In general, the soils of Kerala are acidic, kaolintic and gravelly with low CEC(Cation Exchange Capacity), low water holding capacity and high phosphate fixing capacity. Climate topography, vegetation and hydrological conditions are the dominant factors of soil formation. On the basis of the morphological features and physico-chemical properties, the soils of the State have been classified into red loam, laterite coastal alluvium, riverine alluvium, Onattukara alluvium, brown hydromorphic, saline hydromorphic, Kuttanad alluvium, black soil and forest loam.
Location :- Red loams of Kerala are localized in occurrence and are found mostly in the southern parts of Thiruvananthapuram district.
Formation and Occurance:-These soils occur in catenary sequence along with laterites and are found mainly as deposits by colluviation in foothills and small hillocks.
Colour and texture:- The soils have red colour, which has been attributed to the presence of hematite or anhydrous ferric oxides.
Permeability:- The rapid permeability of the surface soils also has been responsible for the characteristic development of these loamy oils, which are very deep and homogeneous without much expression of horizons.
Fertile Nature:- They are not fertile due to low organic matter content as well as low essential plant nutrients.
Other Features:- These soils are essentially kaolinitic in nature, acidic in reaction and friable.
Location :- This is the most abundant soil seen in Kerala. They cover about 65 per cent of the total area of the State, occupying a major portion of the midland and mid-upland regions and are the most extensive of the soil groups found in Kerala. Laterite soil is predominantly seen in northern parts of Kerala like Kozhikode, Kannur,Mallapuram and Kasaragod.
Formation and Occurance:-Laterite of Kerala is typical kaolinitic weathering products of gneissic and granitic rocks developed under humid tropical conditions. Heavy rainfall and high temperature prevalent in the State are conductive to the process of laterisation.
Colour and texture:- The surface soil, which is reddish brown to yellowish red, is mostly gravelly loam to gravelly clay loam in texture. It shows the development of AB(c) profiles. The profiles have well-developed B-horizon with abundant ferruginous and quartz gravels. The plinthite is characterized by a compact vesicular mass below the B-horizon, composed essentially of a mixture of hydrated oxides of iron and aluminum. The plinthite includes quarriable type that breaks into blocks and also non-quarriable type that breaks into irregular lumps.
Permeability:-They have poor water-holding capacity
Fertile Nature:- Tough this soil, in general, is acidic, poor in available nitrogen, phosphorus and potassium and are low in the bases,it is well drained, widely cultivated and respond to management practices. A variety of crops like coconut, tapioca, rubber, areca nut, pepper, cashew etc can be successfully grown by the proper application of fertilizers and irrigation.
Other Features:- They are generally acidic with pH ranging from 4.5 to 6.2.They have CEC and high P fixing capacity with low organic matter content.
Location :- These soils are seen in the coastal tracts along the West as a narrow belt with an average width of about 10 km.
Formation and Occurrance:- As it has been developed from recent marine deposits Sand is the chief constituent of this type of soil. Soil profile surface is not well developed. They show incipient development.
Colour and texture:- The texture is dominated by sand fraction The A horizon is usually thin and the surface textures observed are loamy sand and sandy loam.
Permeability:-It has very rapid permeability.
Fertile Nature:- These soil are of low fertility level
Other Features:- These soils are acidic (pH value less than 6.5 in most of the areas).They are also low in organic matter, clay and CEC.
Location :- This soil occurs throughout the state cutting across the extensive laterite soils. They are important in Kollam taluk.
Formation and Occurrance:- This type of soil, developed along river valley, occurs mostly along the banks of rivers and their tributaries.
Colour and texture:- They are very deep soils which surface texture ranging from sandy loam to clay loam. Horizon differentiation is not well expressed.
Other Features:- They are moderately supplied with organic matter, nitrogen and potassium. They are acidic and poor in phosphorus and lime. They show wide variation in their physico-chemical properties depending obviously on the nature of alluvium that is deposited and the characteristics of the catchment area through which the river flows.
Location :- These soils are confined to the Onattukara region comprising the Karunagapally, Karthikapally and Mavelikara taluks of Kollam and Alappuzha districts.
Formation and Occurrance:- They occur as marine deposits extending to the interior up to the lateritic belt.
Colour and texture:- The soils are, in general, coarse textured with immature profiles.
Permeability:-These soils have very rapid permeability. In low-lying areas, the water table is high and drainage is a problem.
Fertile Nature:- Addition of sufficient organic matter and irrigation facilities improve the water holding capacity. Coconut, Paddy and Tapioca are the major products derived from these soils.
Other Features:- They are acidic in reaction and are extremely deficient in all the major plant nutrients.
Location :- Hydromorphic soils, as a group, occur extensively in the State. These soils are mostly confined to valley bottoms of undulating topography in the midland and to low lying areas of coastal strip. These are also found in areas of wetland.
Formation and Occurrance:- They have been formed as a result of transportation and sedimentation of material from adjacent hill slopes and also through deposition by rivers. Presence of Lateritic and gravel suggest that these are formed by the action of gravity.
Colour and texture:- As the name implies the soil is deep brown in colour. Compositionally it ranges between sandy loam to clay. Clay is of pottery type.
Permeability:-Drainage is the major problem of this kind of soil.
