## Lesson 11. Soil Reaction and Soil Problems

11.1 INTRODUCTION

Soil reaction is the most important single chemical characteristic influencing many physical and chemical properties of soil. Plant growth and microorganism activity depend upon soil reaction. Three conditions possible in the soil are: Acidity, neutrality and alkalinity.

11.2 SOIL REACTION

Soil reaction is measured by pH of a suspension of a soil in water. The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration (or activity) of hydrogen (H+) or hydroxyl (OH-) ions present in it. Acidity is due to the access of H ions over OH ions, and alkalinity is due to the excess of OH ions over H ions. A neutral reaction is produced by an equal activity of H and OH ion.

The most convenient method of expressing the relationship between H+ and OH- is pH. pH is defined as the logarithm of the reciprocal (or negative logarithm) of the hydrogen ion concentration in gram per litre ; represented in equation form as follows:

At neutrality, the hydrogen-ion concentration is: 0.0000001 or 1 X 10-7 gram of  hydrogen per litre of solution. Substituting this concentration into the formula,

pH = log 10,00,000 = 7

At a pH of 6 there is 0.0000001 gm of active hydrogen, or 10 times more than the concentration of H+ than at a pH of 7. At each smaller pH units, the H+ increases by 10 in concentration. It therefore follows that a pH of 6 is 10 times more than a pH of 7. A pH of 5 is 10 times more acid than a pH of 7; a pH of 5 is 10 times more acid than a pH of 6, and so on.

The pH value, therefore, represents the amount of free or active acidity and not the total quantity of potential (or combined) acidity. In other words, it represents the intensity of acidity of a solution. In this scale, the pH value ranges from 0-14, where pH 0 represents the highest limit of active acidity and pH 14 the highest degree of basicity (or alkalinity). Neutrality represents pH 7 (Fig. 11.1). Therefore, pH 7 shows a neutral reaction. When the pH is less than 7, the solution is acidic, when it is above, it is alkaline.

Fig. 11.1 Ranges in pH

Table 11.1 : Relationship between pH and pOH

 pH Acidity Alkalinity pOH 0 1.0 0.00000000000001 14 1 0.1 0.0000000000001 13 2 0.01 0.000000000001 12 3 0.001 0.00000000001 11 4 0.0001 0.0000000001 10 5 0.00001 0. 000000001 9 6 0.000001 0.00000001 8 7 0.0000001 0.0000001 7 8 0.00000001 0.000001 6 9 0.000000001 0.00001 5 10 0.0000000001 0.0001 4 11 0.00000000001 0.001 3 12 0.000000000001 0.01 2 13 0.0000000000001 0.1 1 14 0.00000000000001 1.0 0

11.3 INFLUENCE OF SOIL REACTION ON AVAILABILITY OF NUTRIENTS

The main effect of soil reaction is on the availability of plant nutrients is the soil. Another indirect effect occurs through the activity of microorganisms. Most microorganism function at their best within a pH range 6.0 to 7.5. If soil reaction is changed beyond this range, the microorganisms become functionless. Consequently, the supply of some of the essential plant nutrients like nitrogen is considerably reduced

1. Nitrogen:

Plant absorbs most of their nitrogen in the form of nitrate whose availability depends on the activity of nitrifying bacteria. The microorganisms responsible for nitrification are most active when the pH is between 6.5 and 7.5. They are adversely affected if the pH falls below 5.5 and greater than 9.0. Nitrogen-fixing bacteria (Azatobactor) also fail to function below pH 6.0 and 7.5. When the reaction is above or below the range, availability is reduced.

2. Phosphorous:

The phosphate ions react with iron and aluminum and insoluble phosphates of these elements are formed and become unavailable.

The phosphates react with hydrated oxides of iron and aluminum and form insoluble hydroxyl-phosphates of iron and aluminum. Unavailability of phosphorus is called phosphorus-fixation.

Fixation of phosphate takes place even when the soil is alkaline (high pH). Phosphate ion combines with calcium ion and calcium (or magnesium) carbonates and form insoluble calcium (or magnesium) phosphate.

3. Potassium:

The availability of potassium does not influence by soil reaction to any great extent. In alkaline soil, particularly if the alkalinity is due to CaCO3 (or is brought about by over liming in add soil), the solubility of soil potassium is depressed (results in non-availability).

4. Calcium and Magnesium:

Acid soils are poor in available calcium and magnesium. In alkaline soil (pH not exceeding 8.5) availability of Ca and Mg. nutrients is always high. When the pH is above 8.5, the availability of these nutrients again decreases.

