Fundamentals of Soil Science (1+1)

Introduction

Importance

• Organic materials are intrinsic and essential component of all soils. Whereas the body of the soil is constituted by the inorganic materials, one may look upon the organic matter as its life.
• Organic matter makes the soil a living, dynamic system that supports all life on this planet.
• Soil Organic Matter (SOM) comprises an accumulation of
• Partially disintegrated and decomposed plant and animal residues
• Other organic compounds synthesized by the soil microbes upon decay.
• SOM is frequently said to consist of humic substances and nonhumic substances.
• The term SOM is generally used to represent the organic constituents in the soil, including undecayed plant and animal tissues, their partial decomposition products, and the soil biomass.

Importance of organic matter

• It is the food source for soil microorganisms and soil fauna. If there is no organic matter the soil would be almost sterile and consequently, extremly infertile . Organic matter also supplies hormones (Auxin ,Gibberellins , IAA) and antibiotics for plant growth.
• Organic matter is an index of the productivity of the soil since it is a store house of essential plant nutrients for plant growth. It functions as a reservoir of nitrogen, phosphorus and sulphur and thereby contribute significantly to the supply of these nutrients to higher plants.
• Humus (a highly decomposed organic matter) provides a storehouse for the exchangeable and available cations.
• Soil organic matter contributes to nutrient release from soil minerals by weathering reactions and thus helps in nutrient availability in soils. Organic acids released from decomposing organic matter help to reduce alkalinity in soils; organic acids along with released CO₂ dissolve minerals and make them more available.
• It acts as a buffering agent which checks rapid chemical changes in pH and soil reaction.
• Organic matter creates a granular condition of soil which maintains favorable condition of aeration and permeability.
• Water holding capacity of soil is increased and surface runoff, erosion etc., are reduced as there is good infiltration due to the addition of organic matter.
• Surface mulching with coarse organic matter lowers wind erosion and lowers soil temperatures in the summer and keeps the soil warmer in winter.
• The organic substances influence various soil processes leading to soil formation
• It is the prime decider of soil health and soil quality.

Sources of Soil Organic Matter
Primary sources

• Plant, animal and microbial materials are the primary source of organic matter. Plant tissues and microbial cells contains approximately 40 to 50 per cent carbon on dry weight basis.

Secondary sources

• On –farm sources: Crop residues,roots,root exduates, organic manures,composts and green manure crops contribute significantly towards build up of soil organic matter.
• Off- farm sources: Biodegradable wastes like agro-industrial wastes and muncipal wastes