* Control of Waterlogging and Salinity Problems
* Irrigation Water Quality
Excess water in the plant root zone restricts the aeration required for optimum plant growth. It may affect the availability of several nutrients by changing the environment around the roots.
Excess salts in the root zone inhibit water uptake by plants, affect nutrient uptake and may result in toxicities due to individual salts in the soil solution. Excess exchangeable sodium in the soil may destroy the soil structure to a point where water penetration and aeration of the roots become impossible. Sodium is also toxic to many plants.
Waterlogging and salinity in the soil profile are most often the result of high water tables resulting from inadequate drainage or poor quality irrigation water. Adequate surface drainage allows excess irrigation and rain water to be evacuated before excess soil saturation occurs or before the water is added to the water table. Adequate subsurface drainage insures that water tables are maintained at a sufficient depth below the soil surface to prevent waterlogging and salt accumulation in the root zone. Salinization of the soil profile is prevented because upward capillary movement of water and salts from the water table does not reach the root zone. Adequate subsurface drainage also allows salts to be removed from the soil profile through the application of excess irrigation water (leaching).
To understand how we may prevent, eliminate or otherwise deal with a waterlogging or salinity problem, we must first understand how crops and soils respond to excess water and salts.
Waterlogging and High Groundwater Tables
The growth of most crops is affected when groundwater is shallow enough to maintain the soil profile in the root zone wetter than field capacity. This excess water and the resulting continuously wet root zone can lead to some serious and fatal diseases of the root and stem. Working the soil when overly wet can destroy soil structure and thus restrict root growth and drainage further. The chemistry and microbiology of waterlogged soils is changed due to the absence of oxygen. This can result in changes which affect the availability of many nutrients. For example, nitrogen can undergo denitrification more readily and be lost to the atmosphere as a gas. The anaerobic (reducing) environment results in changes to metals and other cations that can result in deficiencies or toxicities. For example, sulfide, ferrous and manganese ions will accumulate in waterlogged soils.
Crops vary in their tolerances to waterlogging and a high water table. Some crops, such as rice, are adapted to these conditions and can thrive. The table below presents the different tolerances of some crops.
Tolerance Levels of Crops to High Groundwater Tables and Waterlogging
GROUNDWATER AT 50 CM WATERLOGGING
HIGH TOLERANCE sugarcane, potatoes, rice, willow, plum, broad beans strawberries, some grasses
MEDIUM TOLERANCE sugarbeet, wheat, oats, citrus, bananas, apple, barley, peas, cotton pears, blackberries,
SENSITIVE maize, tobacco, peaches, cherries, olives, peas, beans, date palm
The capillary fringe is a saturated zone that extends some distance above the water table. Water moves into this zone by capillary movement. The roots on many crops do not generally penetrate closer than 30 cm above the water table. The capillary fringe is thinner in sandy soils than in loam or clay soils. Thus the following depths to groundwater are suggested as a minimum for most crops:
Sandy Soils ----------- Rooting Depth + 20 cm
Clay Soils ------------ Rooting Depth + 40 cm
Loam Soils ------------ Rooting Depth + 80 cm
Soil and Water Salinity
Crop yields decrease linearly with increasing salt levels above a given threshold level. This threshold level will vary according to the tolerance of the crop. Yield decreases in the absence of toxic salts such as boron are mainly due to the difficulties the crop has in taking up water due to the high concentration of salt in the soil solution. Often crops present a droughty or dry appearance in high salt soils.
The table below presents the tolerance of different crops to soil and water salinity levels, and the effect that increasing
salinity levels has on yield. In this table, the ECe (Electrical Conductivity of the Saturated Paste Extract) is a measure of
soil salinity, ECw (Electrical conductivity of the Irrigation Water) a measure of water salinity. The Max ECe is the highest
ECe that the plant can tolerate. The Yield Potential is the percent of an optimum yield that can be attained under given
|Crop Salt Tolerance Levels for Different Crops as Influenced by Irrigation Water or Soil Salinity|
|FIELD CROPS||100||90 %||75 %||50%||0%|
An example of how to use this table is as follows: A farmer can produce 50 Kg per Hectare of corn on good soil. The farmer has a field with an ECe of 3.8 which gives him or her many problems. Using the table, an estimate can be made of an expected yield of roughly 37 Kg per Hectare (i.e. a 75% Yield Potential) for this field.