Fig 11.2 Relationship between soil reaction and nutrient availability in so

5. Sulphur:

The availability of sulphur is not affected by soil reaction as sulphur compounds are soluble in the whole pH range. However, it is more soluble in acid soil and lost in leaching.

6. Micronutrients:

In general, the availability of boron, copper and zinc is reduced in alkaline soils and that of molybdenum in acid soils. The availability of boron, copper and zinc progressively decreases as the soil pH increases. Their availability also decreases under highly acid condition when the pH is below 5.0. The availability of molybdenum is more available in neutral and alkaline soils.

11.4 SOIL PROBLEMS

Soil is affected with three types of problems namely (i) Saline soil, (ii) Sodic soil and (iii) Acid soil.

11.4.1 SALINE SOILS

Causes of Salinization:  Salinization or the accumulation of the salts occurs in the following ways:

1.  Primary minerals: During the process of weathering, which involves hydrolysis, hydration, solution, oxidation and carbonation various constituents (Ca, Mg and Na) are gradually released and made soluble.

2.  Arid and semi-arid climate: The low rainfall in these regions is not sufficient to leach out the soluble weathered products and hence the salt accumulates in the soils.

3.  Sea as a source of salts: The Ocean is the source of the salts in low-lying area along the margin of seacoasts. Sometimes salts is moved inland through the transportation of spray by winds be called cyclic salts.

4.  Low permeability of the soil: This causes poor drainage by impeding the downward movement of water. This results into continuous deposition of soluble salts in the soils.

5.  Ground water: Ground water contains large amounts of water soluble salts which depend upon the nature and properties of the geological material with which water remains in contact where water table and evapotranspiration rate is high, salts along with water move upward through capillary activity and salts accumulation on the soil surface.

6.  Irrigation water: The application of irrigation water without proper management (i.e. lack of drainage and leaching facilities) increases the water table and surface salt content in the soils.

11.4.2 SODIC SOILS

Causes of Alkalinity: Process where exchangeable Na content in soil increased due to precipitation of Ca and Mg as carbonate (Na2CO3 or NaHCO3) by low of mass action, Ca and Mg replaced by Na on exchange complex. This soil occurs mainly due to use of alkali or sodic water for irrigation, excessive use of basic fertilizers. It also developed where drainage is defective and where the underground water table is high or close to the surface.

Characterization of salt affected soils:

 Characteristics Saline soil Non-saline alkali soil Saline-alkali soil Content in soil Excess soluble salts Exchangeable sodium on the soil complex Exchange Na on clay as well as soluble salts. ECe (dS/m) > 4 < 4 > 4 Soil pH Less than 8.5 8.5-10 More than 8.5 ESP Less than 15 More than 15 More than 15 Sodium adsorption ratio (SAR) Less than 13 More than 13 More than 13 Total soluble salt content More than 0.1 % Less than 0.1 % More than 0.1 % Dominant salts Sulphate  (SO42-), chloride (Cl-) and nitrates (NO3-) Sodium carbonate (Na2CO3) - Organic matter content Slightly less than normal soils Very low due to the presence of sodium carbonate (Na2CO3) Variable Colour White Black - Physical condition of the soil Flocculated condition, permeable to water and air. Soil structure optimum. Deflocculated condition, permeability to water and air is poor. Very poor soil structure. Flocculated or deflocculated depending upon the presence of sodium salts and Na-clay Other name White alkali (Solonchak) Black alkali (Solonetz) Usar

11.5 CROP GROWTH ON SALT AFFECTED SOILS

1. On saline soils:          The crop growth on salt affected soils is poor due to following reasons:

(A) Water availability theory: Due to high salt concentration plants have to spent more energy to absorb water and to exclude salt from metabolically active sites. At the same time various nutrient elements become unavailable to plants.

(B) Osmotic inhibition theory: The presence of excess solutes in the plant decreases the free energy of unit mass of water.

(C) Specific toxicity theory: According to the specific toxicity theory, soil salinity exerts a detrimental effect on plants through the toxicity of one or more specific ions (cations as well as anions) in the salts present in excess. Accordingly, there may be toxicity of chlorides, bicarbonates and boron.

