not removed initially from the irrigation water.

Bicarbonate Removal Summary: SO2 gas is used to make Sulfurous Acid by burning elemental Sulfur in special SO2 Generators.  The Sulfurous Acid converts most of the irrigation water Bicarbonates to Carbon Dioxide gas.  Lime is then dissolved in the soil when the BiSulfate converts to Sulfate, producing more acid.

OTHER FORMS OF SULFUR

Elemental Sulfur applied directly to the soil or oxidized in an SO2 Generator is converted to an acid.  It is the acid which actually accomplishes the work of eliminating Bicarbonates and/or dissolving Lime.  Elemental Sulfur is inexpensive and more easily handled than an acid.

Sulfate (SO4) occurring naturally in irrigation water is rarely associated with Sulfuric Acid.  Instead, the sulfate is usually half of a Sodium or Calcium salt.  Sulfate in irrigation water will not reduce soil permeability problems unless it is used in conjunction with an SO2 Generator.

ACKNOWLEDGEMENTS

Most of the concepts presented here are not original.  Liberal use of information available from common sources such as the U.S. Salinity Laboratory and the Sulfur Institute has been made.  Information provided by Dr. B. Buhidar and in particular, various publications by J.D. Rhoades and D.L. Suarez of the Salinity Lab, have proven most helpful.