Soil is the reservoir for water in retaining and supplying the soil moisture to plant growth. The periodical recharging of water in soil pore spaces can be made either by irrigation or rainfall. The recharged water has to be supplied to plant system.
This retention capacity and supply capacity varies from soil to soil based on its physical and chemical properties. Based on this, soil classification is made for its suitability for irrigation. This classification is also known as irrigability classification. Generally, soil can be broadly grouped as shallow soil and deep soil.
(i) Shallow soil – It means the actual depth of soil profile to hold moisture is very less and depth of soil medium available for plant to extend its root system for tapping water and nutrients is less.
(ii) Deep soil – The soil profile depth is more to hold moisture and the depth of soil medium available for plant roots to extend its branches to tap water and nutrients is also more. The recent classification of soil for irrigability classes in arid and semi arid regions are as follows.
This classification can be adopted to our country.
Class A – No soil limitation
Class B – Moderate soil limitation
Class C – Severe soil limitation
Class D – Very severe soil limitation
Class E – Not suitable for irrigation
A. Grouping of soil based on their suitability for irrigation Based on the suitability, the soils are grouped into 5 classes as I to V for the purpose of irrigation, survey and mapping as follows.
Group I – It is indicated in green colour in soil mapping. The soil has the characteristic features of:
• Good available moisture holding capacity
• Low water table
• Low salts either soluble or exchangeable
• No soil crust and pan formation
• Negligible sodium amount.
• Negligible sub soil salinity.
• Good Internal permeability.
Group II – Group II is marked in yellow colour. The soil has the following characteristics of:
• Moderately suitable for irrigation
• Relatively higher, salt and exchangeable sodium content is more than group I
• Deep soil with loamy sand texture; some permeable clay may be there
• Subsoil is also permeable in nature.
Group III – It is indicated in red colour.
• Limited irrigation is practiced with limited cropping intensity
• Available soil moisture holding capacity is medium
• Medium water table
• Moderate salt content and exchangeable sodium percentage
• Moderate internal permeability
• No soil crust or pan formation within the root zone
• Sub soil water may be slightly to moderately saline.
Group IV – It is indicated in blue colour. This soil group is usually not suitable for irrigation. Reclamation work such as addition of organic manures, sand, silt, and application of gypsum may bring the soil under irrigation. It has the characteristic features of:
• shallow depth due to rocky substrata
• hard impervious pan formation
• high soil pH
• more soluble salt content (0.5%)
• low moisture supplying capacity
• low internal permeability.
Group V – It is indicated in dark green colour.
• The soil is shallow in depth
• Total soluble salt and exchangeable sodium percentage is high (more than 25%)
• Stony impervious layers
• Severe crust and pan formation are common
• It cannot be reclaimed by normal reclamation work The soil grouping may be again grouped based on the following limitation
• Erosion/drainage which is indicated by the symbol (O)
• Drainage, wetness or overflow indicated by (W)
• Root zone limitation indicted by the symbol (S)
• Climate limitation indicated by the symbol (C) Based on the dominance, the limitation will be ranked serially.
B. Irrigability classes and rating It is very difficult to classify the lands to determine their suitability for irrigation. The bureau of reclamation, USA has developed a system to classify the suitability of various lands for irrigation agriculture. The system uses six classes.
Class I
• Land topographic and drainage characteristic are highly favourable for irrigation.
• Wide range of crops can be cultivated.
• Climate also highly suitable for wide range of crops.
• Higher Yield may be obtained with low cost.
Class II
• Capacity to produce crops may be high as that for class I land.
• Production, drainage and land development costs are higher.
Class III
• The capacity of the soil for crop production is moderately lower than class II.
• More extreme deficiencies or limitations with soil respect to drainage, topographic undulations even though it is suitable for irrigation.
Class IV
• Some lands in this may be costly to irrigate but due to intensive cropping the returns are adequate.
• The reclamation cost will be high in some lands.
• Yields of crops are very low with low cost of production.
Class V
• Normally unsuitable for irrigation: temporarily irrigation may be made under specific condition.
Class VI
• Lands will not pay for irrigation. A wide range of physical and economic constraints are there.
• Reclamation work is very difficult.
• In general the first four groups are suitable for irrigation.
Class V is temporarily suitable and Class VI is considered as unsuitable for irrigation.
Rating – In rating some characteristic features will be given important consideration. Based on their dominance, soil will be rated.
The rated characteristics for each land class are:
• depth
• organic matter content
• fertility
• ability to absorb moisture
• store and release of moisture for crops
• drainage characteristics
• salt content
• response to fertilizers
• erodibility
• workability for implements
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