2. On alkali soils: The reasons of low crop production on such soils are as follows:

(A) Adverse physical conditions: The alkali soils have poor physical conditions. The permeability of air and water and the hydraulic conductivity are at a lower most state due to breakdown of aggregates and dispersion of individual clay-colloids. The breakdown of aggregates is due to dissolution of organic matter, which acts as a cementing agent for binding individual clay particles, due to formation of alkali solution. The dispersed clay plugs all the macro and micro capillaries thereby hampering the movement of air and water. The downward movement of water is practically zero and hence they remain waterlogged when irrigation is given or water is added through rain. On drying, such soils form very large clods, which are very hard in nature. The tillage operations are very difficult to carry due to increase in bulk density, which is due to deflocculation of clay. Because of very adverse physical conditions, the germination as well as the root growth is considerably reduced.

(B)  High sodium on exchange complex: Excess amount of sodium reduces the

crop growth considerably due to plants exhibit the deficiency of Ca.

(C)  Effect of high pH: The high pH reduces the availability of P, Zn, Cu, Mn and Fe. The microbial activity is also at standstill due to unfavourable pH and the processes of mineralization, ammonification or nitrification are practically negligible. The higher concentration of OH ion itself is not favourable to crop growth.

## 11.6 METHODS OF RECLAMATION

I. Mechanical/physical

• Construction of embankment to prevent tidal sea water
• Land leveling and contour bunding.
• Establishment of drainage network
• Breaking of hardpan in the subsurface layer through boring auger hole
• Scrapping of salt crust
• Deep tillage, sub soiling, profile inversion
• Use of soil conditioners e.g. sand, tanch, ash, manures and synthetic polymers like PVAC, PAM, and PVPC
• Use of amendments
• Use of manures
• Green manure
• Selection of salt tolerant crops after afforestation

II.      Chemical

III.     Biological

A. Different types of chemical amendments:

1. Soluble calcium salts e.g.

(i)     Calcium chloride                                 (CaCl2.2H2O)

(ii)    Gypsum                                              (CaSO4.2H2O)

(iii)    Calcium sulphate                                (CaSO4)

2. Acid or acid formers e.g.

(i)     Sulphur                                              (S)

(ii)    Sulphuric acid                                    (H2SO4)

(iii)    Iron sulphate                                      (FeSO4.7H2O)

(iv)    Aluminium sulphate                           (Al2(SO4)3.18H2O)

(v)    Lime sulphur (calcium poly sulphide)  (CaS5)

(vi)    Pyrites                                                (FeS2)

Gypsum is the most common amendment used for reclaiming saline-sodic as well as non-saline sodic soils. It is a low cost amendment and the rate of reaction in replacing Na is limited on its solubility in water, which is about 0.25 % at ordinary temperature. While applying gypsum, mixing it in shallow depth (upper 10 cm depth) is more effective. It is applied by broadcast method or incorporated by disc plough. Gypsum is applied at the time of ponding or leaching. Gypsum directly prevents crust formation, swelling, dispersion and acts as mulch in case of surface application and indirectly increases porosity, structural stability, infiltration and hydraulic properties, soil tilth, drainage and leaching and reduces dry soil strength.

C. Quantity of amendments to be added:

These are evidences to show that even 50 % of the theoretical gypsum requirement for replacement of exchangeable Na in alkali soils has improved their physical properties and assisted response to management practices. Generally, 50 to 75 % of GR (as determined by Schoonover’s method) has been found most satisfactory in many types of soils. The equivalent proportion of different amendments in relation to 1 ton of gypsum is as follows:

 Amendments Weight in tons equivalent to 1 ton gypsum Gypsum 1.000 Sulphuric acid 0.570 Sulphur 0.186 FeSO4.7H2O 1.620 Aluminium sulphate 1.290 Limestone (CaCO3) 0.580 Lime sulphur (Calcium polysulfide containing 24 % S) 0.756

D. The organic amendments: Green manuring with Dhaincha (Sesbania aculeata) has been found most successful. The juice of green plants can neutralize high alkalinity, its initial pH being 4.01, with only slight rise even within a month. In black cotton soil, it thrives well under moderately saline conditions and can with stand high alkalinity, water logging or drought so that it is remarkably suited in that region to alkali soils, characterized by such adverse conditions. Sulphurated hydrogen is generated by the decomposition of Dhaincha. The selection of crop is based on tolerance of a crop to either salinity or sodicity. The list of salt tolerant is given below:

 ECe (dS/m)  (50%  yield reduction) Crop 14-18 Barley, Sugar beet, Cotton, Wheat 8-12 Safflower, Sorghum, Soybean, Rice, Tomato 4-7 Corn, Cabbage, Potato, Sweet potato, Carrot, Onion

The tolerance of various crops to ESP is given below:

 ESP Class Crop 2 – 10 Very sensitive Deciduous fruits, Nuts, Citrus, Avocado 10 – 20 Sensitive Beans 20 – 40 Moderately tolerant Clover, Oats, Tall fescue, Rice, 40 – 60 Tolerant Wheat, Cotton, Alfalfa, Barley, Tomatoes, Beet More than 60 Highly tolerant Crested wheat grass, Fairway wheat grass, Tall wheat grass, Rhodes grass

11.7 ACID SOILS

A. Causes of Soil Acidity

1. Excessive rainfall: The considerable loss of bases due to intensive rainfall and leaching reduces the pH of the soil as well as increase the concentration of H+ on exchange complex.

