Red loams of Kerala are localized in occurrence and are found mostly in the southern parts of Thiruvananthapuram district. These soils occur in catenary sequence along with laterites and are found mainly as deposits by colluviation in foothills and small hillocks. The rapid permeability of the surface soils also has been responsible for the characteristic development of these loamy soils, which are very deep and homogeneous without much expression of horizons. The soils have red colour, which has been attributed to the presence of hematite or anhydrous ferric oxides. These soils are essentially kaolinitic in nature, acidic in reaction, highly porous and friable. They are low in organic matter content as well as in all the essential plant nutrients.
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. The surface soil, which is reddish brown to yellowish red, is mostly gravelly loam to gravelly clay loam in texture. 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. Laterites are in general poor in available nitrogen, phosphorus and potassium and are low in the bases. They have poor water-holding capacity, CEC and high P fixing capacity with low organic matter content. They are generally acidic with pH ranging from 4.5 to 6.2.
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.
These soils are seen in the coastal tracts along the West as a narrow belt with an average width of about 10 km and have been developed from recent marine deposits. They show incipient development. The texture is dominated by sand fraction with very rapid permeability. The A horizon is usually thin and the surface textures observed are loamy sand and sandy loam. These soils are acidic and of low fertility level. They are also low in organic matter, clay and CEC.
These soils occur mostly along the banks of rivers and their tributaries. 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. Horizon differentiation is not well expressed. They are very deep soils which surface texture ranging from sandy loam to clay loam. They are moderately supplied with organic matter, nitrogen and potassium. They are acidic and poor in phosphorus and lime.
These soils are confined to the Onattukara region comprising the Karunagapally, Karthikapally and Mavelikara taluks of Kollam and Alappuzha districts. They occur as marine deposits extending to the interior up to the lateritic belt. The soils are, in general, coarse textured with immature profiles. In low-lying areas, the water table is high and drainage is a problem. These soils have very rapid permeability. They are acidic in reaction and are extremely deficient in all the major plant nutrients.
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. They have been formed as a result of transportation and sedimentation of material from adjacent hill slopes and also through deposition by rivers. 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. Drainage is the major problem. They are moderately supplied with organic matter, nitrogen and potassium and are deficient in lime and phosphorus. Acidity is a problem in some areas.
These soils are usually seen within the coastal tracts of the districts of Ernakulam, Alappuzha, Trissur and Kannur. 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. 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. Electrical conductivity of the soil during this season ranges from 0.1 to 2.0 mmhos/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 to 15 mmhos/cm2. These soils are in general brownish, deep and imperfectly drained. The profile show wide variation in texture, as is common in most of the alluvial soils. 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.
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, the Kayal soils, the karappadam soils and the Kari soils which are dealt with in the zone of Problem Areas.
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. 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.
Being the products of weathering of crystalline rocks under forest cover,
they are restricted in occurrence to the eastern parts of the State.
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.
They generally show vide variation in depth and are dark reddish brown
to black, with loam to silty loam texture. In denuded areas, leaching
and deposition of humus in the lower layers are observed. The B-horizon
usually contains gneissic gravels and boulders. 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.