Interpretation of Soil Map Information

After reading this article you will learn about the interpretation of soil map information for land capability and irrigability classification.

Interpretation of Soil Map Information for Land Capability Classification:

A soil map provides us with detailed information on soils that are utilized for the land capability classification, that, in turn, indicate the suitability or unsuitability of the soil for growing food crops or grasses continuously.

The use of the land may be restricted by one or more of factors like the nature of the soil, the land form or the climate, if these are unfavourable for growing crops. Land capability classification emphasizes that to get the maximum possible return from the land without any loss in its productivity, it must be used only for what it is best suited, and adequate precautions must be simultaneously taken to prevent any damage of the land or decrease in its productivity.

Land capability classification is an interpretive grouping of soils based on inherent soil characteristics, external land features and environmental factors that may restrict the use of the land for growing varieties of crops.

The soil survey of an area is undertaken to establish the soil series of that area which has a definite range of morphological, physical, chemical and mineralogy properties, each of which influences land productivity. Morphological properties include the depth of the soil, and the presence or absence of impermeable layers like clay pan, fragi pan. The soil must be deep enough to encourage the development of a deeper root system.

Impermeable layers like clay pan, and fragi pan prevent the root system from going deeper into the soil. Important physical properties include soil texture, structure, permeability, and colour. The nutrient and moisture retention capacity of the soil depends mainly on the soil texture.

The soils structure and texture effects the circulation of air and water through it. Enough rain water must percolate into the soil to reduce the volume of water running down the slope of the land and causing soil erosion.

The chemical properties of soil include soil reaction, cation exchange capacity and total and available nutrient content. Soil reaction influences the availability of nutrients. Soils of a high cation exchange capacity can store more nutrients than the soils of a low cation exchange capacity.

Fertility of soils is their capacity to supply nutrients in the soluble and available form to crops. The mineralogical properties of soil include the nature of the parent material and primary and secondary minerals present. If the parent material is rich in basic elements and plant nutrients, the resultant soil is expected to be rich in plant nutrients and alkaline in reaction.

Fertile soils with a high moisture re-tension capacity usually contain easily decomposable ferromagnesian minerals and the smectite group of clay minerals. Unfavourable landscape and climatic features may severely restrict the use of land.

If the land slope increases, the soil erosion also increases, which, in turn, decreases the productivity of the land. Many varieties of crops cannot be grown on the land throughout the year in the arid region. Other features like soil salinity and alkalinity, wetness, stoniness and rockiness may also limit the use of the land.

We need the following two kinds of information for land capability classification:

(i) The susceptibility of the soil to various factors that cause soil damage and decrease in its productivity and

(ii) Its potentiality for crop production. We get the first group of information from the soil map but it is not so easy to get the second group, which can only be obtained by growing crops on soils. A soil with capacity to grow varieties of crops for giving high yields, shall qualify to be placed in a better class than another soil which is a little limited in this respect.

The most important characteristic of class I land is that it must be suitable for growing varieties of crops using ordinary good farming practices like use of good manures and fertilizers, crop rotation etc. Paddy is the most important crop in India.

It is preferably grown on lowland clayey soils. Low earthen embankments called bunds are to be erected around the filed which must be puddled to destroy the soil structure in order to retain water within the field for paddy cultivation. So once paddy is cultivated in the field, the field cannot be easily prepared for growing other arable crops like wheat.

Hence a paddy field which gives very high yield of paddy is not placed in class I land, because special farming practices are required for paddy cultivation, and this land not suitable for growing varieties of crops. Thus paddy lands are usually placed in class III land or at the most, in class II land but never in class Hand.

Lands are first tentatively placed in different land capability groups on the basis of slope of the land, erosion and depth of soil which are considered individually as mentioned below:

Lands belonging to slope class A is put in class I lands in black, red and alluvial soil, provided other Factors are favourable. As the black soils are extremely susceptible to erosion, so lands belonging to slope B and C are put in II, III and IV respectively.

Cultivation of lands of more than 5-10 per cent slope is discouraged, and due to their increasing degree of susceptibility to erosion therefore put in classes VI and VII. Black soils of more than 10 per cent slope are usually not found.

As red soils are usually resistant to erosion, the moderately deep and deep red soils can be regularly cultivated for raising grain crops up to D (5 to 10 per cent slope) and E (10-15 E slope) slope class respectively. Lands belonging to D and E slope classes belong to class III lands in moderately deep and deep red soils respectively.

Lands belonging to E & F slope classes are put in class IV in red soil of moderate and good depths respectively as they can be occasionally cultivated for growing grain crop. Lands belong to slope classes F and G and immoderately deep red soil and lands belonging to G and H slope classes in deep red soil are put in class IV and VII respectively.

The effect of climate on land capability classification may be considered only when all other factors are favourable as shown in table 9.1.

Other factors which are to be considered for land capability classification include the nutrient and water retention capacity of soil, its salinity and alkalinity, the available nutrient status of the soil, wetness, stoniness, rockiness, toxicity of certain nutrients.

Each of the above factors considered separately for the tentative placement of lands in capability classes which are then superimposed on each another to get the final land capability classification.

Characteristics of Lands Belonging to Different Capability Classes:

Class I:

Lands are nearly level lands with deep, well drained soils of a satisfactory texture and structure. They are free from the harmful effects of salinity and alkalinity. They are subjected to not more than slight water or wind erosion. They are suitable for growing varieties of crops by adopting ordinary good farming practices. They respond to manuring and possess a high moisture and nutrient retention capacity. They are very fertile lands.