2. Ionization of water: The water may ionize and contribute H+ on exchange complex as follows :

H2O →HOH → H+  OH-  → H+[X] + Bases + OH-

3. Contact exchange: The contact exchange between exchangeable H on root surface and the bases in exchangeable form on soil particle may take as follows

4. Soluble acid production:  The decomposition of organic matter in the soil produces many organic as well as inorganic acids. These acids may contribute H on exchange complex.

5. Use of nitrogenous fertilizers: Continuous use of nitrogenous fertilizers containing NH4-N or giving NH4-N on hydrolysis (i.e. urea) produces various acids in soils.

6. Oxidation of FeS : FeS or iron poly sulphide accumulates under anaerobic conditions as a result of reduction of Fe3+ and SO4. Under aerobic conditions, they will be oxidized and will produce H2SO4.  Under such conditions, soil pH values of 2 to 4 are frequently observed.

7. Hydrolysis of Fe3+ and Al3+: The Fe3+ and Al3+ ions may combine with water and release H+ and produce acidic condition in soil.

8. Acidic parent material: Some soils have developed from parent materials which are acid, such as granite and that may contribute to some extent soil acidity.

9. Acidification from the air : Industrial exhausts, if contain appreciable amount of SO2 may cause acidity in soil in course of time due to dissolution of SO2 in water (rain) as follows :

SO2 + H2O  → H2SO3  (Rain water)

2H2SO3 + O2 → 2H2SO4 (Sulphuric acid)

B. Problems in Acidic Soils

Problems of soil acidity may be divided into three groups:

1. Toxic effects

(a)  Acid toxicity: The higher hydrogen ion concentration is toxic to plants under strong acid conditions of soil.

(b) Toxicity of elements: The concentration of Iron, Manganese and Aluminium ions (Fe2+, Mn2+ and Al3+) in soil increased in acidic condition to a very high and toxicity of these elements develop.

2. Nutrient availability

(a)  Exchangeable bases: Deficiency of bases like Ca2+ and Mg2+ are found in acid soils.

(b) Nutrient imbalances: Phosphorus reacts with Fe, Al and Mn ions and produces insoluble phosphatic compounds rendering phosphorus unavailable to plants. In acid soils having very low pH, the availability of boron, nitrogen, potassium and sulphur become less available.

3. Microbial activity

Bacteria and actinomycetes can not show their activity when the soil pH drops below 5.5. Nitrogen fixation in acid soils is greatly affected by lowering the activity of Azotobacter sp. Besides these, soil acidity also inhibits the symbiotic nitrogen fixation by affecting the activity of Rhizobium sp. Fungi can grow well under very acid soils and caused various diseases like root rot of tobacco, blights of potato, etc.

C. Reclamation of Acidic Soils: The reclamation of acidic soils is done by addition of liming material which may be calcitic limestone (CaCO3) or dolomitic limestone [CaMg(CO3)2]. The rate of lime requirement is determined in the laboratory by method of Shoemaker (1961).

D. Management of Acid Soils: It is done through ameliorating the soils and selecting acidity tolerance crops.

(a) Ameliorating the soils through the application of amendments

(i) Oxides of lime (CaO) : When oxides of lime like CaO is applied to an acid soil, it reacts almost immediately as follows :

(i) Hydroxides of lime [Ca(OH)2] : When hydroxides of lime like Ca(OH)2 is applied for the reclamation of an acid soil, the following chemical reaction takes place :

(i) Carbonates of lime (CaCO3): When carbonates of lime like calcite is applied to an acid soil following reaction takes place.

(b) Selection of crop: The crop should be selected on the basis of their tolerance to acidity. The relative yield of different crops at different pH values is given in following table.

 High acid tolerant crops : Rice, potato, sweet potato, oat, castor, etc. Moderate acid tolerant crops : Barley, wheat, maize, turnip, brinjal, etc. Slightly acid tolerant crops : Tomato, carrot, red clover, etc.