Irrigation types, Irrigation water quality, Salt water problems in drip irrigation and remedies:

IRRIGATION TYPES:-

Whatever the irrigation type, over and under watering is harmful for banana.

• Flood Irrigation:-

         Conventional irrigation whereby field has to be irrigated at a regular interval of 3-5 days depending upon soil type and weather conditions.

• Furrow Irrigation:-

• Trench Irrigation:-

 

 

•  Drip Irrigation:-

1. Drip irrigate the plants at their field capacity everyday. 
2. Water and fertilizer can be given through drip (fertigation). 
3. The quantity of water & fertilizer depends on the age of plant, soil type and climatic conditions. 
4. Apply additional quantity of water during cold spells to avoid chilling injury. 
5. Initially keep the dripper ½ feet – 1 feet away from the plant, 3 months later 1^1/2 feet and 6 months onwards 2 feet away.

WETTING PROFILES FOR DIFFERENT SOILS:

  

 

Water requirement and optimal wetting area for banana:

Crop	Spacing (m)	Rooting pattern		Optimal wetting area (%)	Daily water requirement (litres)	Number of emitters per plant and discharge rate (lph)
		Root radius (m)	Root Area (m2)
Banana	1.8 x 1.8	1	7	96	34	1 x 4

Two lateral lines per row of plants is to get very high yields by adopting to 'Precision Farming Techniques'

Optimal wetting area for banana:

        Age                  Wetting area in Dia around the plant

• 0 to 50 days        -   60 cm dia 
• 51 to 100 days    -   90 cm dia
• 101 to 150 days  -  120 cm dia
• 151 to Harvest    -  180 cm dia 

Optimal wetting area for banana

Comparison of Water saving, Yield and Profit between 

Conventional, Drip and Drip+Fertigation:  

Crop	Water saving (%)	Yield Ton / ha			Profit Rs. / ha
		Conven tional	Drip	Drip + Ferti gation	Conven tional	Drip	Drip + Ferti gation
Banana	35	26	30	37	81,000	98,000	1,20,000

                                                                          

Comparison between flow / Drip irrigation.

 Irrigation water quality:

Both irrigation water quality and proper irrigation management are critical to successful crop production.

The quality of the irrigation water may affect both crop yields and soil physical conditions, even if all other conditions and cultural practices are favorable/optimal. In addition, different crops require different irrigation water qualities.

Therefore, testing the irrigation water prior to selecting the site and the crops to be grown is critical. The quality of some water sources may change significantly with time or during certain periods (such as in dry/rainy seasons), so it is recommended to have more than one sample taken, in different time periods.

The chemical characteristics of irrigation water refer to the content of salts in the water as well as to parameters derived from the composition of salts in the water; parameters such as EC/TDS (Electrical Conductivity/ Total Dissolved Solids), SAR (Sodium Adsorption Ratio) alkalinity and hardness. 

Standard irrigation water testing will usually include the following chemical parameters:

Name of the element

Calcium (Ca)

Magnesium (Mg)

Sulphate (SO4)

Bicarbonates (HCO3)

Carbonates (CO3)

Sodium (Na)

Iron (Fe)

Manganese (Mn)

Boron (B)

Fluoride (F).

Nitrate (NO3)

Phosphate (PO4)

Chlorides (Cl)

Electrical conductivity

pH

The chemical quality of your irrigation water is important for the following reasons: 

1. Determines the suitability of the water for irrigation – your water may have high salinity (high EC), high SAR level or contain harmful elements in levels that might be toxic to your crop. Read more about irrigation water quality.
2. Affects crop yields – the total salinity of the irrigation water and the level of particular elements can reduce the crop yield, if a certain threshold is exceed. This threshold is crop specific.
3. Influences the fertilization program – your water may contain essential plant nutrients, like calcium, magnesium, sulphur and boron. Adequate levels of these nutrients in the irrigation water can save on fertilizer use, as nutrients present in the irrigation water are available to the plant.
4. Affects the irrigation schedule - high salt level in your water may require higher application amounts in order to prevent salts to accumulate in the root zone.

