Soils

Definiton of Soil

Soil may be defined as a thin layer of earth crust which serves as a natural medium for the growth of the plants''.

Soil Structure

It refers to the arrangement of soil particles. It is one of the important property of soil, since it influences aeration, permeability and water capacity.

Types of structure

Platy - Horizontal alignment

Prism like - Columnar type

Block like - Angular or sub- angular types

Spiroidal - Granular and crumb types

Soil Texture

The varying proportions of particles of different size groups in a soil constitute is known as soil texture.

The principle textural classes are clay, clay loam, sandy clay, silt clay, sandy clay loam, silty clay loam, sandy loam, silt loam, sand, loamy sand and silt.

Soil Profile  

" It is the vertical section of the soil through all its horizons from the surface to the unaffected parent materials''. Generally the profile consists of three mineral horizons viz., A, B and C.

The surface soil or that layer of soil at the top which is liable to leaching and from which some soil constituents have been removed is known as horizon 'A' or the horizon of eluviation. The intermediate layer in which the materials leached from horizon ' A' have been re-deposited is known as horizon ' B' or the horizon of illuviation. The parent material from which the soil is formed is known as horizon ' C'.

The soil in each of these horizons is usually uniformly developed and presents a more or less homogeneous character. Each layer or horizon develops specific morphological features such as the size and shape of particles, their arrangement, colour, consistence etc. which distinguish from one horizon to another.

Study of soil profile is important since it reveals the characteristics and qualities of the soil.

Soil Composition  

Soil consists of

organic matter

Soil organisms - Micro flora and Micro fauna.

Soil water

Soil air

Inorganic matter - Macro nutrients and Micro nutrients

Organic Matter

The plants and animals grown in weathered material and the organic residues left behind decay with time and become an integral part of the soil. The main source of soil organic matter is plant tissue. Animals are subsidiary source of soil organic matter.

The micro flora like bacteria, fungi, algae, actinomycetes, and micro fauna like protozoa, nematodes, macro fauna like earthworms, ants etc. play an important role in formation of organic matter.

The organic matter influences the soil in respect to colour, physical properties, supply of available nutrients and adsorptive capacity.

Soil Organisms

Soil is the habitat for enormous number of living organisms. Some of these organisms are visible to naked eye where as others can be seen by microscope only.

Roots of higher plants are considered as soil macro flora while bacteria, fungi, algae and actinomycetes are considered as soil micro flora. Protozoa and nematodes are the significant soil micro fauna where as the earthworms, moles and ants constitutes soil macro fauna.

Soil Water

In order to function as a medium for plant growth, soil must contain some water. The main functions of water in the soil are as follows:

Promotes many physical and biological activities of soil.

Acts as a solvent and carrier of nutrients.

As a nutrient itself.

Acts as an agent in photosynthesis process.

Maintains turgidity of plants.

Acts as an agent in weathering of rocks and minerals.

Soil Air

Oxygen is essential for all biological reactions occurring in soil. Its requirement is met from the soil air.

The gaseous phase of soil acts as a path way for intake of oxygen which is absorbed by soil micro organisms, plant roots and for escape of carbondioxide produced by the plants.

This two way process is called soil aeration. Soil aeration become critical for the plant growth when water content is high, because water replaces soil air.

Soil Inorganic Matter

The inorganic constituents of the soil comprises carbonates, soluble salts, free oxides of iron, aluminium and silica in addition to some amorphous silicates.

The inorganic constituents forms the bulk of the solid phase of the soil. Soils having more than 20% of the organic constituents are designated as organic soils.

Soils where inorganic constituents dominates they are called mineral soils. The majority of the soils in India are mineral soils.

Soil pH  

The negative logarithm of hydrogen ion ( H +) concentration is called pH. Soil pH may be acidic, basic or neutral.

Soil Fertility

Soil fertility deals with the nutrient status or ability of soil to supply nutrients for plant growth under favourable environmental conditions such as light, temperature and physical conditions of soil.

Soil Productivity  

Soil productivity is defined as the capability of the soil for producing a specified quantity of plant produce per unit area and the ability to produce sequence of crops under a specified system of management.

Acid Soils

Those soils with pH less than 6.5 and which respond to liming may be considered as acid soils.

Reasons for Acidity

Humus decomposition results in release of large amounts of acids. There by lowering the pH.

