There are three major reactions found in soils:
1. Acidic reaction – formation of Acidic Soils
2. Alkaline reaction – formation of Alkaline Soils
3. Saline reaction – formation of Saline Soils
1) Acid Soils
The soils with pH less than 6.5 and which respond to liming may be considered as acidic soils.
a) Reasons for Acidity
Humus decomposition results in release of large amounts of acids. There by lowering the pH.
- In areas with more than 100 cm rainfall associated with high R.H., Ca, Mg is dissolved in water and leached out due to this base saturation of soil decreases.
- Application of elemental sulphur under goes reactions resulting in formation of H2SO4.
- Continuous application of acid forming fertilizers like ammonium sulphates or ammonium
chlorides results in depletion of Ca by CEC (cation exchange capacity) phenomenon.
Parent Material: Generally rocks are considered as acidic, which contain large amount of silica (SiO2) when this combined with water, acidity increases.
- pH is less than 6.5. These soils are open textured with high massive Structure.
- Low in Ca, Mg with negligible amount of soluble salts.
- These soils appear as brown or reddish brown, sandy loams or sands.
c) Injury to Crops
i) Direct Affects
- Plant root system does not grow normally due to toxic hydrogen ions.
- Permeability of plant membranes are adversely affected due to soil acidity.
- Enzyme actions may be altered, since they are sensitive to pH changes.
ii) Indirect Affects
- Deficiency of Ca and Mg occur by leaching.
- Al, Mn and Fe available in toxic amounts.
- All the micro nutrients except molybdenum are available.
- So ‘Mo’ deficiency has been identified in leguminous crops.
- Phosphorous gets immobilized and its availability is reduced.
iii) Effect on Activity of Microorganisms Most of the activities of beneficial organisms like Azatobactor and nodule forming bacteria of legumes are adversely affected as acidity increases.
d) Crops Suitable for Cultivation in Acidic Soils
|pH Level||Acidic Soils|
|4.5 :||Citrus, Blue berries|
|5.0 :||Tobacco, Apple, Grapes, Plum, Watermelon|
|5.5 :||Cowpea, Soybean, Cotton, Wheat, Oat, Peas, Tomato, Sorghum|
|6.0 :||Peanut, Cabbage, Carrot, Onion, Radish, Spinach, Cauliflower|
|6.5 :||Alfalfa, Sugarbeet|
Lime as reclaiming agent:
- Lime is added to neutralize acidity and to increase the pH, so that the availability of nutrients will be increased.
- Basic slag obtained from Iron and steel industry can be substituted for lime.
- It contains about 48–54 per cent of CaO and 3–4 per cent MgO.
- Ammonium sulphate and Ammonium chloride should not be applied to acid soils but urea can be applied.
- Calcium Ammonium Nitrate (CAN) is suitable to acidic soils.
- Any citrate soluble phosphate fertilizer is good source of phosphorus for acidic soils. e.g. Dicalcium phosphate (DCP), Tricalcium phosphate (TCP) potassium sulphate is a suitable source of ‘K’ for acidic soils.
- But MOP is better than K2SO4 because Cl– of MOP replaces -OH ions, their by release of -OH ions tends to increase the pH.
2) Alkaline Soils
- Alkali soils are formed due to concentration of exchangeable sodium and high pH.
- Because of high alkalinity resulting from sodium carbonate the surface soil is discoloured to black; hence the term black alkali is used.
a) Reasons for Alkalinity
- The excessive irrigation of uplands containing Na salts results in the accumulation of salts in the valleys.
- In arid and semi arid areas salt formed during weathering are not fully leached.
- In coastal areas if the soil contains carbonates the ingression of sea water leads to the formation of alkali soils due to formation of sodium carbonates.
- Irrigated soils with poor drainage.
Saline soil have soil pH of more than 8.5 Ec is less than 4.0 m.mhos/cm ESP (exchangeable sodium per cent) is more than 15 It has black colour that why it is also called as Black alkali
c) Injury to Crops
- High exchangeable sodium decreases the availability of calcium, magnesium to plants.
- Dispersion of soil particles due to high exchangeable ‘Na’ leads to poor physical condition of soil, low permeability to water and air, tends to be sticky when wet and becomes hard on drying.
- Toxicity due to excess hydroxyl and carbonate ions.
- Growth of plant gets affected mainly due to nutritional imbalance.
- Restricted root system and delay in flowering in sensitive varieties.
- Typical leaf burn in annuals and woody plants due to excess of chloride and sodium.
