27 Biological Properties of Soil

Meenakshi Nandal

Biological Properties of Soil

Objectives:

• To study Soil flora and fauna.
• To study the function of flora and fauna.
• To study the factors effecting the soil organism.

28.1 Introduction

Climatic condition, organisms present, land form, and parent material has profounder effect on building of specific type of soil .With the interaction of these factors soils develop characteristic horizon. The processes of mineral weathering and the formation of humus due to the decomposition and incorporation of organic materials into the mineral matrix results into the formation of different soil profile. Soil microbe play important role in these process like decomposition and weathering. These living organisms are present in diverse form in the soil. These organisms not only interact with one another but also with plants and small animals forming biological active complex web. Soil is the most biologically diverse form present on the Earth. The soil food web includes beetles, springtails, mites, worms, spiders, ants, nematodes, fungi, bacteria, and other organisms. These organisms help soil resistance to erosion, break down of organic matter, improve the infiltration and storage of water capacity, plant nutrition. Soil organisms contribute from season to season a wide range of essential services like checks and balances to the soil food web through population control, mobility to the sustainable function of all ecosystems. Soil organisms plays other important roles (fig.28.1) also like

1. a) They act and control different nutrient cycle b) Soil organic matter formation is being regulated by them. c) soil carbon sequestration and greenhouse gas emission d) modifying Soil physical structure and water regimes is modify by them e) enhancing the amount and efficiency of nutrient attainment by the vegetation and promoting plant health. All these properties are not only for essential functioning of natural ecosystems but on the other hand constitute an important resource for the sustainable management of agricultural systems. The soil environment is teeming with biological life and is one of  the most abundant and diverse ecosystems on earth. Soil biota, including flora (plants), fauna (animals) and microorganisms, perform functions that contribute to the soil’s development, structure and productivity.

28.2 Soil Flora

Both animals (fauna/micro-fauna) and plants (flora/micro-flora) are important in the overall fertility and stability of soil. Soil organisms include plants and animals i.e., micro flora and micro fauna are equally important for soil. Plants help in aiding structure, porosity and in supplying SOM via shoot and root residue in the soil. Plant roots are constantly growing & dying in soil, thereby supplying soil organisms with food and energy. During the root decomposition root channels can remain open for some time, allowing the path for water and air movement. Roots system also stabilizes soil through  aggregation and integral root systems can decrease soil erosion. Bacteria, fungi and viruses are three main forms of micro flora in soils. Weathering of rocks and minerals, breakdown of organic matter, nutrient cycling etc. all transformation process are being carried by the bacteria. Fungi plays important role in the decomposition of organic matter and also in stabilizing soil aggregates. Mycorrhizal fungi play a major part in securing nutrients for plant production and many plants are dependent on such relationships. On the other hand Actinomycetes belonging to bacterial group grow as hyphae like fungi are especially important in degrading hard-to-decompose compounds, such as chitin and cellulose, and another important fact they are active at high pH levels. Micro flora mainly act through chemical decomposition also are responsible for further decomposing organic material in animal feces and dead bodies of animals. Therefore they are also known as the ultimate decomposers. Viruses are extremely small creatures and yet remain undiscovered for some time instead full importance of these tiny creatures. The ‘rhizosphere,’ the narrow zone of soil directly surrounding plant roots, is the most biologically active region of the soil. It contains sloughed root cells and secreted chemicals (i.e., sugars, organic acids) that provide organisms with food.

8.3 Soil Fauna:

Soil fauna also known as soil engineers, helping in the breakdown of dead plant and animal material, ingesting and processing large amounts of soil. While burrowing into the soil forms ‘biopores’ for water and air movement and mixing soil layers increasing aggregation. Important soil fauna include earthworms, insects, nematodes, arthropods and rodents. Earthworms are considered one of the most important soil fauna. Through the process of burrowing, they provide channels that increase a soil’s porosity, water holding capacity, and water infiltration. They also increase further biotic activity by breaking down large amounts of SOM through digestion and supplying nutrient-rich secretions in their cast’s .Furthermore, earthworms are able to build soil by moving between 1 to 100 tons of subsoil per acre per year to the surface, possibly helping offset losses by erosion. They are key components of soil food web also supporting and regulating several ecosystem services.

In general, soil invertebrates are classified according to their size in micro fauna, meso fauna,  macro fauna and mega fauna .

• 1) Micro fauna: Just one group organisms protozoa, is found wholly within this category whose body size is between 20 μm and 200 μm among the others, small mites, nematodes, rotifers, tardigrades and copepod crustaceans all fall within the upper limit.
• 2) Mesofauna: Micro arthropods such as mites and springtails are the main representatives of this group, which also includes nematodes, rotifers, tardigrades, small araneidae, pseudo scorpions, opiliones, enchytraeids, insect larvae, small isopods and myriapods. Organisms falls in this has bodysize is between 200 μm and 2 mm.
• 3) Macrofauna: This category includes certain earthworms, gastropods, isopods, myriapods, some araneidae and the majority of insects. Organisms whose size is between 2 mm and 20 mm are observed here.
• 4) Mega fauna: large size invertebrates (earthworm snails, myriapods) and vertebrates (insectivores, small rodents, reptiles and amphibians) fall in this category and whose size exceeding 20 mm.

