Ganpat Louhar, Kavitha P Jadhav, Ragini S Patiland Athulya S
Abstract
Soil organic matter (SOM) is a key determinate for the productive soil.
SOM comprises microorganisms, fresh residues and humus fractions. SOM has positive effects on physical, chemical and biological properties of soil. It regulates the supply of air, water and nutrients to the plants. SOM provides a food source for the microorganisms and also involved in the nutrient cycling of N, P, S and K. SOM contribute approximately over half of the nitrogen (N) and a quarter of the phosphorous (P) which is required by the crops. A soil with less active clays and low amount of clay (kaolinites), soil organic matter contribute significantly in cation exchange capacity (CEC) and buffering capacity against acidification. SOM significantly increase plant available water by 2-3 mm/10cm for every 1% increase in soil organic carbon (SOC). Several management practices influence the SOM in the soil such as crop rotation, different tillage practices, residue management, cover cropping and use of manure or compost. Decomposition of freshly added residues and SOM is affected by physico-chemical characteristics as well as by moisture, temperature, nutrients and other factors that affect biological activity directly. Therefore, we need to improve the organic matter levels in soil.
Keywords: biological properties, CEC, microorganisms, physico-chemical properties, SOM
Introduction
Soil organic matter (SOM) comprises all of the organic materials found in soils irrespective of its origin or state of decomposition. It can be living organic matter (floral, faunal and microbial biomass), dissolved organic matter (DOC), particulate organic matter (POM), humus and inert or highly carbonized organic matter (charcoal). The functionally soil organic matter excludes organic materials more than 2 mm in size (Carter, 2002). Soil organic matter differs from soil organic/inorganic carbon (Figure 1) can be
defined as: Soil organic matter encompasses of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P) and sulphur (S). For practical purpose, most of the analytical methods are used to analysis of soil organic matter which actually analyzes the amount of soil organic carbon in the soil.
Based on the carbon content in the soil organic matter, the level of soil organic matter in the soil is analyzed by a conversion factor. It is assumed that the organic matter of soil contains 58% of carbon. The general conversion factor is 1.72 given by Van Bemmelen. Soil organic matter can be calculated by multiplying 1.72 and the soil organic carbon. However this conversion factor may vary from 1.72 to 2.0 depending on the origin and nature of the soil organic matter (Carter, 2002). Soil inorganic carbon especially occurs in arid areas of soil and more association with mafic parent materials such as limestone, basalts etc. Most common types are calcium carbonate as concretions, nodules or as diffuse carbonate, dolomite or magnesium carbonate. Carbonates can be pedogenic origin (formed in the soil) or a lithogenic origin (formed from parent material). Inorganic carbon doesn’t add to the soil organic matter (Monger et al., 2015).
Fig 1: Constituents of soil carbon pool Where,
DOC: Dissolved organic carbon POC: Particulate organic carbon
MOC: Mean oxidation number of organic carbon Sources of organic matter
The primary sources of organic matter are plants, animals and microbial materials. Plant tissues and microbial cells both are contain approximately
40-50% carbon on dry weight basis (Loveland and Webb, 2003). The amounts and types of materials that are added to the soil depend upon the C:
N ratio (Table 1) climatic conditions and vegetation type. Forest system contributes larger amounts of OM to soils as compared to grasslands systems. These sources can be broadly grouped into two classes
i) On-farm sources such as crop residue, roots, roots exudates, livestock manure, green manure
ii) Off-farm sources such as sewage sludge, processing waste, agro industrial waste, municipal wastes etc. Decomposition of OM depend upon amounts and types of materials, C: N ratio, nutrient content (Table 2) and form of materials (liquid and coarse materials) that are added to the soil. Organic materials (OM) having less bulk density than other soil materials (Table 3)
Table 1: C: N ratio of different organic materials (Murphy, 2014)
Organic material C:N ratio
Soil organic matter 10:1
Composted manure 10-30:1
Alfalfa 12:1
Corn stover 60:1
Grain straw 80:1
Saw dust 100-400:1
1. If the ratio is <20:1, the residue has >2% nitrogen and N will be quickly available to plants.
2. If the ratio is >40:1, the residue has <1% nitrogen and N will be tied up for a few weeks resulting unavailable to plants.