Other Features:- They exhibit wide variation in physico-chemical properties and morphological features.The development of the soil profiles has occurred under impeded drainage conditions. These soils,therefore, exhibit characteristic hydromorphic features like grey horizons, mottling streaks, hard pans, organic matter depositions, iron and manganese concretion, etc. They are moderately supplied with organic matter, nitrogen and potassium and are deficient in lime and phosphorus. Acidity is a problem in some areas.
Location :- These soils are usually seen within the coastal tracts of the districts of Ernakulam, Alappuzha, Trissur and Kannur.
Formation and Occurrance:- The origin, genesis and development of these soils have been under peculiar physiographic conditions. They are, therefore, not comparable with the saline soils occurring in the other parts of the Country. The network of backwaters and estuaries bordering the coast serves as an inlet of tidal waters to flow into these areas, causing Salinity. During the period of March – April the soil is characterised by high salinity, but in June – July they are devoid of salts. Wide fluctuation in the intensity of salinity has been observed. During the rainy season, the fields are flooded and most of the salt is leached out, leaving the area almost free of the salts.
Colour and texture:- These soils are in general brownish and deep. The profile show wide variation in texture, as is common in most of the alluvial soils.
Permeability:- This soil is imperfectly drained.
Fertile Nature:- In these soils one crop of Paddy is cultivated during Auguast – December.
Other Features: - Electrical conductivity of the soil during June – July season ranges from 0.1 to 2.0 m mhos/cm2. The maximum accumulation of toxic salts is observed during the summer months from March to April when electrical conductivity rises to the range of 10 - 15m mhos/cm2. Being developed in areas with relatively high ground water table, these soils show aquic properties. In some areas,undecomposed organic matter is observed in the lower layers, causing problems of acidity. The Pokkali (Ernakulam district) and kaipad (Kannur district) soils come under this category.
Kuttanad alluvium/Acid saline soil
Characteristics:- The Kuttanad region covering about 875 km2 is a unique agricultural area in the world. A good portion of this area lies 1-2 m below MSL and is submerged for major parts of the year. The area is susceptible to seasonal ingress of saline water as a result of tidal inflow from the sea. During the monsoons, the rivers and rivulets pour fresh water into the area. As the North East monsoon recedes, seawater again enters the Vembanad Lake and the whole area becomes saline. Hence, the soils of Kuttanad area are faced with the serious problems of hydrology floods, acidity and salinity. Consequent on the construction of the Thanneermukkam bund, salinity hazards have been considerably reduced. The soils of Kuttanad form the typical waterlogged soils and are entirely different from normal well-drained soils in their morphological, chemical and physical characteristics.
They can be grouped into three categories which are dealt with in the zone of Problem Areas.
The Kayal soils
Location :- This type of soil is seen in the reformed lake bed of Kottayam and Alappuzha districts.
Colour and texture:- This soil is dark brown in colour but a white colour seen on the surface due to presence of salt. They contain Alluvial silt and Clay Loams some lime shells, organic matter,calcium, little nutrients and some salts on the surface.
Permeability:- They are poorly drained soils.
Other Features: -They are slightly acidic in nature and are deeply situated. These soils show some aquic features like grey colour, red and brown streak, as the clay content decreases with depth.
The karappadam soils
Location :- These occur in the flat lands of large part of upper Kuttanad and also along the inland stream and rivers.
Formation and Occurrance:- They are formed by the transportation activity of rivers. These are situated 1 – 2m below sea level.
Colour and texture:- This soil contain Clay Loams, Silty clay sub soils, some sands, decayed organic matter, and little amount of nutrients Phosphorus and Calcium.
Permeability:- They are poorly drained soils.
Other Features: -The important features of these soils are high salt content and presence of decomposed organic materials
The Kari soils
Formation and Occurrance:- These are situated 1–2m below sea level.
Location:- Black soils are restricted in their occurrence to Chittoor taluk of Palakkad district. They are found to occur in patches and are considered as extension of the black cotton soils observed in the adjacent Coimbatore district of Tamil Nadu.
Characteristics:- These soils are dark, low in organic matter, calcareous, neutral to moderately alkaline (PH 7.0 to 8.5) and high in clay content and CEC. Hence they exhibit the characteristic cracking during dry periods. They are usually located in gently sloping to nearly level lands. The levels of potassium and calcium are moderate and those of nitrogen and phosphorus, low. In a relatively small area of 1000 ha. in Chittoor block, a highly dispersed soil termed as "Poonthalpadam" soil, is seen. This soil occurs as a slushy layer to a depth of about 0.5m to 1.5m. A bed of limestone is seen beneath the slushy layer. The physical properties like plasticity, cohesion, expansion and shrinkage are similar to those of the regular soils of the Deccan.
Location:- They are found in the eastern parts of the State.
Formation and Occurrance:- They are the products of weathering of crystalline rocks under forest cover.
Colour and texture:- They generally show vide variation in depth and are dark reddish brown to black, with loam to silty loam texture.They have immature profiles with shallow soils, followed by gneissic parent material in various stages of weathering. In areas with lesser canopy cover, signs of laterisation have been observed in the profiles. In denuded areas, leaching and deposition of humus in the lower layers are observed. The B-horizon usually contains gneissic gravels and boulders.
Fertile Nature:-The soil is quite fertile under forest cover and promotes prolific undergrowth. This soil is often found under vegetation
Other Features: - These soils are generally acidic with PH ranging from 5.5 to 6.3. They are rich in nitrogen; but poor in the bases because of heavy leaching.