The role of soil flora and fauna in soil ecosystem processes:

Soil organisms more commonly are heterotrophic consumers of preformed organic materials but can be primary producers of organic materials (e.g., phototrophic algae and bacteria). These heterotrophic organisms act on plant materials and are essential in the cycling of nutrients of many elements (nitrogen, sulphur, carbon) and transfer of energy. Soil organisms also play major roles in soil formation and soil structural development by forming biotic pores, transforming soil minerals and organic matter into stable aggregates, and catalyzing mineral weathering processes. Soil fauna works mainly through mechanical action, fungi and bacteria carried out chemical degradation process in both free and intestinal symbionts of other organisms during digestion process. On the other hand so produce organic substances are enriched by enzymes that are dispersed in the soil along with the faeces further contributing to humification of soil.

In soil ecosystem organisms perform many important environmental functions, including (1) regulation of carbon cycling from plant detritus back to the atmosphere, (2) through decomposition and biological nitrogen fixation process provides inorganic nitrogen to plants, (3) mitigation of water contamination through transformation of nitrate via denitrification, (4) biodegradation of natural and synthetic contaminants in soil, and (5) while percolating water through soil into groundwater purification of water also occurs. Along with other soil disciplines soil biology is recognized as a key component for the understanding and improvement of land management systems by human society to help maintain and develop ecosystem functioning on local, regional, and global scales

Soil Microorganisms are invisible to the naked eye. However, their effects on numerous soil properties are far-ranging. Microorganisms represent the largest and most diverse biotic group in soil, with an estimated one million to one billion microorganisms per one gram of agricultural top soil (Tugel and  Lewandowski, 1999). Microbes aid soil structure by physically surrounding particles and ‘gluing’ them together through the secretion of organic compounds, mainly sugars. This contributes to the formation of granular structure in the A horizon where microbial populations are greatest. Soil microbes include bacteria, protozoa, algae, fungi and actinomycetes (Figure 28.2). In ecosystem services these organisms play diverse and often critical roles like affect the porosity and aeration as well as the infiltration and distribution of organic matter within soil horizons Their is vast metabolic diversity of soil microbes in our soil ecosystem and their activities drive or contribute to the cycling of all major elements (e.g. C, N, P), and this cycling of nutrient maintain the structure and the functions of soil ecosystems as well as the ability of soils to provide services to people. Bacteria are the smallest and most important diverse form present in soil and they are important to carry out various functions like SOM decomposition, nutrient transformations and small clay aggregation. Some bacteria carry out very special roles in the soil, such as Rhizobia, the nitrogen-fixing bacteria associated with legume roots. Protozoa (e.g., amoebas, ciliates, flagellates) are mobile organisms that feed on other microbes and SOM. Algae, like plants, photosynthesize and are found variable, and they may only be economical for small-scale, high-value crops. For encouraging mycorrhizal symbioses in agricultural can be done by increasing SOM content, reducing tillage and other soil disturbances and eliminating long fallow periods .

28.4 Biological Activity

Biological activity of soil is controlled by various factors such as residue and SOM quantity and quality, primarily nitrogen (N) content, are major limiting factors for soil organism activity. Other essential factors that promote various activities in the soil are sufficient levels of oxygen, near-neutral pH, temperatures between 85-95°F, and 50-60% moisture. Combinations of these factors will result in maximum activity. These organisms have adapted to extreme environmental conditions, whereas activity generally decreases when conditions fall outside of these ideal ranges. For example, if a soil becomes too wet, oxygen diffusion is impeded and overall activity slows since oxygen is required by most organisms. Management practices can affect soil organism activity through changes in aeration and structure, cropping systems, and inputs. Tillage typically accelerates short-term bacteria and protozoa activity by increasing aeration and breaking up residue into smaller particles that are more exposed to microbial attack. Conversely, fungal biomass has been shown to increase in conservation tillage systems, possibly as a result of less tillage disrupting fungal hyphal networks and/or increases in SOM levels. With increase in SOM levels and minimize soil disturbances earthworm populations also tend to increase. On the other hand crop rotation systems also help in increasing more organism diversity and activity than monoculture systems due to increased and more diverse residues and specific interactions occurring between certain plants and organisms. Fertilizer applications can also influence soil organism populations and activity. With the addition of fertilizers, particularly those containing N will likely increase biotic activity in soils having low fertility or SOM content microbes populations will eventually stabilize as N is consumed and condition of soil improves. Whereas in addition, certain fertilizer applications, anhydrous ammonia, can temporarily harm some soil organisms at the injection site. However, most organism populations will recover with time.