This table represents general information about relative tolerances to salt, but varietal differences are also very important. Much effort has been put into developing salt tolerant varieties of many crops because of the worldwide salinity problem. In some cases, minor problems can be alleviated by selecting the correct variety.
Electrical Conductivity (EC) is the reciprocal of Resistance (1/ohms), and is measured in mmhos/cm or in dS/m (dS/m = mmhos/cm). EC is measured with a salinity or conductivity meter, which is a standard piece of equipment in all soil labs and can often be purchased at a reasonable price for field use. ECw (salinity of the water) is measured by simply inserting the conductivity meter in the irrigation water, with adjustment made for temperature. ECe (soil salinity) is a little more complicated, requiring a saturated paste of the soil from which the water is then extracted and the salts measured.
Exchangeable sodium in the soil becomes a problem when the predominant salts in irrigation water or in the soil solution are sodium salts. Soil constituents which determine soil structure, such as clays and organic matter (soil colloids), have negative charges (exchange sites) on their outer surface which loosely attach to positive ions and molecules (cations) such as Calcium (Ca++), Ammonium (NH4+), and Sodium (Na+) (see Figure 7.1). These cations can readily be replaced by other cations (they are exchangeable). If there is excessive sodium in the soil solution, it will take over most of the exchange sites. Sodium is a small cation, so when present in large quantities on the exchange sites, it destroys the separation between soil particles. What happens then is that the clay or organic matter collapses on itself leaving no air spaces or pores (deflocculation). In some cases, the structureless organic matter is dispersed and can be lost in the drainage water, hence the old-fashioned term for these soils is Black alkali soils.
Sodium is measured as the Exchangeable Sodium Percent (ESP) or as the Sodium Absorption Ratio (SAR ). The ESP is simply the percent of all the exchange sites in the soil which are holding sodium on them. The SAR is more complicated, and is merely an index of the extent of the problem.
Very high sodium levels not only affect soil structure, but are toxic to many crops.
Classification of Salt Affected Soils
These soils contain sufficient amounts of soluble salts to interfere with germination, growth and yield of most crop plants. They do not contain enough exchangeable sodium to alter soil characteristics. Technically, a saline soil is defined as a soil with an ECe greater than or equal to 4 mmhos/cm and an Exchangeable Sodium Percent (ESP) less than 15. The soil pH is usually less than 8.5. These soils may have a white crust or white salt crystal accumulation on the surface (salt blooms) so they are sometimes called "white alkali soils". Excess soluble salts can be removed by leaching if drainage permits as will be discussed.
These soils contain soluble salts and exchangeable sodium in sufficient quantities to interfere with the growth of most crops. Technically, a saline-sodic soil is defined as a soil having an ESP greater than 15 and an ECe greater than or equal to 4 mmhos/cm. The soil colloids (charged particles) are collapsed (deflocculated), and drainage and aeration are very poor. pH is usually in the range of 8-10.
These soils contain sufficient exchangeable sodium to interfere with the growth of most crops, but do not contain appreciable quantities of soluble salts. Technically, they are soils with an ESP greater than 15 and an ECe of less than 4 mmhos/cm. Drainage and aeration are very poor because soil colloids are very dispersed. The pH is generally above 8.5. These soils are sometimes called "black alkali soils". High pH values generally can be used as a indicator of possible sodium problems, but this is not always true.
Evaluating Waterlogging and Salinity Problems
The evaluation of the extent of waterlogging and salinity problems can usually be conducted through simple observation, communication and possibly some soil analysis. The following steps can be followed:
1) Interview local agronomists, agricultural technicians, and agribusiness personnel. Ask them questions about water table depths, salinity problems etc. If such problems exist, how are local farmers taking care of them?
2) Conduct a field reconnaissance to find out if the problem exists in your area. Wells, gravel pits and deep channels which show the depth to groundwater should be observed. If there are few of these, then install pits or auger small observation wells into the soil to depths of 30 to 80 cm below the expected rooting depths (30 cm for sandy soils, 80 cm for loams and fine textured soils). If soil horizons are reached which are grey, wet and may contain black or red mottles, you have hit "gleyed" or waterlogged horizons. You can assume at this point that soils are poorly drained at this level.
