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Chapter 12: Salinity and alkalinity problems

Salinity and alkalinity problems are most likely to occur under 2 conditions:

• Irrigated soils in semi-arid and arid regions (less than 500 mm annual rainfall) where rainfall or irrigation isn't sufficient to leach accumulated salts out of the root zone. The salts are released by decomposing rock and other parent material below the subsoil and are also brought in by irrigation water and additions of chemical fertilizers and manure.

• Intrusion of salt water into low-lying areas near oceans and seas.

In humid regions, there's usually enough rainfall to flush the salts downward out of the root zone. In low-rainfall areas, irrigation may move salts downward, but they move back up again as the soil dries out between irrigations unless enough extra water is applied. The very high evaporation rates common to these drier regions aggravate this tendency. In many cases, subsurface drainage is also poor, which makes matters worse. Bringing land under irrigation may raise the water table to within a meter or so of the surface, enabling salts to move upwards by capillary action the same way kerosene travels up a lamp wick.

Saline and alkali (sodic) soils fall into 3 classes according to the amount of soluble salts and adsorbed (held by clay and humus particles) sodium they contain: (These soils are also referred to as halomorphic soils)

• SALINE SOILS: These contain enough neutral soluble salts to harm plant growth much like fertilizer burn does. The salts are mainly chlorides and sulfates of sodium, calcium and magnesium. Less than 15% of the soil's exchange capacity (see Chapter 6) is occupied by adsorbed sodium ions, and the pH is usually below 8.5. Saline soils are also called white alkali soils, because the salts tend to accumulate on the soil surface. The usual causes are lack of enough water for adequate leaching, poor drainage, or both.

• SALINE-ALKALI SOILS (Saline-Sodic Soils): These soils not only contain excessive anount of soluble salts, but also harmful amounts of adsorbed sodium (i.e. plus-charged sodium ions that adhere to the negatively-charged clay and humus particles). More than 15% of the soil's exchange capacity is occupied by sodium ions. Although sodium is strongly basic, the pH of these soils is usually below 8.5 due to the buffering influence of the the neutral soluble salts.

• NON-SALINE ALKALI SOILS (Sodic Soils): These soils contain only low levels of soluble salts but have excessive amounts of adsorbed sodium. More than 15% of the soil's exchange capacity is occupied by adsorbed sodium ions held by clay and humus particles. The pH is above 8.5 and often as high as 10, because there aren't enough soluble salts to exert a buffering effect. Sodic soils have very poor physical condition due to their high sodium content; it disperses and puddles (breaks down) soil aggregates (crumbs and clumps of soil particles), making the soil rather impervious to water. Sodic soils are also called black alkali soils, since their surfaces are often black due to the accumulation of dispersed humus brought to the surface by the upward capillary movement of water (from a high water table) and by evaporation.

How salinity and alkalinity harm crop growth

• Osmotic Effect of Salts: Soluble salts in the soil water reduce the ability of plants to absorb water through their root hair membranes (a process called osmosis). If the salt concentration is high enough, water actually starts moving out of the plant roots back into the soil, and the plant may soon die; this is called plasmolysis. At lower salt levels, plants may suffer leaf tip burn, stunting, and defoliation. Germinating seeds and young seedlings are the most sensitive to this osmotic effect. As shown in Table 12-2, crops vary considerably in their salinity tolerance.

• Effect of Sodium: Sodic soils harm plant growth mainly through the toxic effect of sodium itself, the high alkalinity (pH 8.5-10), and the toxicity of the bicarbonate ion with which the sodium is often associated. Germinating seeds and young seedlings are the most sensitive.

• Boron Toxicity: Most irrigation water contains boron which becomes toxic above 1-2 parts per million. Boron is not easily leached from the soil. Irrigation water with a high boron content may limit farming to boron tolerant crops. As shown in Table 12-3, crops vary considerably in their tolerance to boron.

• Rainfall-induced injury: If high evaporation and lack of sufficient leaching allow a high level of salts to accumulate at the soil surface over the weeks, an unseasonal, light rain shower cay move these salts only as far as the crop root zone and cause injury. This is mainly a problem on peat soils when sub-irrigation is used. (Sub-irrigation consists of running water down wide canals through the field to raise the water table enough to irrigate plants by upward capillary movement; It's commonly used on peat soils, which tend to have high water tables.)

Lab diagnosis of salinity and alkalinity

Soil Analysis

Soil testing labs can measure the soluble salt content of soils through electrical conductivity tests. Since salts are electrolytes, the higher the salt content, the higher the electrical conductivity (EC or Ece). The readings are expressed in millimhos or micromhos. Readings can range fro. 0 to over 16 millimhos (16,000 micromhos).