Rainfall : 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 (Si o2) when this combined with water, acidity increases.

Characteristics

PH is less than 6.5

This soils are open textured with high massive Structure.

Low in Ca, Mg with negligible amount of soluble salts.

This soils appear as brown or reddish brown, sandy loams or sands.

Injurity to Crops

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.

Indirect Affects

Deficiency of Ca, Mg occur by leaching.

Al, Mn, 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.

Actvity of Micro Organisms

Most of the activities of beneficial organisms like Azatobacter and nodule forming bacteria of legumes are adversely effected as acidity increases.

Crops Suitable For Cultivation in Acid Soils

Amelioration

Lime as reclaiming agent : Lsime 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% of Cao and 3-4% 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 acid soils.

Any citrate soluble phosphate fertilizer is good source of phosphorous for acid soils.

Eg. Dicalcium phosphate ( DCP), Tricalcium phosphate (TCP)

Potassium sulphate is a suitable source of 'K' for acid 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.

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.

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.

Characteristics

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 get 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 effect the solubility of zinc( Zn).

Crops Suitable for Cultivation in Alkaline Soils

Barley, Sugarbeet, Cotton, Sugarcane, Mustard, Rice, Maize, Redgram, Greengram, Sunflower, Linseed, Sesame, Bajra, Sorghum, Tomato, Cabbage, Cauliflower, Cucumber, Pumpkin, Bitterguard. Beetroot, Guava, Asparagus, Banana, Spinach, Coconut, Grape, Datepalm, Pomegranate.

Amelioration

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

It is slightly soluble in water. So it should be applied well in advance.

Requrement

For every 1 m.e of exchangeable Na per 100 gm of soil, 1.7 tonns of Gypsum/ acre is to be added.

Application

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 (Fe S2)

Sulphur present in pyrites causes decrease in pH of soil due to formation of H2So4.

H2So4 + Ca Co3 -- Ca S04 Ca So4 + Na --- Na So4 + Ca ( leachable)

Application of sulphur.

Application of molasses.

Drainage channels must be arranged around the field.

Growing the green manure crops and incorporate in the field.

Problem Soils  

The soils which owe characteristics that they can not be economically used for the cultivation of crops without adopting proper reclamation measures are known as problem soils''.

Saline Soils  

The saline soils contains toxic concentration of soluble salts in the root zone. Soluble salts consists 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.

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.

Characteristics

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 % of seeds is reduced.

Crops Suitable For Cultivation In Saline Soils

Barley, Sugarbeet, Cotton, Sugarcane, Mustard, Rice, Maize, Redgram, Greengram, Sunflower, Linseed, Sesame, Bajra, Sorghum, Tomato, Cabbage, Cauliflower, Cucumber, Pumpkin, Bitterguard. Beetroot, Guava, Asparagus, Banana, Spinach, Coconut, Grape, Datepalm, Pomegranate.

Amelioration

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 this 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 cms 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 Diancha can be grown upto 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% salt solution for 2 to 3 hours.

Soil Testing  

Need : When land is brought under cropping, grain or fruit and sometimes the entire plants are removed (harvested) from the land. Hence, the soil losses a considerable amount of its nutrients (up take by plants). If cropping is continued over a period of time, without nutrients being restored to the soil, its fertility will be reduced and crop yields will decline.

Apart from removal by crops, nutrients may also be lost from the soil through leaching and erosion. Even to maintain soil productivity at the existing levels, it is necessary to restore to the soil, the nutrients removed by crops as also those lost through leaching and erosion.

Continued maintenance of a high level of soil fertility is an indispensable for profitable land use and sustained agricultural production. From time to time the inherent fertility of soil has to be evaluated. There are different methods for soil fertility evaluation as listed below:

Visual method of diagnosis

Plant analysis (Analysis of whole or part of plant growing on the soil in question).

Biological tests in which higher plants or certain micro organisms are used.

Soil tests.

Field experiments.

Advantages

Among the different methods soil testing is a better method for the following reasons.

Soil testing, being a rapid method, is an advantage over the biological methods which are relatively elaborate and time consuming. It is also better than deficiency symptoms and plant and tissue analysis, because the needs can be determined before the crop is planted while in the other methods the crop needs can be ascertained only after the crop is grown, by which time it may be late to correct any nutritional deficiencies that may be indicated.