As part of the reconnaissance, observe fields for signs of excess water or salinity such as:
a) White crusts on the soil surface. There may be a problem even when these are not present.
b) Plants which are stunted, appear droughty or irregular even though the soil is fairly moist. In cases of high salinity, the leaves may be curled up and yellow. The margins of the leaves may burn, a reddish color is often seen and in some cases the plant may actually die during or shortly after germination and emergence.
c) Use of drainage water, tailwater or water which has been used extensively for washing, irrigation or industrial purposes before reaching the field. This may be a problem when the farmer is a tail-end user on a major irrigation system. This water can accumulate salts.
d) Soils with poor structure, which appear sticky and plastic when wet and which do not grow a crop. Hard, structureless soil pans can develop at different depths in sodic soils.
e) Standing water or wet spots in parts of the field where crops grow poorly. Standing water in spots after a prolonged drying period are also useful indicators.
f) When soil is dry and smooth or has slicked over areas without vegetation, sometimes with a thin peeled up skin, it can indicate infiltration and sodic soil problems
g) Absence of field drains for removing excess water.
h) Condition of field drains: Are surface drains full of vegetation or plugged up? Are surface and subsurface drains operating properly?
i) If the opportunity presents itself, take soil samples and have them analyzed if you suspect a salinity problem, or look at past samples if any are available.
Surface and Subsurface Drains
The first requisite in the prevention or elimination of waterlogging and salinity problems is an adequate drainage system. Very often, the natural drainage in an area along with good water management is sufficient to eliminate excess water and to preclude the need for expensive subsurface drainage systems. However, almost every farmer who applies water by surface irrigation or who deals with significant rainfall should have adequate surface drainage facilities to remove excess water. This will allow the farmer to avoid waterlogging and possible salinity problems at the tail end of borders, furrows or basins after irrigation or intense rainstorms. It will also allow the prevention of erosion associated with natural movement of the excess water over the soil surface.
Surface drains are open channels which collect water as it runs off of, or into irrigated fields. These drains convey water to a stream or channel where it can be carried safely. The design procedures for these drains are the same as for any open channel (see Chapter 5). The main requirement is that they are able to convey the maximum expected flow rate without erosion. At the tail-end of irrigated fields, these drains are often broad and shallow to allow farm machinery to operate efficiently.
Subsurface drainage may be accomplished either through the construction of open trenches or through buried clay or concrete tiles or perforated pipe. Subsurface drainage systems can be classified as Natural, Herringbone, GridironorInterceptor (Cutoff) types.
The Natural systems are used in fields where there are small and isolated wet areas. The buried drain lines follow natural draws or depressions.
The Herringbone systems are useful in situations where the land slopes toward a draw on either side. The main line
follows the draw, and the laterals empty into this from both sides.
The Gridiron systems are similar to the Herringbone except that they enter the main drain from only one side.
Interceptor drains are installed across a slope to intercept the passage from higher ground. These drains can prevent the waterlogging of soils below irrigation ditches, springs or at the foot of a hill. They can be useful in collecting water for recycling into the irrigation system.
The design, drain size, spacing and depth are a function of the water table depth desired, the soil permeability (hydraulic conductivity), amount of water to be drained, economics of construction, etc. Generally, the deeper the drains are
installed, the wider the spacing between drains can be. In humid regions, drain spacings of 10 to 50 meters (30 to 150 feet) are common. The closer spacing is used in heavier soils with higher value crops and greater rainfall. In more arid irrigated areas, spacings of 50 to 200 meters (150 to 600 feet) are common.
Tile drain is common in 10, 13 and 15 cm (4, 5 and 6 inch) sizes, but can be obtained in greater sizes as can corrugated drainage pipe. Minimum grades are sometimes based on a minimum velocity of 0.45 m/s (1.5 feet per second) at full flow. Surface inlets, outlets and cleanouts, envelope filters and other structures must be properly designed if the drain system is to operate correctly.
The design of subsurface drains is generally more complex than for surface drains and requires significant knowledge of groundwater hydrology. Thus the reader should seek the assistance of a drainage engineer before undertaking the design of expensive subsurface drains. The one possible exception is the Interceptor drain which can be installed as an open channel below the level of an irrigation canal to provide drainage to land which would otherwise be waterlogged by the canal.