TABLE 12-1

Soluble Salt Content of Soils and its Effect on Crop Growth

Electrical Conductivity (EC)

Millimhos

Micromhos

Diagnosis

Less than 2

Less than 2000

No adverse effect.

2

2000

Yields of some salt sensitive crops are affected.

Below 8

Below 8000

Crop must be moderately salt tolerant.

8-16

8000-16000

Crop must have good salt tolerance.

Above 16

Above 16000

No profitable cropping possible.

Labs can also measure the amount of adsorbed soil sodium to determine alkali danger.

Irrigation Water Analysis

The soluble salt, sodium, and boron content of irrigation water can also analyzed.

SOLUBLE SALTS: The U.S. Salinity Lab classifies irrigation water in 4 categories according to soluble salt level:

Class 1: Low Salinity

100-250 micromhos/cm of depth (0.1-0.25 millimhos/cm). Safe to use on nearly all crops and soils. Some leaching is needed to keep salts moving downward. Salinity problems may develop on poorly drained soils.

Class 2: Medium Salinity

251-750 micromhos/cm (0.25-0.75 millimhos/cm). Can be used to irrigate relatively permeable soils. Crops need to have medium salt tolerance, and leaching will be needed.

Class 3: High Salinity

751-2250 micromhos/cm (0.75-2.25 millimhos/cm). Only for salt-tolerant crops. Adequate drainage is a must as well as ample leaching.

Class 4: Very High Salinity

Above 2250 micromohos/cm (2.25 miIlimhos/cm). Can't be used for irrigation except under certain these conditions permeable soil, good drainage, and high water application rates to obtain good leaching.

SODIUM IN WATER: The sodium content of irrigation water can be accurately measured, but the potential toxicity depends on the proportion of sodium in the water relative the combined calcium and magnesium content. This is known as the Sodium Adsorbtion Ratio (S.A.R.) and is determined by the lab. As with soil, irrigation water can be grouped into 4 categories, ranging from Class 1 (low-sodium water) to Class 4 (high-sodium water), but the actual interpretation isn't as simple as it might appear.

MANAGING; SALINITY AND ALKALINITY PROBLEMS

Reclaiming Saline Soils

Since saline soils contain only soluble salts, leaching is the cure. In many cases, however, salinity is caused by a high water table, and leaching won't be effective until artificial drainage has been installed such as underground tile drains. Deep ripping (subsoiling) may be needed to break up any hardpan that is restricting drainage.

Either periodic leaching or continuous flooding may be used. Salt-tolerant crops like beets, cotton, and barley can be grown during reclamation if flooding isn't used. The amount of water needed for leaching depends on the soluble salt content of the soil and water and the final salt level desired. As a rough guide, about 50% of the soluble salts in the root zone can be removed with 15 cm of water applied per 30 cm depth of soil (15 cm = a layer of water 15 cm deep or the equivalent of 150 liters per sq. meter). About 80% of the salts can be removed by applying 30 cm of water per 30 cm depth depth of soil (300 liters per sq. meter).

The leaching requirement is the ratio of the salt content of the irrigation water to that of the soil. For example where an EC of 8 can be tolerated in the root zone and the irrigation water has an EC of 8 the leaching requirement is 2/8 or 25%. This means that 25% more water should be applied to a crop than is used up by evaporation and plant transpiration. (To help determine crop water needs, see Chapter 5 on water management).

Reclaiming Non-Saline Alkali Soils (Sodic-Soils)

Leaching alone won't remove the adsorbed sodium and insoluble sodium carbonate and bicarbonate. You need to add a soil amendment such as gypsum (calcium sulfate) first which reacts with the soil in 2 ways:

• It converts insoluble sodium salts like sodium bicarbonate into soluble sodium sulfate which is mobile and leachable.

• It also detaches the adsorbed sodium ions adhering to the clay and humus particles and replaces them with calcium. This also results in a lowering of soil pH after leaching.

The gypsom needs to be finely ground and should be hoed oe harrowed into the surface rather than turned under with a moldboard plow. (The turning action of a moldboard ends up leaving the gypsum poorly distributed in slots.)

The soil should be kept moist to promote the reaction of the gypsum and soil. At least 5000 kg/ha (500 g/sq. meter) of gypsum is needed, but the soils lab will determine the exact amount.

