Chapter 8

SOIL pH AND SOLUBLE SALTS

Soil water contains suspended solids and dissolved chemicals, both inorganic and organic. Thus,
the soil water is actually a solution of ions, many of which are plant nutrients. In this exercise we will look at the concentration of the total ions in the soil solution (soluble salts) and pay particular attention to the concentration of hydrogen ions (pH).

Hydrogen Ion Concentration (pH)

The concentration of H⁺ has a significant effect on soil chemical characteristics. As the concentration of H⁺ decreases, soil becomes less acidic (more basic). Changes in acidity determine the availability of plant nutrients and other soil chemical properties, such as cation exchange capacity. Thus, H⁺ is an important indicator of soil chemical character and its measurement is one of the most fundamental of soil analyses.

The concentration of H⁺ is usually expressed in terms of pH. The pH of a solution is simple the negative logarithm of the H⁺ concentration.

pH = - log (H⁺) = log __1__

(H⁺)

Water dissociates into H⁺ and OH^- ions. When pure water dissociates there are equal amounts of these ions and the solution is considered neutral (pH = 7) (Table 8.1). An acid solution contains more H⁺ than OH^- ions and has a pH less than 7. A basic solution contains less H⁺ than OH^- ions and has a pH greater than 7. The pH scale is logarithmic, indicating that a unit change in pH is equivalent to a ten-fold increase or decrease in the concentration of H⁺ or OH^- (Table 8.1). Notice that as H⁺ concentration increases, OH^- concentration decreases. The relationship between the concentrations of H⁺ and OH^- is based on the disassociation constant of water (K_w).

(K_w) = (H⁺) (OH^-) = 10^-14

At neutral pH: (H⁺) = (OH^-) = 10^-7; therefore pH = 7

Table 8.1 Relationship of pH and to H⁺ and OH^- concentration.

Active versus Reserve Acidity

Soil pH is a measure of the H present in the soil solution or active acidity (Figure 8.1). These are the H+ ions which have disassociated from the cation exchange complex. The greater the percentage H⁺ saturation of the exchange complex, the greater the disassociation of H⁺, and the more acidic the soil solution becomes. Soil pH measurements only indicate the degree of active acidity or active H⁺.

Reserve or potential acidity refers to the H⁺ adsorbed onto the exchange complex (Figure 8.1). Soil pH determinations do not measure reserve acidity, but they are correlated to reserve acidity. The degree of disassociation of H⁺ will determine the amount of H⁺ in solution, active acidity. The amount of reserve H⁺ is usually many times greater than the active H⁺; this depends upon the cation exchange capacity of the soil. Because reserve acidity is the major source of H⁺ ions in the soil, these H⁺ ions must be accounted for when treating soils to change their pH. Lime is commonly used to raise the pH of acidic soils. Only a few grams of lime per hectare are needed to neutralize soil active acidity, but several metric tons per hectare are usually required to neutralize reserve acidity. Thus, active acidity (through dissociation of reserve acidity) indicates the need for liming, but reserve acidity determines the amount of lime needed.

Figure 8.1. Relationships between active and reserve acidity.

Adjusting Soil pH

Liming acid soils to increase pH is one of the most fundamental and widely practiced chemical treatments in agriculture. Agricultural lime is usually CaCO₃, but MgCO₃, CaMg(CO₃)₂, and Ca(OH)₂ are also commonly used. These materials are inexpensive, easy to handle, and environmentally safe. When CaCO₃ is added to an acidic soil, both active and reserve acidity are neutralized. The reactions which take place are shown below.

Soil-H + CaCO₃ ---> Soil-Ca + 2H⁺ + CO₃^-2

Soil-Ca + 2H⁺ + CO₃^-2 ---> Soil-Ca + CO₂ + H₂0

In this typical cation exchange reaction, Ca²⁺ replaces 2H⁺ on the exchange complex, forcing the H⁺ ions into solution. The carbonate ions (CO₃^-2) react with the 2H⁺ ions to produce carbon dioxide (CO₂) and water in a neutralization reaction. In areas having acid soils, a limestone requirement is often determined by soil testing. The lime requirement gives a measurement of the amount of lime necessary to raise the soil pH to a predetermined level (usually 6.5-7.0). Such measurements take into account active, as well as reserve acidity.

Lowering the pH of a basic soil is often more difficult than raising the pH of an acidic soil. Basic soils, especially those high in organic matter, are well buffered and resistant pH change. Leaching a basic soil will help remove basic cations, such as Ca²⁺, Mg²⁺, and Na⁺, from the soil solution. But if the soil has a high cation exchange capacity, these ions can quickly be replaced by additional basic cations disassociating from the exchange complex. This is what is meant by buffering capacity. Soils become acidic when the supply of basic cations has been depleted.