Reclamation of Salt Affected Soils
The chemical and physical analysis of soils provides a basis for the diagnosis, treatment and management of salt affected soils. After diagnosing the problem but before actual reclamation, two steps must be observed.
1. Establishing adequate drainage in the area. The water table should be lowered if it is high and water should be at least 3 to 4 meters below the surface.
2. The land should be level or contour farmed so that the surface of the soil will be covered uniformly by water.
If the soil is only saline, it can be reclaimed simply by leaching the excess salts below the root zone. The quantity of water depends on the texture of the soils, the concentration of salts in the soil and in the leaching water (the higher, the more water needed) and the amount of salts to be leached. On the average, 0.5 to 1.25 meters of water are required.
Saline Sodic Soil and Sodic soil
If leaching is conducted on a saline-sodic soil, the soil will become sodic and could present more problems than it would have originally. Saline-sodic soils require the leaching process to be accompanied by the application of amendments. The amendments that are used are the same ones that would be utilized on a sodic soil. Sodic soils are generally very poor in infiltration, so amendments are slow to enter soil. For this reason, both compacted saline-sodic soils and sodic soils should undergo deep cultivation such as deep ripping to break up hardpans which prevent infiltration.
Correcting Sodium Problems with Amendments:
The presence of lime (free calcium carbonate) in soil allows for the widest choice of amendments. To test for this, a
spoonful or clod of soil is treated with a few drops of sulfuric acid or hydrochloric acid. If bubbling or fizzing occurs
where the acid drops fall, then lime is present. The greater the fizzing, the more lime is present. If the soil contains lime,
any of the amendments listed in Table 7.3 can be used. If no lime is present, then only amendments containing soluble
calcium are recommended.
|Commonly Used Amendment Materials and Their Equivalent Amendment Values|
|Tons of Amendment Material Equivalent to:|
|Amendment||Chemical Formula||1 Ton of||1 Ton of|
|(100% Basis)||Pure Gypsum||Soil Sulfur|
The percent purity is generally given on the bag.
Types of Amendments
Calcium containing amendments such as gypsum react in the soil as follows:
GYPSUM + SODIUM-SOIL _ CALCIUM SOIL + SODIUM SULFATE
Leaching is then undertaken to wash out the sodium sulfate. Repeated applications are necessary in many cases. The amount of gypsum used is substantial, often 1.5 or more tons of material per hectare, because it is not highly water soluble, and in many cases, the reaction described above takes a long period of time. It needs to be incorporated to speed up reaction. A more precise measurement of the "gypsum requirement" is available from most soil labs, assuming a material of 100% purity.
Acids such as sulfuric acid undergo a two step process:
1. SULFURIC ACID + SOIL LIME _ GYPSUM + CO2 + WATER
2. GYPSUM + SODIUM-SOIL _ CALCIUM SOIL + SODIUM SULFATE
Acids are dangerous and corrosive, so handling can be a problem. The volume applied has to be controlled because of excessive frothing. Occasionally, cheap industrial sources are available but must be used with caution because of the potential for heavy metal contamination. An analysis of spent acids is recommended. They are much faster than other reclamation procedures because the reaction is instantaneous.
Acid forming materials such as sulfur are much slower because they undergo a three step process, the first step requiring microbial intervention in the oxidation reaction:
1. SULFUR + OXYGEN + WATER _ SULFURIC ACID
2. SULFURIC ACID + SOIL LIME _ GYPSUM + CO2 + WATER
3. GYPSUM + SODIUM-SOIL _ CALCIUM SOIL + SODIUM SULFATE
These steps can take years.
Effectiveness and Amount of Amendments:
In the absence of a soil analysis for gypsum requirement, a rule of thumb is that something is better than nothing. Gypsum is usually used in large quantities, so 0.5 to 2 metric ton applications per hectare are not unusual. To convert the gypsum requirement to an amount of some other amendment, Table 7.3 offers a simple guideline. Simply multiply the gypsum ton equivalent by the gypsum requirement.
If the material being considered is not 100% pure, a simple calculation will indicate the amount needed to be equivalent to 1 metric ton of pure material:
100 % / % purity = m Tons per 1 m ton of pure material.
For example: If gypsum is 60 percent pure, the calculation would be 100/60 = 1.67 m tons. In other words, 1.67 tons of 60 percent pure gypsum is equivalent to 1 m ton of 100% material.