On those sodic soils containing calcium carbonate (lime), sulfur can be used instead of gypsum. The soil bacteria convert it to sulfate which then reacts with the calcium carbonate in the soil to form gypsum. Much less sulfur than gypsum is needed (1000 kg of sulfur have the effect of 5380 kg of gypsum). Allow 2-3 months between sulfur application and leaching to allow the conversion to gypsum. The presence of calcium carbonate can be detected by adding several drops of sulfuric or hydrochloric acid to a small amount of soil. If fizzing occurs, this indicates calcium carbonate.

An alternative to soil amendments: Recent research by the USDA has shown that sorghum-sudangrass hybrids can be effective in freeing adsorbed sodium on sodic soils containing calcium carbonate. Sorghum-sudangrass (often called sordan) roots release unusually high amounts of carbon dioxide; the CO2 dissolves soil calcium which can then displace the adsorbed sodium ions. Heavy irrigation is needed to flush away the freed sodium. Where suited, the use of sordan is much cheaper than gypsum and reclaims a greater depth of soil; it is also a nutritious cattle feed. However, plants less than 45 cm tall or those that have been drought-stricken or frosted contain toxic levels of hydrocyanic acid (prussic acid).

Reclaiming Saline-Alkali Soils (Saline-Sodic Soils)

As with sodic soils, leaching alone isn't effective on saline-sodic soils, because it removes only the soluble salts, leaving behind the adsorbed sodium. Free fro. the buffering effect of the soluble salts, the sodium can now exert its full effect in raising the pH and deteriorating soil physical condition. In other words, leaching, by itself, converts a salinesodic soil into a sodic soil! In a few cases, leaching without using gypsum or sulfur may be effective when the soil contains a large amount of soluble calcium or magnesium which can displace the adsorbed sodium.

Controlling the Buildup of Salinity and Alkalinity

It's seldom possible to permanently rid an affected soil of salinity and alkalinity, especially if the irrigation water is one of the causes. The beat strategy is to use management practices that favor crop growth on these soils and help keep salt content at tolerable levels. Here are some guidelines:

• Treatment of irrigation water: There's no economically feasible way to reduce the soluble salt content of irrigation water, but the sodium hazard can be virtually eliminated by adding gypsum to the water. Automatic gypsum metering devices can be bought or built, or a sack of gypsum placed in the irrigation ditch.

• Use of soil amendments: Gypsum or sulfur may need to be periodically applied where conditions are favorable for the development of sodic or saline-sodic conditions.

• Crop selection: Choose crops that are tolerant to saline and alkali conditions. Boron tolerance may be an important consideration too. Tables at the end of this chapter list co on crops and their tolerances.

• Improving soil drainage: Deep ripping of the soil or double-digging by hand (see Chapter 4) may be needed to break up hardpans or compacted layers to facilitate downward water movement.

• Improving soil physical condition: Organic fertilizers and soil conditioners (see Chapter 8) have an especially beneficial effect.

• Land leveling: This will smooth out depressions and rises to give more even distribution of irrigation water which will help prevent the development of pockets with high salt contents,

• Watering management: There are several watering practices that are vital in controlling salinity buildup in susceptible soils:

•• Water frequently to maintain a good soil moisture content to facilitate plant water uptake, which is hampered by the osmotic "pull" of the salts.

•• When watering, apply about 10-20% more water than needed in order to produce enough leaching for salt removal. Leaching assures that the soil salt content will never be higher than that of the irrigation water. Without leaching, soil salt content will gradually increase well above that of the water as salts accumulate instead of being flushed out.

•• Where furrow irrigation is used, increasing the water level in the furrows will aid in seed germination. Placing the seed or transplants just above the water line will help lessen salt buildup around them. ( See Figure 12-1.)

•• Some sources recommend drip irrigation as a way to efficiently maintain good soil moisture and leaching of salts in the immediate plant area. Others caution that drip irrigation can produce salt buildup just as other methods do, especially at the borders between the wetted area and dry soil.

• Seedbed design: Furrow irrigation will cause salts to accumulate near the germinating seeds or the plants unless special attention is given to seedbed design. Figure 12-1 shows how bed design and irrigation techniques determine where salts accumulate and where seeds and plants should be situated.


FIGURE 12-1: Seedbed design and irrigation technique influence where salts accumulate. Single-row beds (A) with every furrow irrigated cause salt accumulation around plants. Conventional, multiple-row beds (B) or slanted, multiple-row beds (C) provide crop space in areas of lower salt concentration. Alternate furrow irrigation (D) concentrates salts in the unirrigated furrow away from crop roots.


TABLE 12-2 Relative Tolerance of Crops to Salinity' (Listed in order of decreasing salt tolerance within each "high", "medium", and "low" grouping)


TABLE 12-3 Relative Tolerance of Some Crops to Boron

(Listed in order of decreasing tolerance to boron within each group)