The main purpose of soil testing is to evaluate the fertility status of the soil. It provides a basis for fertilizer, lime and gypsum recommendation. Laboratory test is a means of making an inventory of the chemical conditions of soil and determining treatments, if any, are needed. Soil test information is then used along with an evaluation of specific crop requirements, cropping history and physical characteristics of the soil for determining the exact amounts of different nutrients and soil amendments, if any, needed for a certain crop or cropping sequence. With this objective in view, a number of soil testing laboratories have been established in the country by the State Governments, Agricultural Universities and fertilizer industry for making fertilizer recommendations to farmers on the basis of the fertility status of their soils. This service is generally rendered free of cost.

Methodology  

Soil Sampling : Soil tests and their interpretation are based on the soil samples sent in for analysis. It is, therefore, important that soil sample should be properly collected and be representative of the area to be tested. Soil tests and their interpretation are as reliable as the soil samples drawn.

Sampling Procedure

Each field should be sampled separately. When the areas within the field distinctly differ in crop growth, in the appearance of soil, in elevation or area known to have been manured or cropped differently in the field should be divided suitably and each area sampled separately.

Drawing samples from spots which do not represent the field should be avoided. Such spots may be old bunds, marshy spots, hedges, areas previously occupied by compost heaps, etc. Sampling should not be done in a field within three months of the application of lime, ash or fertilizer.

Proper sampling tool should be used. Samples can be satisfactorily taken with a soil tube, an auger, a kassi (spade) or khurpi. In a very friable soil, a large spoon can also be used.

A composite sample may be taken from each area. After scrapping the surface litter, a uniform core or a thin slice of soil from the surface to plough depth (15 to 22 cm deep) from 15 to 20 spots should be taken. In a hard soil, a small pit of about 15 cm x 15 cm and of about 15 cm in depth be made. Than a v-shaped slice from one of the slides be taken.

Where crops have been planted in lines, sampling may be done between the lines.

Individual cores or slices should be collected in a clean container. All lumps should be broken and mixed well in the container or on a clean cloth. The size of the composite sample should be reduced by successive quartering to about half a kilogram.

The sample should be dried in shade for an hour or two before putting it into a bag and dispatching it to the nearest soil testing laboratory. Alkathene bags which are available from soil testing laboratories or ordinary clean cloth bags may be used.

Each sample should be identified by name or number to correspond to the field name or number and also by the cultivator's name.

The information sheet furnished by the soil testing laboratory should be filled up completely. This is important and will help the chemist to schedule a more accurate fertilizer recommendation. The information sheet along with the soil sample container should be sent to the soil testing laboratory.

If standard information sheets are not available, information may be given on the following points.

Legal or revenue description of the land (survey number and name of the village)

Crops grown in the last two years:

Date of lost ploughing of the field:

Quantity of lime, gypsum and fertilizer used and when:

Date of legumes last grown on the field:

Whether green manuring practiced, if so, when:

Lay of the land, degree of erosion, drainage, crop growth etc;

Crops to be grown in the next year:

In the soil testing laboratory, soil samples are analyzed for the following five individual soil properties :

PH or soil reaction which indicates whether the soil is acidic, alkaline or normal

Total soluble salts which indicates whether the soil is saline or normal :

Organic carbon (as a measures of available nitrogen)

Available phosphorus

Available potash

Whenever facility exists samples are also analyzed for micro nutrients, especially for Zinc.

Soil Testing

Based on analysis, soils are classified into three categories i.e., low, medium and high, in respect of their available content of each nutrient according to the ratings of soil reaction.

Soil Test Interpretation and Fertilizer Recommendations

From the results of analysis of soil samples sent by the farmer and information sheet supplied by him, soil test reports are prepared in the laboratories. Copies of these reports are sent to the concerned farmer.

Soil test reports are usually in three main parts. First part indicates results of analyses of the soil sample. Most laboratories give actual analyses as well as the ratings. Second part is fertilizer recommendations for the crop based on soil analyses, history of the field like cropping pattern, manures and fertilizers earlier applied, etc. This part indicates quantities of nitrogen (N), Phosphate ( P205), Potash(K20), Zinc (Where facilities exist ) and also of lime or gypsum to be applied per hectare. Most laboratories also show in the report optimum quantities of organic manures as per recommendations of the Agriculture departments.