Sulfur presents an additional challenge, since not only purity but the fineness of the granules must be accounted for. The finer the material, the faster microbial oxidation will occur. Coarse grade materials are highly insoluble and may take years to be active.
Management of Saline and Sodic Soils
Often, it is too expensive or impractical to reclaim saline or sodic soils, or even to maintain them at low salinity levels. It may be impossible to adequately drain an area, amendments may not be available or may be too expensive, or the water used for irrigation may be of poor quality.
In these situations, there are various management practices that will aid in controlling or reducing the impact of salts or sodium:
1. Selection of crops or crop varieties that have higher tolerances for salt or sodium (See Table 7.2)
2. Use of special planting procedure that will minimize salt accumulation around the seed. (See Figure 7.2)
3. Use of the appropriate irrigation method for the root characteristics of the crop (See Figure 7.3).
4. Use of sloping beds and other special land preparation procedures and tillage methods to provide a low salt environment
5. Use of irrigation water to maintain a high water content to dilute the salts or to leach the salts out for germination or from the root zone.
6. Use of physical amendments such as manure, compost, etc. for improving soil structure and tilth. Conservation tillage to incorporate crop residues will help create drainage.
7. Deep ripping of soil to break up sodic and other hardpans or other impervious layers to provide internal drainage.
8. Use of chemical amendments as described.
9. Good, sound farming practices and careful fertilizer management.
An understanding of the quality of the irrigation water is essential in any salinity or sodium control program. Often, poor quality water is the source of the salinity or sodium problem. Table 7.4 presents some quality guidelines for evaluating the riskiness of the water. If water is of poor quality, tactics such as dilution with other water sources, or applications of larger leaching amounts can be implemented.
|Effect of Irrigation Water Quality on Soil Salinity, Permeability, Toxicity|
|Salinity ECw (mmhos/cm)||< 0.75||0.75 - 3.0||> 3.0|
|Permeability ECw (mmhos/cm)||> 0.50||0.50 - 0.20||< 0.2|
|Montmorillonite 1||< 6.0||6.0 - 9.0||> 9.0|
|Illite 2||< 8.0||8.0 - 16.0||> 16.0|
|Kaolinite 3||< 16.0||16.0 - 24.0||> 24.0|
|Toxicity (most tree crops)|
|Sodium (adj. SAR) 4||< 3.0||3.0 - 9.0||> 9.0|
|Chloride (meq/l) 5||< 4.0||4.0 - 10.0||> 10.0|
|Boron (mg/l)||< 0.75||0.75 - 2.0||> 2.0|
|Nitrogen (mg/l) 6||< 5.0||5.0 - 30.0||> 30.0|
|Bicarbonate (HCO3)||< 1.5||1.5 - 8.5||> 8.5|
|pH||Normal Range: 6.5 - 8.4|
1 Temperate clay soils, highly expandable, not suited for ceramics or clay tiles.
2 Temperate clay soils or tropical soils in low rainfall or wet/dry climates. Not highly expandable. Can be used for ceramics.
3 Tropical clay soils in high rainfall areas. Usually have a distinct red or yellow color.
4 For most field crops
5 Sprinkler irrigation may cause leaf burn when >3 meq/l.
6 Excess nitrogen causes excessive vegetative growth, lodging, and delayed crop maturity.
Salinity problems can occur due to saline water being used in irrigation. Decreased soil infiltration rates can be the result of irrigation water which is low in salts but high in sodium, or water which has a high sodium to calcium ratio. If infiltration problems are due to high sodium water, the effect will be noticed in the surface few centimeters of the soil.
Other water quality problems to be on the look-out for include:
1. Water high in iron, bicarbonate or gypsum which can result in unsightly deposits on cash crops.
2. Highly acid (low pH) or corrosive water which can result in severe corrosion of irrigation hardware such as pipelines and wells.
3. Other pH abnormalities (high or low) which can result in encrustation or other effects on crops.
4. Risks from diseases such as Bilharzia (schistosomiasis), malaria and lymphatic filariasis; or risks from vectors of diseases such as mosquitoes. Vector breeding can often originate in situations where there is low water infiltration rates, use of wastewater for irrigation or poor drainage.
5. Sediments which can clog up irrigation structures, build films on leafy cash crops which make them unacceptable for
marketing and seal-off soils due to the depositing of structureless silt on soil surfaces.
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