Table 2: Ratio of carbon, nitrogen, phosphorus and sulfur and average mineralization factor (Murphy, 2014)
Carbon Nitrogen Phosphorus Sulphur Ratio in soil organic matter 100 10 1 0.25-0.50 Carbon: Element ratio 1:1 10:1 100:1 200-400:1 Element % of soil organic matter 58% 5.8% 0.58% 0.15-0.29
Annual mineralization factor Mineralization factor for soil organic matter nutrients: 1.5% per annuam
Table 3: Average bulk density for soil minerals (texture) and organic matter (Murphy, 2014)
Class (texture, organic matter) Average Bulk Density (g/cm3)
Organic matter 0.22
Sand 1.56
Loamy sand 1.54
Sandy loam 1.50
Loam 1.45
Silt loam 1.20
Sandy clay loam 1.63
Silty clay 1.55
Clay loam 1.45
Silty clay loam 1.40
Soil organic matter (SOM) pools
The functional pools of SOM (Figure 2) may be grouped into two classes (Krull et al., 2004).
A) Decomposable
i) Metabolic litter: (Plant and animal residues such as cellulose having turnover period 1 to 6 months)
ii) Structural litter: (Plant residues such as lignin having turnover period 3 months to 2 year)
B) Resistant
i) Active soil carbon pool: Labile fraction such as microbial biomass having turnover period 1.5 years
ii) Slow soil carbon pool: Labile fraction such as particulate OM having turnover period 8-50 years
iii) Passive soil carbon pool: Humus having turnover period up to 2000 years
Fig 2: Soil organic matter (SOM) pools (Murphy, 2014)
Active SOM: The active SOM pool has primarily composed of soil organisms (Watt et al., 2006) and recent applied plant residues that are in the early stages of decomposition. It is very important pool for nutrient release and helps to develop a slow SOM pool.
Slow SOM: Slow pool of organic matter strongly influenced the physical condition and nutrient buffering capacity of soil. It is also a source of nitrogen and phosphorous.
Stable/passive SOM: Passive pool of organic matter is highly recalcitrant pool (resistant to decomposition). It strongly influenced the cation exchange capacity (CEC) of the soil and plays an important role in soil physical processes such as aggregation. The amount of stable SOM is less affected by recent management practices and tends to increase with increasing the clay content of the soil (McLaughlin et al., 2011). However, different management practices like addition of crop residue, burning, tillage and cover cropping helps in addition of relative and absolute amounts of stable.
Fractions of soil organic matter (SOM) pools: Different fractions of soil organic matter are classified as following (Murphy et al., 2011)
Dissolved organic matter: Organic materials <0.45mm in solution
Particulate organic matter (POC): Organic fragments with a recognizable cell structure
Litter organic materials: Located at the surface and devoid of mineral particles
Macro-organic matter: Fragments of organic material >50μm
Light fraction: Organic materials separated from soils by flotation
Humus: Amorphous organic materials
Non-humic biomolecules: Organic molecules that can be placed into discrete classes of biopolymers
Polysaccharides
Proteins
Waxes
Lignin
Humic substances: Organic molecules that are biopolymers without discrete structures
Humic acid
Fulvic acid
Humin
Inert organic matter: Highly carbonized organic materials including charcoal and charred plant materials
A graphical representation of soil organic matter losses and gains in response to tillage (Figure 3 & 4)
Fig 3: Effect of tillage on level of soil organic matter with time (Fenton and Helyar, 2007)
Fig 4: Effect of tillage on level of soil organic matter pools with time (Fenton and Helyar, 2007)
Soil organic matter and functional soil properties
Organic matter is a fundamental component of the soil because of it plays an important role in physico-chemical and biological processes within the soils (Bouajila and Sanaa, 2011).