For proper operation and maintenance of drip irrigation systems, following parameters have to be tested for the water. Clogging or Plugging of drip irrigation lines are based on the concentration of minerals present in the irrigation water as shown here:

 

Plugging of drip irrigation lines hazard is based on Concentration of water
	Concentrations
Factor	Slight	Moderate	Severe
Physical
Suspended solids(filterable)	< 50	7.0 to 7.5	> 7.5
Chemical
pH	< 7.0	7.0 to 7.5	> 7.5
Dissolved solids	< 500	500 to 2000	> 2000
Manganese	< 0.1	0.1 to 1.5	> 1.5
Iron	< 0.1	0.1 to 1.5	> 1.5
Hydrogen sulfide	< 0.5	0.5 to 2.0	> 2.0
Hardness as  ppm CaCO3	< 150	150 to 300	> 300
Biological
Bacteria (population)	< 10,000	10,000 to 50,000	> 50,000
If the mineral concentration in the water is severe then the clogging will also be severe in the  drip irrigation PVP pipes, laterals and emitters.

All the parameters need not be studied for all situations. If the source of water is surface water like rivers and lakes, mostly biological impurities like algae and bacterial slime are predominant. In such cases, other tests need not be carried out. 

When water is taken from deep wells, the presence of Hydrogen Sulfide is very less. In such cases, this test can be avoided. The presence of hydrogen sulfide can be easily known if the water has rotten egg smell.

The presence of Iron can be easily known if the water has a brown colour.

Problems Related to Irrigation Water Quality:

1.  Salinity - The main problem related to irrigation water quality is the water salinity. Water salinity refers to the total amount of salts dissolved in the water but it does not indicate which salts are present in it.

High level of salts in the irrigation water reduces water availability to the crop (because of osmotic pressure) and causes yield reduction. 

Salinity Stress in banana

Above a certain threshold, reduction in crop yield is proportional to the increase in salinity level. Different crops vary in their tolerance to salinity and therefore have different thresholds and yield reduction rates.

The most common parameters used for determining the irrigation water quality, in relation with its salinity, are EC and TDS.

TDS ppm or mg/L	EC dS/m	Salinity hazard
<500	<0.8	Low
500 - 1000	0.8 - 1.6	Medium
1000 - 2000	1.6 - 3	High
> 2000	> 3	Very high

 

In case the irrigation water salinity exceeds the threshold for the crop, yield reduction occurs. Equations were developed to estimate the yield potential, based on the irrigation water salinity.

How Salty Water Can Be Used ?

Conventional Method

In conventional method, normal Irrigation interval is when the moisture content reaches 50% of available soil moisture. Let us assume a soil mass with following data:

Concentration of salt water used for irrigation       = 0.015 kg/m3

Volume of salt water just after irrigation in soil mass         = 1 m3

Volume of salt water just before irrigation in soil mass      =  0.5 m3

Salt concentration in soil mass                             =  0.0 15kg/ 0.5m3

=  0.03 kg/m³

Note that the concentration of salt water has increased twice.

Drip Irrigation

Normally very short irrigation interval is adopted in drip irrigation. Let it be when soil moisture reaches 90% of available soil moisture.

Volume of salt water just before just next irrigation is soil mass =  0.9 × l m3

Salt water concentration                                                  =  015 kg / 0.9 m3

       =  0.016 kg / m3

The concentration of salt water increases very slightly. When the salt content of soil water is high, the crops do not adsorb nutrients and water and crop growth gets affected.

How In Salty Soil The Drip Method Is Useful ?

When water moves over the root zone, the salts are dissolved from root zone and taken to periphery of wetting zone. When water gets evaporated from soil surface, it leaves the salt at locations as shown in Figure here below. Salt build up is away from the root zone. 

If very small rain less than 5 mm occurs, there may be a possibility of salt build up moving into the root zone. Because of this fact, in saline lands plants appear wilted after a small rain. The remedy for this is, if rain is less, irrigate through drip sufficiently.