Addition of ammonium fertilizers or elemental sulfur has a net acid-producing effect on sols. The reactions of elemental sulfur in soils are mediated by microorganisms.

2NH₄⁺ + 20₂ ----> 2NO₃^- + 8H⁺

2S + 0₂ + 2H₂0 ----> 2H₂SO₄

Application of sulfuric acid to basic soils is commonly practiced where economically feasible

Soil-Ca + H₂SO₄ ----> Soil-H + CaSO₄

The net result of these reactions is an increase in both active and reserve H⁺. However, often the decrease in pH is of small proportion and short duration. Most basic soils are so well buffered that they resist pH change. Soils of the arid Southwest have large accumulations of basic cations and additional salts are added in irrigation water.

Measurement of Soil pH

The measurement of pH is deceptively simple. The results of any pH determination depend to a large degree on the method of measurement being used. Colorimetric methods are often used in the field and involve color changes of chemical indicators or treated papers. These papers are commonly referred to as litmus paper. More accurate measurements can be made using electronic pH meters. Meters are available for use both in the field and in the laboratory. Electrodes sensitive to H⁺ are inserted into soil solution. The H⁺ concentration, in pH units, is read from a dial or a digital display. Investigators disagree on the techniques used in electrometric pH determination. Different techniques result in a different values for the pH of the same soil, and the results vary considerably. Most of the difficulties arise from the amount and type of diluent used with the soil.

The pH electrode must be in complete contact with the soil solution to function properly. Thus, water (or other diluent) must be added to insure contact. However, the soil pH changes (usually increases) with increasing dilution. This increase in pH upon dilution may be over one pH unit and be partially due to the disassociation of dissolved basic cations. In recent years, the trend has been to use narrower ratios of soil to water, such as the soil saturated paste instead of a 1:5 dilution. In any case, the dilution used is always reported as part of the test results. Diluents other than water have been used in pH determinations. Weak solutions of neutral salts, such as CaCl₂ and KCl, have been used to decrease the observed seasonal fluctuations in soil pH. If a diluent other than water is used in the pH measurement, it must also be reported with the test results. Other problems associated with pH determinations contribute to varying results. Such problems include positioning of the electrode, stirring time, and soluble salt concentration.

Soluble Salts

In soil science, soluble salts refer to the inorganic soil constituents that are soluble in water. Soluble salts are readily accumulated in arid regions where leaching is limited. Salts may be released by weathering of parent material or they may be transported to a soil by wind or water. Soils low in soluble salts may develop salt problems after irrigation with water that is saline (salty).

Plant growth is often restricted in soils high in soluble salts. The osmotic pressures of the soil solution is directly proportional to the amount of dissolved salts. As the osmotic pressure increases, water and nutrient uptake is impeded. The salt concentration of the soil solution is inversely proportional to the water content of the soil. A reduction in the soil water content, through evaporation or transpiration, causes an increase in the concentration of the salts and a further increase in osmotic pressure. Thus, sandy soils are often more prone to salt damage than clayey soils, since sands hold less water than clays.

Soils high in soluble salts are found primarily in arid and semiarid regions. Occasionally, localized areas of soil in humid regions have salinity problems. Such areas are called slick-spots because of their lack of vegetation and poor physical condition. Such spots are usually less than about a half hectare and are often only about 50 m² in size.


Measurement of soluble Salts

The measurement of soluble salts is generally simple, rapid, and accurate. The method is based on the conduction of electricity. Pure water is a poor conductor of electricity. Water containing dissolved salts conducts current approximately in proportion to the amount of salt present in the solution. In the measurement of total soluble salts, a soil-water extract is tested for its ability to conduct electricity by measuring the electrical conductance between parallel electrodes immersed in the solution. As the salt concentration of the solution increases, the conductance of electrical current increases. The conductance (mhos) is measured across a short distance, so the units of electrical conductivity (EC) are mhos/cm. To make the values of EC easier express, a smaller unit is normally used when reporting EC values for soil extracts (mmhos/cm). The equivalent S.I. unit is dS m^-1. To convert EC to ppm soluble salts multiply the EC by 640. Thus, a water sample with an EC of 2 dS m^-1, has 2 x 640 or 1280 ppm soluble salts. The response of plants to different ranges of electrical conductivity is shown below (Table 8.2).

Table 8.2. Plant response to EC of soil saturated paste extract.