Class II:

Lands can also regularly cultivated for growing grain crops although some easily applied special conservation practices like contour farming are to be followed for maintaining the productivity of lands. They are gently sloping lands with deep soil a satisfactory texture and structure. They are slightly affected by a salinity, alkalinity, wetness erosion etc. which can be easily corrected. They have poor quantity of available nutrients.

Class III:

Lands can also be regular cultivated for raising grain crops by following some intensive soil conservation practices like terracing. They are moderately sloping lands with a moderately deep soil. They are subjected to frequent wetness. They are more severely affected by salinity and alkalinity and soil erosion than class II land s. They have a poor nutrient content and water retention capacity.

Class IV:

Lands are used to cultivate grain crops once in four years. These lands should be left under grasses during rest of the time. They are strongly sloping lands with shallow soils. They are severely affected by soil salinity and alkalinity, soil erosion and wetness. Class IV lands usually possess sandy or concretionary soil of a poor moisture retention capacity.

Class V:

Lands are nearly level lands which are not subjected to more than slight erosion. Cultivation of grain crops on this land is not possible due to some permanent obstruction like rockiness, stoniness. These lands are best suited to growing grasses and forest trees.

Class VI:

Lands are even limited for growing grasses or forest trees. The limitations of class VI land include steep land of shallow soils which are highly susceptible to erosion they are usually dry gravelly/ sandy/loamy soils.

Class VII:

Lands are severely limited even for growing grasses and forest trees. They are steep rough stony lands of extremely shallow soil due to past erosion. Lands might have been extremely damaged by salinity and alkalinity.

Class VIII:

Lands are not suitable even for growing grasses and forest trees because all the soil has been eroded away. So they are steep rough stony mountains.

Interpretation of Soil Map Information for Land Irritability Classification:

Land irritability maps indicate the suitability of lands belonging to different irritability classes for irrigation farming, the precautions that have to be taken for growing different crops on lands and the necessity of applying soil amendments like gypsum or lime to irrigated lands.

The classification of land according to its irritability is based on inherent soil characteristics like soil depth, presence of impermeable layer (clay pan, fragi pan) below the surface of the land, soil texture and structure, infiltration permeability of soils, available moisture characteristics, the cation exchange capacity of soil, the soluble salt content and pH of soils and external land features like slope, erosion stoniness, rockiness, presence of saline and alkaline patches of lands.

Each of these factors indicates the suitability of different lands for irrigation farming. Deep root systems develop in deep soils of a favourable texture and structure. Crops develop a shallow root system if any impermeable layers like clay pan, fragi pan etc. are present near its surface. Sufficient water will not infiltrate in the soil if the texture and structure of the surface soil is impermeable.

Similarly, drainage of the land depends on the permeability of the sub soil which, in turn, depends on the texture and structure of land. Soils should be of a medium texture that allows water to percolate down the soil at a reasonable depth, yet stores enough water for crop growth.

The soil texture mainly affects the water holding capacity of the soil, which is also modified by soil structure, and humus content and the nature of gravels and stones present in the soil.

The available moisture retention capacity of medium textured soils like silty loam and very fine sandy loams is higher than that of the finer textured soils like clay and clay loam. A high cation exchange capacity of a soil is responsible for its high nutrient—storage and buffering capacity.

Such soils are placed in a better land irrigibiality class than the soils of a low nutrient storage and buffering capacity, because they are highly productive and will not be damaged even if moderate saline water is applied to them. Saline lands qualify to be placed in the inferior land irrigability classes due to the higher cost of reclaiming these lands than the non-saline lands.

Similarly, strongly saline and alkaline soils of very high pH and strongly acidic soils of very low pH are placed in land irrigability classes inferior to the neutral soils, due to the high cost of reclamation of saline alkali and acidic soils.

It is very convenient to evenly distribute irrigation water to nearly level land where slope percentage of the land does not exceed 0.2 per cent. The suitability of different lands for land irrigability classification should be determined by the following criteria.

Criteria for land irritability classification

Characteristics of Lands Belonging to Different Irritability Classes:

Class I Lands are excellent lands with no limitations. They are deep level lands where the land slope does not exceed 0.2 per cent. They are moderately permeable to water. The soil texture varies from very fine sandy loam to silty loam or silty clay loams which are not adversely affected by soil salinity and alkalinity or erosion. They possess a high available water holding capacity.

Class II Lands are slightly limited for irrigation. They may be very gently sloping land of deep soil which possesses fairly high available water holding capacity. The soil texture may be sandy loam, sandy clay loam or clay loams which are slightly affected by soil salinity or alkalinity. Permeability may be moderately slow or moderately rapid.

Class III Lands are moderately limited for irrigation. They are gently sloping lands with a fairly deep soil which is moderately affected by soil salinity and alkalinity wetness or doughtiness.They possess a low to moderate available water holding capacity.

Class IV Lands are severely limited for irrigation. They are moderately sloping lands with shallow soils with low available water holding capacity; they are severely affected by soil salinity, alkalinity, wetness or doughtiness.

Class V Lands are not suitable for irrigation because they are strongly sloping and have very shallow soil with a low available water holding capacity. They are very severely affected by soil salinity and alkalinity.