The third part of the report usually indicates time and methods of fertilizer application and other practices required to make the fertilizer use more efficient.

During the relatively short period that soil testing service has been in operation in this country a large number of soil samples have been analyzed in various laboratories. Based on the results of these analyses , soil fertility maps have been prepared indicating the nutrient status of nitrogen, phosphorus, potassium and zinc in different parts of the country. It must however, be noted that this is only a broad classification , since it is based on limited soil sample analysis.

Rating Chart for Soil Test Data

Classification of Indian Soils  

There are 8 major group of soils in India which are furnished below

Red Soils

Red colour is due to various oxides of iron. They are poor in N, P, K and with pH varying 7 to 7.5. These soils are light textured with porous structure. Lime is absent with low soluble salts.

Red soils occurs extensively in Andhra Pradesh , Assam, Bihar, Goa, Parts of kerala, Maharastra, Karnataka, Tamilnadu and West Bengal. Most of the red soils have been classified in the order ' Alfisols'.

Lateritic Soils

Seen in high rainfall areas, under high rainfall conditions silica is released and leached down wards and the upper horizons of soils become rich in oxides of iron and alluminium. The texture is light with free drainage structure.

Clay is predominant and lime is deficient. pH 5 to 6 with low in base exchange capacity, contained more humus and are well drained. They are distributed in summits of hills of Daccan karnataka, Kerala, Madhyapradesh, Ghat regions of Orissa, Andhra pradesh, Maharastra and also in West Bengal, Tamilnadu and Assam.

Most of the laterite soils have bee classified in the order ' ultisols' and a few under ' oxisols'.

Alluvial Soils

These are the most important soils from the agriculture point of view. The soils are sandy loam to clay loam with light grey colour to dark colour, structure is loose and more fertile. But the soils are low in NPK and humus.

They are well supplied with lime; base exchange capacity is low, pH ranges from 7 to 8. These soils are distributed in Indo-Gangetic plains, Brahmaputra valley and all most all states of North and South. Most of the alluvial soils have been classified in the orders ' Entisols', ' Inceptisols' and ' Alfisols'.

Black Soils

This is well known group of soils characterised by dark grey to black colour with high clay content.

They are neutral to slightly alkaline in reaction. Deep cracks develop during summer, the depth of the soil varies from less than a meter to several meters. Poor free drainage results in the soils, base exchange is high with high pH and rich in lime and potash. Major black soils are found in Maharastra, Madhyapradesh, Gujarat and Tamilnadu.

Cotton is most favourable crop to be grown in these soils. These soils are classified in the order 'Entisols', ' Inceptisols' and ' vertisols'.

Forest Soils

This group of soils occur in Himalayas. Soils are dark brown with more sub-soil humus content. They are more acidic.

Desert Soils

These soils are mostly sandy to loamy fine sand with brown to yellow brown colour, contains large amounts of soluble salts and lime with pH ranging 8.0 to 8.5. Nitrogen content is very low.

The presence of Phosphate and Nitrate make the desert soils fertile and productive under water supply. They are distributed in Haryana, Punjab, Rajasthan. They are classified in the order ' Aridisols' and ' Entisols'.

Peaty and Marshy Soils

These soils occur in humid regions with accumulation of high organic matter. During monsoons the soils get submerged in water and the water receipts after the monsoon during which period rice is cultivated. Soils are black clay and highly acidic with pH of 3.5. Free alluminium and ferrous sulphate are present.

The depressions formed by dried rivers and lakes in alluvial and coastal areas some times give rise to water logged soils and such soils are blue in colour due to the presence of ferrous iron.

Peaty soils are found more in Kerala and marshy soils are found more in coastal tracks of Orissa, West Bengal and South - East coast of Tamilnadu.

Saline - Sodic Soils

Saline soils contain excess of natural soluble salts dominated by chlorides and sulphates which affects plant growth. Sodic or alkali soils contain high exchangeable sodium salts.

Both kinds of salt effected soils occur in different parts of India like Uttarpradesh, Haryana, Punjab, Maharastra, Tamilnadu, Gujarat, Rajastan and Andhra pradesh. These soils are classified under ' Aridisols', ' Entisols' and ' Vertisols'.