 

Salt accumulation in soil when saline water is given through Drip irrigation.

2.  Sodium hazard and irrigation water infiltration - The parameter used to determine the sodium hazard is SAR - Sodium Adsorption Ratio. This parameter indicates the amount of sodium in the irrigation water, in relation to calcium and magnesium. Calcium and magnesium tend to counter the negative effect of sodium. 

High SAR levels might result in a breakdown of soil structure and water infiltration problems. Soil tends to seal and to become hard and compact when dry. 

Ironically, higher salinity reduces the negative effect of sodium on soil structure. So, when sodium levels in the soil are high in relation with calcium and magnesium, i.e SAR is high, flushing the soil with good irrigation water quality will only worsen the problem.

3.   Toxicity of specific ions

The quality of the irrigation water can be also determined by toxicity of specific ions. The difference between a salinity problem and a toxicity problem is that toxicity occurs within the plant itself, as a result of accumulation of a specific ion in the leaves. The most common ions which might cause a toxicity problem are chloride, sodium and boron. The same as with salinity, crops differ in their sensitivity to these ions. 

Special attention should be given to boron because toxicity occurs in very low concentrations, even though it is an essential plant nutrient. 

Toxic levels of even a single ion in the irrigation water might make the water unsuitable for irrigation. Nevertheless, there are some management practices that can help in reducing the damage.

These practices include proper leaching, increasing the frequency of irrigations, avoiding overhead irrigation, avoiding the use of fertilizers containing chloride or boron, selecting the right crops, etc.

4.   Alkalinity and pH

Alkalinity is the sum of the amounts of bicarbonates (HCO₃^-), carbonates (CO₃^2-) and hydroxide (OH^-)  in water. It is expressed as mg/l or meq/l CaCO₃.

Alkalinity buffers the water against sudden changes in pH. If the alkalinity is too low, any addition of acidic fertilizers will immediately lower the pH. In container plants and hydroponics, ions released by plant roots may also rapidly change the pH if alkalinity is low. 

Soil amendments and Irrigation Water Quality:

The purpose of some soil amendments is to counter the effect of sodium, by increasing the soluble calcium content or by increasing the salinity of the irrigation water. 

Gypsum and other calcium supplying materials - Gypsum is the most commonly used soil amendment. Since water infiltration problems, caused by sodium, affect mainly the upper few centimeters of soil, repeated small applications of gypsum, incorporated at lower rates into a shallow depth, are preferred over a single large application.

If the salinity of the irrigation water is low (EC<0.5 ds/m), gypsum can be added to the irrigation water at rates of 1-4 meq/l of dissolved calcium.

Other amendments - when lime (CaCO3) is present in soil, some acids or acid-forming amendments can be used. These amendments cause calcium to be released to soil solution. Examples for such amendments are elemental sulfur, sulfuric acid and ferric sulfate. 

Organic residues - these amendments improve soil structure and water infiltration, by keeping the soil porous.

Blending Irrigation Water Sources

Water infiltration can be improved either by increasing the irrigation water salinity or reducing the SAR.

By diluting the irrigation water source with water of lower sodium concentration, the SAR of the irrigation water is reduced, even if calcium and magnesium concentrations are higher.

TDS and Electrical Conductivity

Since the electrical conductivity is a measure to the capacity of water to conduct electrical current, it is directly related to the concentration of salts dissolved in water, and therefore to the Total Dissolved Solids (TDS). Salts dissolve into positively charged ions and negatively charged ions, which conduct electricity.

Since it is difficult to measure TDS in the field, the electrical conductivity of the water is used as a measure.

The electrical conductivity of the water can be determined in a quick and inexpensive way, using portable meters. 

Distilled water does not contain dissolved salts and, as a result, it does not conduct electricity and has an electrical conductivity of zero.