Physical properties
Water holding capacity: A general relationship between plant available water and texture of soil is that available water from 10kPa to 1500kPa increased more for sandy soils with increasing soil organic carbon than for clayey soils (Benjamin et al., 2003). The general trend is shown below for making specific recommendations such as:
In sandy soils for every 1% increase of soil organic carbon contains available water about 3 mm/100mm of soil
In loam soils for every 1% increase of soil organic carbon contains available water about 2.5 mm/100mm of soil
In clayey soils for every 1% increase of soil organic carbon contains available water about 2 mm/100 mm of soil
Any changes in water holding capacity (WHC) associated with increasing SOM are affected by the effects of SOM on bulk density (Raut et al., 2012). Organic matter acts as a sponge having ability to absorb and hold the sufficient quantity of water for plants. Sandy soils absorb less amount of
water which is easily available to plants but in case of clay soil which holds larger quantities of water but less available to the plants (Agehara and Warncke, 2005).
Soil structure and aggregate stability: Aggregate stability is a most important fundamental property of the soil that influences many other physical properties of soil (Haynes, 2000). Relationship found that good aggregate stability is required to maintain the good soil structure and a suitable soil physical condition of the soil for plant growth, infiltration and control of erosion (Abrishamkesh et al., 2011). A level of soil organic carbon (SOC) 2 to 2.5% is considered as necessary to maintain good aggregate stability and aggregate stability is deteriorate rapidly when SOC level falls below 1.2 to 1.5% (Boix-Fayos et al., 2001). The role of soil organic matter in maintaining aggregate stability varies with texture of the soil. Soil organic matter in sandy soils, cation balance in clayey soils and both cation balance and soil organic matter in loamy soils is the most important factor for aggregate stability (Baldock, 2002). Organic matter helps in soil aggregation, which improves soil structure. A better soil structure improves infiltration of water through the soil in turns improving ability of soils to absorb and hold the water in soils.
Compaction and friability: Compaction of soils, friability and the atterberg limits are mainly affected by the soil organic matter content (Keller and Dexter, 2012). When the SOC less than 1% results compaction and friability are most limiting factor for plant growth and tillage operations (Turski, 2002). Soils with more amount of clay are less affected by soil organic matter (Keller and Dexter, 2012). Higher amount of organic matter levels tends to reduce the soil compaction and soil capping particularly on fine textured clayey soils through an improved soil structure.
Soil erodibility: Organic matter helps in better water infiltration and more stable soil structure tend to reduce erosion losses of soil. Soil erodibility is only one factor in determining the potential for water erosion with rainfall erosivity, length and degree of slope and land management factors. When SOM less than 2% (SOC <1.2%), erodibility of soil is increased (Rosewell and Loch, 2002). Good aggregate stability (amount of aggregates >125m) also reduced the erosion losses of soil (Berry et al., 2002).
Chemical properties
Nutrient supply/cycling: Organic matter acts as a reservoir of nutrients that can be released to the soil through the mineralization process. Organic
matter helps in limiting pH fluctuations (pH Buffering). SOM holds a relatively constant ratio of the different nutrients for example each % of SOM contains approximately 1000kg of organic nitrogen (N) in addition to other nutrients (Kirkby et al., 2011). For every 1 tonne of SOC contains 83 kg, 20 kg and 14 kg of N, P and S, respectively (Brady and Weil, 2002).