The sodium adsorption ration (SAR) is a measure of the amount of sodium in soils and waters relative to the amount of calcium and magnesium. SAR is gradually replacing exchangeable sodium percentage (ESP) in predicting the sodicity hazard of soils and water. The utility of SAR is based on the fact that divalent cations, such as CA²⁺ and Mg²⁺, will be preferentially adsorbed on colloids over monovalent cations, such as Na⁺ and K⁺. If irrigation water has a high content of Ca²⁺ and Mg²⁺, and a high content of Na⁺, the Na⁺ will not be readily adsorbed. Therefore, the Na⁺ can be easily leached and will cause few problems. If the amount of Na⁺ is much higher than the amount of CA²⁺ and Mg², then the Na⁺ is adsorbed and dominates the soil system. This can cause serious problems. The method used to calculate SAR is shown below.

SAR = [Na⁺]

(([Ca⁺²] + [Na⁺])/2)^1/2

Classification of Salt-Affected Soils

Three types of soils with salt problems are usually described, depending on the kinds and amounts of salts present (Table 8.3):

Saline soils - Saline soils contain large quantities of neutral salts which interfere with plant growth. The salts present may include bicarbonates, carbonates, borates, chlorides, or sulfates of calcium, magnesium, potassium, or sodium. However, in saline soils, sodium comprises only a small percentage of the exchangeable cations present (SAR < 13). The salts are called neutral salts because the pH of these soils is usually no greater than 8.5, yet the electrical conductivity is high, greater than 4 dS m^-1 (4 mmho/cm). Saline soils are sometimes called white alkali soils because of the surface accumulation of salt. Surface deposition of salt is often seen in irrigation furrows in the Mesilla Valley. However most Mesilla Valley soils are not saline. Saline soil reclamation involves leaching the soil with enough water to remove the salts from the rooting zone.

Sodic soils - Sodic soils contain large amounts of exchangeable sodium (SAR > 13). The pH of sodic soils is usually high, greater than 8.5, due to the formation of NaOH. But the electrical conductivity of sodic soils is low, less than 4 dS m^-1. Restricted plant growth is due to high concentrations of Na⁺, not soluble salts. Sodic soils are noted for their poor physical condition brought about by the dispersing action of Na⁺. These dispersed soils are impervious to water movement into or through the profile. Humic material may be dispersed and sometimes results in a black surface deposit, hence the name black alkali soils. Reclamation of sodic soils is more difficult than saline soil reclamation. First, the sodium ions must be replaced with a flocculating cation, such as calcium, in order to aggregate the soil and allow for free movement of water. Finally, the soil is thoroughly leached to remove the soluble sodium salts. Reclamation of sodic soils usually takes several years.

Saline-sodic soils - Saline-sodic soils contain appreciable quantities of both neutral salts and sodium (SAR > 13). Although many of the exchangeable ions are sodium, the pH of these soils is likely to be below 8.5 due to the presence of the other neutral salts. Plant growth is impaired because of both high soluble salt and high sodium concentrations. Leaching saline-sodic soils removes the flocculating cations first, leaving a sodic soil. Thus, reclamation first involves treatment with a flocculating cation and then leaching.

Table 8.3 Summary of soil properties of normal and salt-affected soils.

Laboratory Exercise

A. pH

1. Mix soil and deionized water in three dilutions:

a. Soil paste (100 g soil and deionized water)

b. 1:2 (10 g soil and 20 ml water)

c. 1:5 (10 g soil and 50 ml water)

2. To make a saturated paste, slowly add deionized water to soil with continuous stirring until the soil surface becomes shiny and the soil mass begins to flow. Mix all dilutions thoroughly for 2 minutes and let stand for 15 minutes before taking measurements.

3. The instructor will demonstrate the calibration and use of the pH meter.

4. Determine the pH of each of the soil:water dilutions and record on the data sheet.

5. Determine the pH of deionized water, tap water, and an irrigation water sample and record on the data sheet.

B. Soluble Salts and Electrical Conductivity

1. Place each (3) of the dilutions for pH from Part A in to separate Buchner funnel and extract the soil solution under vacuum until about 10 mls are collected for each sample.

2. The instructor will demonstrate the use of the Solubridge.

3. Determine the EC of each extract and record on the data sheet.

4. Determine the EC of deionized water, tap water, and an irrigation water sample and record on the data sheet.

5. Calculate the ppm and pounds per acre furrow slice of soluble salts in each of the extract.

Example: EC = 2 dS m^-1

2 dS m^-1 x 640 mg = 1280 mg/l

1280 mg = 1280 mg = 1280 g = 1280 ppm

liter 1000 g 1,000,000 g soluble salt

1280 x 2 = 2560 pp2m or 2650 lbs per Acre furrow slice

C.

Soil

Sample pH EC Classification

Soil paste, rep. 1

Soil paste, rep. 2

Soil paste, rep. 3

Soil paste, rep. 4

1:2 dilutions

1:5 dilution

deionized H₂O

tap H₂O

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