Mechanism behind Plugging of Emitters/Drippers in Drip Irrigation Sysytem:

Chemical Reactions

Water pumped out from deep bore well by air compressor is normally ‘Hard’ with more EC than the same water pumped out using submersible pump. 
REASON:

Water is often referred to as the universal solvent since almost everything is soluble in it to some extent. The solubility of a any material in water is controlled by variations in temperature, pressure, pH, redox potential, and the relative concentrations of other substances in solution. Three gases (oxygen, carbon dioxide, and hydrogen sulfide) are important in determining the solubility characteristics of water. These gases are very reactive in water, and they determine to a significant extent the solubility of minerals within a given water source. 

Carbon dioxide gas is of particular importance in the dissolution and deposition of minerals. Water absorbs some CO₂ from the air, but larger quantities are absorbed from decaying organic matter as water passes through the soil. Under pressure, as is in groundwater, the concentration of CO₂ increases to form carbonic acid. This weak acid can readily dissolve mineral compounds such as calcium carbonate to form calcium bicarbonate which is soluble in water. This process allows calcium carbonate to be dissolved, transported in the irrigation water. The bicarbonates under certain conditions again get deposited on pipes and emitters as calcium carbonate. 

 

In the wells with Air Lift pumps, since the water is lifted by supplying air to the well water, the amount of CO₂ dissolved is very high. Therefore, the amount of calcium bicarbonate dissolved from the subsurface formations is also high. Therefore, in air lift pumps, the pipes get deposition of calcium carbonates significantly.

Chemical plugging usually results from precipitation of one or more of the following minerals: calcium, magnesium, iron, or manganese. The minerals precipitate from solution and form encrustations that may partially or completely block the flow of water through the emitter. Water containing significant amounts of these minerals and having a pH greater than 7 has the potential to plug emitters. Particularly common is the precipitation of calcium carbonates, which is temperature and pH dependent. An increase in either pH or temperature reduces the solubility of calcium in water, and results in precipitation of the mineral. 

When groundwater is pumped to the surface and discharged through a micro irrigation system, the temperature, pressure, and pH of the water often changes. This can result in the precipitation of calcium carbonates or other minerals to form scale on the inside surfaces of the irrigation system components.  

A simple test for identifying calcium scale is to dissolve it with vinegar. Carbonate minerals dissolve and release carbon dioxide gas with a hissing sound. 

Iron is another potential source of mineral deposit that can plug emitters. Iron is encountered in practically all soils in the form of oxides, and it is often dissolved in groundwater as ferrous bicarbonate. When exposed to air, soluble ferrous bicarbonate oxidizes to the insoluble or colloidal ferric hydroids and precipitates. The result is commonly referred to as 'red water,' which is sometimes encountered in wells. Manganese will sometimes accompany iron, but usually in lower concentrations. 

Presence of Hydrogen sulfide will minimize the precipitation of calcium carbonate (CaC0₃) because of its acidity. But if Iron is present, Iron sulphate is precipitated.

Bio-Chemical Reactions

Algal and bacterial growth are major problems associated with the use of surface water. Whole algae cells and organic residues of algae are often small enough to pass through the filters. These algal cells  form aggregates that plug emitters. At the exit of the dripper, due to sunlight, the plugging is very fast. Residues of decomposing algae can accumulate in pipes and emitters to support the growth of slime-forming bacteria. Chemical precipitation is normally not a major problem when using surface water. 

Certain bacteria can cause enough precipitation of manganese, sulfur, and iron compounds to cause emitter plugging. In case of Iron, soluble ferrous in the water is oxidized by certain bacteria into insoluble ferric form. Hydrogen sulfide is oxidized by some bacteria into elemental Sulphur slimes.

Chemical Reactions of Fertilizers with Irrigation Water

• In water with calcium, magnesium and bicarbonates, if phosphate  containing fertilizers are mixed, calcium and magnesium phosphates are formed. By reducing pH, this effect can be reduced. 
• If polyphosphate fertilizers are mixed with water with Calcium and Magnesium ions, gel suspensions are formed which clog filters and drippers. 
• When water contains Calcium and if sulphate fertilizers are mixed with it, Calcium sulphate is precipitated. With rise in temperature, this problem would get aggravated. 
• When water contains Calcium and bicarbonate ions, mixing with Urea precipitates Calcium Oxide commonly known as lime. 
• When Phosphoric acid is mixed with water containing Calcium, Calcium Phosphate is formed. 
• If water contains more than 0.1 ppm of Iron, and if Calcium or Phosphate fertilizers are injected, then  Iron precipitate is formed.