- Bronzing of leaves in citrus.
- It effects the solubility of zinc (Zn).
d) Crops Suitable for Cultivation in Alkaline Soils
Barley, Sugarbeet, Cotton, Sugarcane, Mustard, Rice, Maize, Red gram, Greengram, Sunflower, Linseed, Sesame, Bajra, Sorghum, Tomato, Cabbage, Cauliflower, Cucumber, Pumpkin, Bittergaurd. Beetroot, Guava, Asparagus, Banana, Spinach, Coconut, Grape, Datepalm, Pomegranate.
The process of amelioration consists of two steps: To convert exchangeable sodium into water soluble form. To leach out the soluble sodium from the field. Amendments used for reclamation of Alkali soils.
Gypsum – For every 1 m.e. of exchangeable Na per 100 g of soil, 1.7 tones of Gypsum/acre is to be added.
- If the requirement is 3 tonnes/acre- apply in one dose.
- If the requirement is 3 to 5 tonnes/acre- apply in 2 split doses.
- If the requirement is 5 or more tonnes/acre – apply in 3 split doses.
- Use of Pyrites (FeS2).
- Sulphur present in pyrites causes decrease in pH of soil due to formation of H2SO4. Application of sulphur.
- Application of molasses.
- Drainage channels must be arranged around the field.
- Growing of green manuring crops and incorporate them in the field.
3) Saline Soils
- The saline soils contain toxic concentration of soluble salts in the root zone.
- Soluble salts consist of chlorides and sulphates of sodium, calcium, magnesium.
- Because of the white encrustation formed due to salts, the saline soils are also called white alkali soils.
a) Reasons for Salinity
- In arid and semi arid areas, salts formed during weathering are not fully leached.
- During the periods of higher rainfall the soluble salts are leached from the more permeable high laying areas to low laying areas and where ever the drainage is restricted, salts accumulate on the soil surface, as water evaporates
- The excessive irrigation of uplands containing salts results in the accumulation of salts in the valleys.
- In areas having salt layer at lower depths in the profile, seasonal irrigation may favour the upward movement of salts.
- Salinity is also caused if the soils are irrigated with saline water.
- In coastal areas the ingress of sea water induces salinity in the soil.
- Saline soil have soil pH of less than 8.5 EC is more than 4.0 m.mhos/cm ESP (exchangeable sodium per cent) is less than 15 Dominated by sulphate and chloride ions and low in exchangeable sodium Flocculation due to excess soluble salts.
- High osmotic pressure of soil solution Presence of white crust It has white colour that why it is also called as White alkali
c) Injury to Crops
- High osmotic pressure decreases the water availability to plants hence retardation of growth rate.
- As a result of retarded growth rate, leaves and stems of affected plants are stunted.
- Development of thicker layer of surface wax imparts bluish green tinge on leaves.
- Due to high EC germination per cent of seeds is reduced.
d) Crops Suitable For Cultivation in Saline Soils
Barley, Sugarbeet, Cotton, Sugarcane, Mustard, Rice, Maize, Red gram, Greengram, Sunflower, Linseed, Sesame, Bajra, Sorghum, Tomato, Cabbage, Cauliflower, Cucumber, Pumpkin, Bitterguard. Beetroot, Guava, Asparagus, Banana, Spinach, Coconut, Grape, Datepalm, Pomegranate.
- The salts are to be leached below the root zone and not allowed to come up.
- However this practice is some what difficult in deep and fine textured soils containing more salts in the lower layers.
- Under these conditions, a provision of some kind of sub-surface drains becomes important.
- The required area is to be made into smaller plots and each plot should be bounded to hold irrigation water.
- Separate irrigation and drainage channels are to be provided for each plot.
- Plots are to be flooded with good quality water upto 15 – 20 cm and puddled.
- Thus, soluble salts will be dissolved in the water.
- The excess water with dissolved salts is to be removed into the drainage channels.
- Flooding and drainage are to be repeated 5 or 6 times, till the soluble salts are leached from the soil to a safer limit.
- Green manure crops like Daincha can be grown up to flowering stage and incorporated into the soil.
- Paddy straw can also be used. Super phosphate, Ammonium sulphate or Urea can be applied in the last puddle.
- MOP and Ammonium chlorides should not be used.
- Scrape the salt layer on the surface of the soil with spade.
- Grow salt tolerant crops like sugar beet, tomato, beet root, barley etc. Before sowing, the seeds are to be treated by soaking the seeds in 0.1 per cent salt solution for 2 to 3 hours.
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