During the decomposition of the SOM, these nutrients will be released into the soil and uptake by the plants depends on the various factors. Thus, SOM acts as a sink and source of nutrients in the soil. Nitrogen is a dynamic nutrient recycled between the atmosphere, within and outside the soil through the soil solution, SOM, plant materials and soil microorganisms (Abu-Zahra and Tahboub, 2008). A soil microorganism plays an important role in the transformation of nitrogen between these pools and made available to plants. Mineralization is mainly depends upon the C: N ratio and its lignin content of the organic materials. Organic materials with high C: N ratios such as cereals in which N being fixed in the soil organism pool and becoming unavailable to plants. Decomposition of SOM under aerobic conditions gives nitrate and ammonium ions but in anaerobic conditions N2O and N2 can be formed rather than mineral N (Bengtston, 2003). Phosphorus present in different pools in the soil and in many soils the P from SOM may be only one of these pools (Shen et al., 2011). Phosphorus from SOM contains about 40% of total P in the soil, but this can vary from 20 to 80%
depending on the soil type. SOM reduce the P fixation by the preventing complex formation of P with Fe and Al minerals (Franzluebbers, 2010) and keeping them in available pools for plants. The major source of sulphur in the soil is SOM, although some soils have a high mineral level of S such as those high in gypsum and some volcanic soils and some soils associated with marine deposits having been associated with acid sulphate soils (FAO, 2004).
Cation exchange capacity (CEC)
SOM have an effect on cation exchange capacity but it is complex due to dependent on the soil texture and pH of the soil (Krull et al., 2004). Most of the SOM fraction that contributes to the cation exchange capacity of the soil has variable charge and this is why the effect of soil organic matter on CEC is pH dependent. Soil pH less than 5.5 appears to be SOM doesn’t contribute significantly to the CEC.
Biological properties
Soil organic matter plays a vital role in soil biodiversity by source of energy and nutrients for soil organisms. Several management practices are
affect the microbial community in soil, those factors are crop rotation, tillage, manures, compost, high analysis chemical fertilizers, herbicides and water content etc. Vegetation is also provides carbon sources for soil microorganisms. Microbes required active SOM to habitat in the soil. A No-tilled long term soil contains higher levels of microbes, more active carbon and their storage and more SOM as compared to conventional tillage soils (Kesik et al., 2010). Bacteria are the primary microbes to decompose recent applied organic plant and animal residues in the soil. About 2 to 5% and 5 to 10% of the total carbon assimilated by anaerobic and aerobic bacteria, respectively and leaving behind many waste of carbon compounds and inadequately using energy stored in the SOM (Biederbeck et al., 2005).
Fungus more efficiently assimilating 40 to 55% of the total carbon and captures more energy from the SOM (Garbeva et al., 2004). As SOM products are added, consumed and recycled in the soil results microorganism population changes rapidly. Microbes help in buildup of SOM and storage the beneficial nutrients in the soil.
Management of soil organic matter
Soil organic matter accumulation in the soil is a long term process. It has many beneficial roles in soils in many aspects (Gardiner and Miller, 2004).
So therefore, we need to improve the organic matter level in soils. These can be done by following options:
Incorporation of crop residue/straw: Cutting and chopping the crop residue and straw and their reincorporation into the soil improves the organic matter content in the soils
Application of farmyard manure (FYM)/compost: Timely applications of FYM or different type of compost such leaf manure, vermicompost, rural and town compost will gradually improve organic matter content as well as supply sufficient amount of beneficial nutrient for plant growth and development
Application of organic fertilizers: Application of cattle, goat, poultry and pig manure into the soil significantly improve the organic matter levels
Cover crops: Growing green cover crops such as groundnut, black gram, green gram, lobia etc. can be helps in buildup of soil organic matter in soil particularly where they are ploughed in as green manure. Best results can be achieved by cover crops are combined growing with other remedial measures
Crop rotation: Crop rotation including growing legumes and grass in the sequence of crop cycle improves the organic matter levels by addition of OM
Reduced tillage operations: Minimum disturbance of soil surface by minimum tillage or no-tillage systems combined with other options such as straw incorporation, green manure etc. will help to increase the organic matter contents in soil
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