Note: As it is difficult to find out what kind of chemical reaction that would occur when mixing with any fertilizer, normally it is recommended that a following test is conducted. The fertilizer solution equal to the same concentration with which, it would be applied to field is kept in a container for sometime under the same environment as that of the field. If, some precipitate get formed, then that fertilizer should not be applied through fertigation.

MANAGEMENT OF  CLOGGING OF DRIPPERS AND DRIP LINES: 

Chlorination:

If the source of irrigation water is dam, river, irrigation channel, water would contain organic matter (algae, bacterial slime). In such situations, chlorination is inevitable. Chlorine is an oxidizing agent which kills bacteria, algae and  organic matter. 

Normally used chlorine compounds are sodium hypochlorite (liquid) and calcium hypochlorite (solid). Sodium hypochlorite contains 10% chlorine. Calcium hypochlorite may cause calcium precipitation in alkaline irrigation water, so one should be careful in using this or the pH should be brought down by acidification. Venturi or fertilizer tank  can be used for injecting the dissolved chlorine compounds into the water.

Acid Treatment:

Addition of  acid to irrigation water is done for the following situations:

• Lowering the water pH can enhance the effectiveness of chlorine.
• The pH-lowering power of acid can prevent precipitation of solid compounds, particularly calcium carbonate (CaCO3).

• Citric acid prevents iron scale formation when continuously injected at 25 ppm.

It is always recommended to neutralize 80% of the bases (carbonates and bicarbonates) in the water to eliminate carbonate precipitation.

Caution :

Always add acid to water; do not add water to acid. Adding water to acid can cause a violent reaction, and may cause the acid to splash on the person pouring the water. Individuals working with acids should wear protective clothing and eyewear.

Acids normally used are as follows:

• Hydrochloric acid ( 32% Strength)
• Sulfuric acid (93 % strength)
• Phosphoric acid (85 % strength)

Sulphuric acid is highly toxic and hence it is not used popularly. 

Phosphoric acid should not be used if there is more than 50 ppm Ca in the irrigation water because calcium phosphate will precipitate. Calcium phosphate is nearly insoluble and does not readily dissolve. 

Hydrochloric acid is very much commonly used.

 

Acid Treatment for Prevention of Carbonate scales:

Acid treatment can be continuously done to prevent the scale formation due to carbonate salts in the water. If these salts plug the emitters and pipe, reclamation by acid treatment becomes difficult. It is because the emitters will be discharging the water and it takes more time for removal of the salts. Hence, if the carbonate content is more than a critical level continuous treatment with acid for all irrigations is recommended.

  

A water quality analysis usually lists electrical conductivity in micromhos per centimeter (mho/cm). To estimate parts per million (ppm) dissolved solids as shown in Table.1, multiply mho/cm by 0.64. For example, if the electric conductivity meter reads 1000 mho/cm, then dissolved solids can be estimated as 640 ppm.

 

Generally it is recommended that 1 litre of HCl should be done for 1000 litres of water. The acid treatment should be done for half an hour. The acid treatment should be done  at the end of any irrigation. The water in the pipes should be left as such for overnight and next day the system should be flushed. During flushing first main lines should be flushed and then laterals. Otherwise, there will be possibility of emitters getting clogged with the removed scales. 

Algae Control in Ponds and Wells:

Copper sulphate can be used for controlling the algae growth in the ponds and wells. But if the water is used for domestic purposes and also for animals, it is better not to use it.  Copper sulphate can be taken in porous cloth or gunny bag and tied in such a  way that the salt is floating on the surface of the water. The concentration of copper sulphate  may be in the range of  0.5 to 2 ppm. Following table provides the safe levels of concentration that does not affect fish life.

Posted in: 11. Irrigation types - Water quality - Salt water problems in drip irrigation and remedies