SOIL RELATED CONSTRAINTS

The state is located in the humid tropical belt characterized by high rain fall and temperature conditions. The distinct altitudinal variations and undulating topography has a modifying effect on the various factors of the soil formation. Different combinations of the soil forming factors have resulted in the development of a variety of soils and soil related constrains which have a direct bearing on crop production. The major soil related constraints from the point of view of soil health are:

Physical

• Steep slopes and erosion
• Root zone limitation
• Gravel content and hard pans
• Poor drainage and hydrology
• Drought stress

Chemical

• Soil acidity
• Acid sulphate soils
• Aluminium and iron toxicity
• Subsoil acidity
• Low nutrient reserves and retention
• High phosphorus fixation
• Secondary and micro nutrient status
• Soil salinity through sea water intrusion
• Biological

GRAVEL CONTENT AND HARD PANS

The gravel content in laterite soils of Kerala ranges from 15 to 70 per cent. The gravelliness factor depending on the soil texture increases the bulk density which adversely affects the foraging capacity of plant roots for nutrients. The decrease in soil volume also leads to poor inherent soil fertility, lesser available water capacity and other hydrological properties. The low water holding capacity of the low activity clays also brings about water stress in the root zone during summer months. This is especially severe in the northern districts of the state.

POOR DRAINAGE AND HYDROLOGY

Majority of the soils of the state are well drained and they do not pose serious crop management problems. The coastal alluvial soils are excessively drained and needs careful water management strategies for crop production. Poor drainage conditions are consequent on high water erosion and water logging in the lower landscape positions resulting in flooding. Water logging is a characteristic feature of the Kuttanad, Kole, Pokkali and Kaippad lands which have topographic disadvantage of lying below sea level.

DROUGHT STRESS

The low available water capacity as a result of gravelliness of the laterite soils, presence of laterite pans and consequent reduced soil volume has been responsible for intense moisture stress in the root zone leading to drought. The erratic monsoon rains also contribute to severe drought stress in the state

ALUMINIUM AND IRON TOXICITY

Majority of the soils of the state are rich in iron and aluminium oxides and strongly acid with pH less than 5.5 resulting in high levels of soluble Al which accumulates in the root tip and causes injury and inhibits root growth. Aluminium toxicity occurs in the potential acid sulphate soils of Kuttanad, Kole, Pokkali and Kaipad lands. Under aerobic conditions, pyrities mineral get oxidized to jarosite which is hydrolyzed to iron hydroxide and sulphuric acid causing drastic reduction in pH values. In the laterite region, the sub soil layers of the profile with pH less than 5.5, the clay minerals are solubilized bringing into solution K, Mg, Fe and Al. Concentration of Fe and Al increases to toxic levels in soils.
Iron toxicity is often encountered in flooded anaerobic situation especially in rice ecosystem. The reducing conditions bring about solubilisation of oxides of iron, which compete with other cations like K+, Ca2+, Mg2+ and Zn2+, leading to deficiency of these elements. High concentration of soluble Fe also causes precipitation of soluble P into insoluble forms making it unavailable to plants. Toxic levels of Fe are also harmful to the roots. Iron toxicity is more observed under poorly drained conditions and in soils with low cation exchange capacity.

SUBSOIL ACIDITY

The subsoil layers of the laterite soil are often very strongly acid (pH 4.5 and less) resulting in solubilisation of exchangeable Al and deficiency of Ca and Mg. Exchangeable Al in toxic levels inhibits root growth and the target is the root apex causing stunting of primary roots and growth inhibition of the lateral roots. Combined application of gypsum (CaSO4•2H2O) and dolomitic limestone is recommended to neutralize exchangeable Al levels in subsoil and alleviate deficiency of Ca and Mg. Gypsum and its combination with liming materials enables substantial movement of Ca and Mg to lower levels due to the presence of stable and mobile SO42- ion. Gypsum (phosphogypsum), a byproduct from fertilizer industry is available in the state.

LOW NUTRIENT RESERVES AND RETENTION

In general, the soils of the state are highly weathered and dominated by low activity clays with acidic pH, very low cation exchange capacity (CEC) and base saturation, low nutrient reserves and low water holding capacity. The high rainfall conditions result in intense leaching leading to further decrease in nutrient status. The dominance of low activity clays with low CEC and poor carbon reserves contribute to poor nutrient retention in soils.
Low organic matter content of the soil is another factor responsible for the poor nutrient and water holding capacity of soils. Rregular application of organic manures and green manuring helps in improving carbon reserve and thereby improving nutrient and water holding capacity

PHOSPHORUS FIXATION

The important factor controlling the availability of P in soils is the maintenance of soil pH between 6.0 and 7.0. Soils of the state are acidic and rich in sesqi-oxides with resultant P fixation to the tune of 90 -95 %. Around 64 per cent of the cultivated soils of Kerala are high in available phosphorus. This is indicative of the high doses of applied P fertilizers which get fixed initially in the colloidal fraction of the soil and subsequently getting solubilised resulting in high P levels in soils.

SECONDARY AND MICRO NUTRIENT STATUS

Extensive deficiencies of secondary nutrients (Ca and Mg) and micro nutrient (boron) have been observed in Kerala soils. Application of liming materials such as dolomite, calcium carbonate and magnesium sulphate are recommended for alleviating deficiency of Ca and Mg. Application of copper sulphate, zinc sulphate and borax is recommended for correcting deficiencies as per the recommendations of Kerala Agricultural University.

SOIL SALINITY THROUGH SEA WATER INTRUSION

Salinity of soils is mainly a problem in the rice ecosystem in areas adjacent to the coast through sea water ingress. The problem is only seasonal and aggravates during summer months at high tidal flow. The monsoon rain often washes out the accumulated salts leading to low salinity levels enabling cultivation of rice. Rice is being cultivated in Kuttanad region where saline intrusion is protected through construction of bunds and barrages.
Cultivation of salinity resistant rice varieties is common in these areas. However, no protection has been provided in the Pokkali and Kaipad Lands against sea water intrusion and rice cultivation is carried out after the monsoon season when salinity levels are low due to washing out of salts.

BIOLOGICAL

The soil micro flora utilizes organic matter as source of energy and high levels of organic matter are often conducive to intense biological activities. The high temperature and destruction of vegetative cover has accelerated the oxidative process due to exposure leading to depletion of the organic carbon stock. Recycling of organic matter in crop lands are not sufficient to compensate for the oxidative losses.
Soils of Kerala being strongly acidic, alleviation of soil acidity through liming has to be a regular practice for sustaining the soil biological balance. Maintenance of soil organic matter at satisfactory level is also an equally important management requirement for sustaining the biological bio diversity of the soil.