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Popular Kheti Volume -2, Issue-1 (January-March), 2014 Available online at www.popularkheti.info © 2014 popularkheti.info ISSN:2321-0001
Soil Organic Matter for Sustaining Crop Productivity and Soil Health Kailash Prajapat*1, G. L. Choudhary2, Sita Kumawat3, T. A. Jadhav4 and S. R. Rundala5 1, 2 & 4 Division of Agronomy, IARI, New Delhi 3&5 Dept. of Soil Science and Agricultural Chemistry, SKN COA, Jobner, Rajasthan *Email of corresponding author:
[email protected]
Soil organic matter is natural source of plant nutrients. Organic matter makes a soil healthy, a healthy soil produces a crop healthy, and healthy crops nourish people well. This article discusses little about role of organic matter in soil health. Soil Organic Matter Organic material is anything that was alive and is now in or on the soil. Organic material is unstable in the soil, changing form and mass readily as it decomposes. As much as 90 percent of it disappears quickly because of decomposition. For it to become organic matter, it must be decomposed into humus. In its broadest sense, soil organic matter comprises all living soil organisms and all the remains of previous living organisms in their various degrees of decomposition. Soil organic matter consists of a variety of components. These include, in varying proportions and many intermediate stages, an active organic fraction including micro-organisms (10–40 percent), and resistant or stable organic matter (40–60 percent), also referred to as humus. The living organisms can be animals, plants or micro-organisms, and can range in size from small animals to single cell bacteria only a few microns long. Non-living organic matter can be considered to exist in four distinct pools: • Organic matter dissolved in soil water • Particulate organic matter ranging from recently added plant and animal debris to partially decomposed material less than 50 microns in size, but all with an identifiable cell structure. Particulate organic matter can constitute from a few percent up to 25% of the total organic matter in a soil • Humus which comprises both organic molecules of identifiable structure like proteins and cellulose, and molecules with no identifiable structure (humic and fulvic acids and humin) but which have reactive regions which allow the molecule to bond with other mineral and organic soil components. These Fig.1: Different components of soil organic matter molecules are moderate to large in size
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Popular Article
(molecular weights of 20,000 – 100,000). Humus usually represents the largest pool of soil organic matter, comprising over 50% of the total • Inert organic matter or charcoal derived from the burning of plants can be up to 10% of the total soil organic matter. When plant and animal debris is added to soil, it is broken down by macro and micro-organisms, initially into particulate organic matter, and finally into humus. The raw materials can vary greatly in their resistance to breakdown. Woody organic substances like lignins are very resistant, while more simple compounds like sugars are readily utilised. Along the way, microbial populations increase. Usually, only about 5 percent of OM mineralizes yearly. That rate increases if temperature, oxygen, and moisture conditions become favourable for decomposition, which often occurs with excessive tillage. Effects of Organic Matter on Soil Environment Soil is a living, dynamic ecosystem and healthy soil is the foundation of the food system. It produces healthy crops that in turn nourish people. Maintaining a healthy soil demands care and effort from farmers because farming is not benign. Plants obtain nutrients from two natural sources: organic matter and minerals. Soil organic matter (SOM) and specifically soil organic carbon (SOC) are known to play vital role in the maintenance as well as improvement of soil environment. Soil organic matter is a critical component of the soil resource base, which affects the biological, chemical, and physical processes of the soil and, through the effect on these processes, fulfils a very wide range of functions. A. Effects on soil physical environment 1. Soil structure and aggregate stability: Soil structural stability refers to the resistance of soil to structural rearrangement of pores and particles when exposed to different stresses. Soil structure is stabilized by a variety of different binding agents. Soil organic matter is a primary factor in the development and modification of soil structure. While binding forces may be of organic or inorganic origins, the organic forces are more significant for building large, stable aggregates in most soils. Examples of organic binding agents include plant and microbially derived polysaccharides, fungal hyphae, and plant roots. Both the stable and the active fraction of SOM contribute to and maintain soil structure and resist compaction. 2. Soil water regimes: It has been observed that soils with higher SOM are “fluffier” or have better “tilth” than soils with less SOM. This is because SOM is less dense than the mineral soil particles per unit of volume, and therefore provides greater pore space for water and air to be held. The result of increasing SOM is greater soil pore space, which provides an area for water to be stored during times of drought. A unique characteristic of the pore space in SOM is that the pores are found in many different sizes. The large pores do not hold water as tightly, and thus will drain more readily. The medium and small-sized pores will hold water more tightly and for a longer period of time, so that during a dry period the soil retains moisture and a percentage of that water is made available over time for plant uptake. The benefit of leaving residue on the soil surface and increasing soil organic matter is that water infiltration is increased, soil crusting is decreased, and
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ISSN:2321-0001
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Prajapat et al., 2014, Pop. Kheti, 2(1):100-104
Popular Article
the soil can hold more of the water that infiltrates and will eventually make it available for plant use. 3. Soil temperature: SOM imparts dark brown or black soil colour which is important for ensuring good thermal properties, which in turn contribute to soil warming and promote biological processes. Only about 10% of the solar energy reaching the earth’s surface is actually absorbed by the soil, which can be in turn used to warm the soil. Naturally, dark coloured soils absorb more energy than light coloured ones. However, this does not imply that dark coloured soils are always warmer, since dark coloured soils usually have a higher amount of organic matter, which holds comparatively larger amounts of water; a greater amount of energy is required to warm darker soils than lighter coloured ones. Thus, the thermal property of soil is to a large degree influenced by water content, Db, soil texture (fine versus coarse) and soil colour. In addition, the surface cover of soil affects the heat transfer in and out of a soil. As bare soils warm up and cool off more quickly than those with a vegetation or mulch cover. Thus presence OM acts as a insulator, retarding heat movement between the soil and atmosphere. B. Effects on soil chemical environment 1. CEC: The density of cation exchange capacity (CEC) of organic matter is greater than it is for clay minerals (Table 1). It has thirty times more cation absorption capacity than that of mineral colloids. The different functional groups viz. carboxylic acids (54%), phenolic and hydroxil groups (36%) and amide groups (10%) of SOM are believed to be one of the main contributors to CEC as they provide negatively charged sites. Table 1: Cation exchange capacity of different soil particles Soil particle CEC (cmol/kg) Humus 100-300 Smectites (black swelling clays) 60-150 Kaolinite (white potter’s clay) 2-15 Iron and aluminium oxides (from ferrosols)
