Management of acidic, saline and alkaline soils

Introduction

There is a great variety in their physical, chemical and biological properties, but they can be made to yield a good crop by

1. adopting proper management practices and soil and water conservation measures,
2. applying organic manures and chemical fertilisers and
3. supplementing monsoon rain with irrigation water.

But there are some extreme soil conditions which do not provide a suitable medium for the growth of plants and desirable micro organisms. Such soils are either too acidic or alkaline and special measures have to be adopted to bring them in proper condition before any crop can be raised on them satisfactory and economically. In high rainfall areas, acidic soils are formed due to leaching of bases or salts. In arid regions where rainfall is low and temperature high, soils become saline or alkaline due to alkaline due to accumulation of salts in the surface soil.

Acid soils

The soil may become acidic due to the following factors:

1. Leaching due to heavy rainfall.

The rainfall carries lime and other bases downwards beyond the reach of plant roots. In areas where rainfall is more than 1,000 mm., there is heavy leaching of calcium and other bases and saturation of the exchange complex by hydrogen ions takes place.

2. Nature of parent material

Some soils have developed from parent material which is acidic in nature, e.g. granite.

3. Use of acid forming fertilisers

Some fertilisers like ammonium sulphate and ammonium chloride increase soil acidity. If there is no sufficient free lime in the soil, continuous use of ammonium sulphate or chloride may cause soil acidity.

4. Microbiological action

Micro-organisms decompose organic matter in the soil and organic acids are continuously being formed. If these acids are not neutralized by free lime or bases to make the soil acidic.

1. It has toxic effect on root tissues and adversely affects the permeability.
2. It disturbs the balance between basic and acidic constituents of the plant affects growth of plants.
3. It affects enzymic changes which are particularly sensitive to pH changes.
4. It affects the beneficial activity of soil micro-organisms.
5. Elements like aluminum, manganese andirons are highly soluble in acid medium excess amount causes toxic effect.
6. Due to soil acidity, calcium and potassium may be deficient.
7. It affects the availability e.g. phosphorus, copper and zinc.
8. Plant diseases are more prevalent in acidic soils.

Management of soil acidity

It can be corrected by liming. The liming material may be ground lime stone (CaCO₃), burnt lime (CaO) or hydrated lime Ca (OH)_2- When lime is added, the soil solution becomes charged with calcium ions which replace hydrogen ions in the exchange complex. When burnt (CaO) or slaked or hydrated lime Ca (OH)₂ is added to acidic soil, it changes into calcium bicarbonate, which in solution reacts with soil colloids.

• Lime requirement of soil

1. pH or intensity of soil acidity. In order to raise the pH by one unit i.e., from 5.0 to 6.0 in one hectare about 1,500 kg., of CaCO₃ would be required.
2. Texture of the soil: Clay loam soils require more lime than sandy soils due to higher buffer capacity.
3. Purity of the liming material.
4. Degree of fineness finer the material is ground, more rapidly it goes in solution and is more effective. Material passing through 60 mesh sieve is considered to be standard and quite effective.
5. Chemical composition. The neutralizing value of CaCO₃ is taken as 100.

• Method of applying lime

Lime should be applied before ploughing or on ploughed land well before cropping and should be thoroughly mixed with the soil by discing. Efficient way to use lime is to apply it in small quantities every year or once in two years. If the quantity is not more than five tons per hectare it can be applied in one dose. If the dose is more one half is applied before ploughing and the remaining half applied and worked in after ploughing.

• Tolerance of soil acidity by crops

For medium acidic soil with pH upto 5.6- jowar, maize, wheat, sweet potato, tobacco, tur, velvet bean, turnip. In strongly acidic soils with pH upto 5.1.- potato,rice, oats, rye,cowpea, linseed, grasses, tea and coffee.

Saline and alkali soils

Saline and alkali soils are generally noticed in arid and semi arid regions where there is low rainfall and high temperature causing intense evaporation.

• Alkali soil

The percentage of exchangeable sodium is more than 15. The pH is 8.5 and 10.0.and electrical conductivity less than 4 mmhos/cm., at 25⁰C.

• Saline soil

The exchangeable sodium percentage is less than 15 and the pH below 8.5 and electrical conductivity value of 4 or more mmhos/. cm. at 25⁰C. It indicates white incrustation and therefore called as white alkali soil.

• Saline-alkali soil

It contains exchangeable sodium in excess of 15 p.c. The pH is 8.5 or more and an electrical conductivity more than 4 mmhos/cm., at 25⁰C.

• Use of gypsum

In these soils the exchangeable sodium is so great as to make the soil almost impervious to water. Large quantities of gypsum are applied to replace sodium and leached downward and out of reach of plant roots. Gypsum is applied on the soil surface and mixed by harrowing tow to four weeks before sowing @ 2.5 to 5.0 tons per hectare.

• Leaching of salts

Leaching and draining away of salts by rain and irrigation water.

• Scraping

Scraping off surface salts from highly saline patches.

• Use of sulphur

In extreme cases sulphur is used to reduce alkalinity. Ground sulphur is incorporated into the soil several weeks before planting of the crop. The quantity of sulphur required is depending upon the soil and climatic conditions. The pH upto 8-1.25to 2.5 tons of sulphur and 5 to 7.5 tons of organic matter per hectare will be required to reduce the pH to about 6.5.

• Use of acidifying fertilisers

Example of superphosphate or ammonium sulphate which increases acidity in soil and maintains fertility of soil impoverished by leaching and cropping.

• Green manuring

In alkali soil green manuring of dhaincha has been found to be beneficial along with gypsum in resorting physical condition and enriching the soil in nitrogen and organic matter.

• Use of molasses

Molasses are used to reclaim alkali soils @ 5 tons/ha.alongwith with 2.5 to 5 tons of press mud and 50 to 100kg. It provide source of energy for micro-organisms and on fermentation, produce organic acids which reduce alkalinity while press mud help in reducing exchangeable sodium.

• Growing salt tolerance crops

For moderately alkaline (pH upto 8.4) – Barley, sugarbeet, cotton, sugarcane. For salty soils- date palm, coconut, arecanut. Garden beet, spinach and asparagus are very salt tolerant while radish and beans are sensitive to salt.

Ag.
Technologies
(Soil Magt.)