During the last decades of the twentieth century there was an awareness of the importance of the soil as an environmental component and recognition of the need to maintain or improve its capacity to allow it to perform its various functions. The soil, if used improperly or poorly managed, its characteristics can be lost in a short period of time, with limited opportunities for regeneration and the resource become exhaustible due to over exploitation. Soil is characterized as a complex and dynamic system. It is constituted by several layers that differ in relation to the physical, chemical, mineralogical and biological nature, which are influenced by the climate and activities of the living organisms. Besides contributing to the maintenance of all forms of life that occur in the terrestrial surface, soil plays an important role in protecting the groundwater acting as a collector filter of organic and inorganic residues, helping in sequestering possible toxic compounds.

28.5 Soil organic matter

On the basis of organic matter content, soils are characterized as mineral or organic. Mineral soils contain from a trace to 30 percent organic matter in most of the world’s cultivated land. Organic soils (Figure 28.3) are naturally rich in organic matter due to favorable climatic condition. Although they contain more than 30 percent organic matter, but not considered vital for cropping soils. Reason may  be soil organic matter affecting biological activity in soils. It is the carbon source for not only the amount, but also the type of organic compounds in the soil determines its biological activity; e.g., microbial activity is greatly increased by incorporating fresh organic residues (such as green manure or crop residues), which can be readily mineralized by microbes.

Figure 28.3 Soil organic matter and its composition. Source: https://www.pinterest.com/pin/368169338269509675/

Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process. The main source of soil organic matter is plant tissue containing 60-90 percent moisture. And the other constituent of dry matter includes carbon (C), oxygen, hydrogen (H) and rest little amounts of calcium (Ca) and magnesium (Mg) sulphur (S), nitrogen (N), phosphorus (P), and potassium (K). For soil fertility management, these nutrients are very important although present in small amounts. Soil organic matter consists of many intermediate stages, such as (10-40 percent) active organic fraction consisting of microorganisms, and resistant or stable organic matter (40-60 percent), known as humus consisting of varying proportions of a variety of components. For practical purposes Tate (1987) and Theng (1987) has given classification of soil organic matter which may be a) above ground organic matter comprises of plant residues and animal residues; b)belowground organic matter consists of living soil fauna and micro flora, partially decomposed plant and animal residues, and humic substances. The type of material and ease of decomposition depends upon the C: N ratio i.e. hard woody materials with a high

• C: N ratio being more resilient than soft leafy materials with a low C: N ratio. Soil organic matter serves several functions within soil and in agricultural it is important for two main reasons:
• (i) It act as a “revolving nutrient fund”
• (ii) Considered to be an agent to improve soil structure, maintain tilth and minimize erosion.

Further organic matter as a revolving nutrient fund, serves two main functions:

From plant residues soil organic matter is derived containing all of the essential plant nutrients.Therefore, plant nutrients organic matter is also known as storehouse of plant nutrients.

The stable organic fraction (humus) adsorbs and holds nutrients in plant-available form .Upon decomposition organic matter releases nutrients in a plant-available form. In order to maintain the nutrient cycling system the rate of organic matter addition from crop residues, manure and any other sources must equal to the rate of decomposition and also take into account the rate of uptake by plants and losses by leaching and erosion. Soils nutrients are present in solution form or bound to soil particles depending upon their chemical nature. In the “soil pools” exchange of nutrients between different forms is governed by physical, chemical, or biological processes. Due to human activities the soil and global nutrient cycles are affected.

Role of Organic Matter in Soil Fertility

Organic matter is obtained from dead plant roots, various organic manures like farmyard manure, compost and green manure, crop residues and fungi, bacteria, worms, insects etc.although forms a very small but an important fraction of soil.(fig.28.4)

Functions of organic matter

Soil physical condition particularly the structure is being improved by organic matter. Soil structure refers to the naturally occurring array of soil particles into aggregates. Soil aggregates are formed by the chemical and physical interaction of microbial and plant derived organic matter (such as polysaccharides and humic acids) with soil clay particles. With the time and action of physical forces such as drying and rewetting, and the action of soil biota, build these organo-mineral complexes into progressively larger aggregates. These aggregates are essential for all soil biological processes because they help to develop the pore size in soil for water and air movement, which in turn controls microbial activity and soil organic matter.

• 1) For bacteria, fungi and other organisms decaying organic matter acts as a food material.
• 2) Presence of organic matter dissolves many insoluble soil minerals and make them available to plants
• 3) It plays an important role in the nutrient supplying power of soil as it has got high cation exchange capacity (CEC)
• 4) It increases the water holding capacity of the soil, particularly in sandy soils.
• 5) It improves aeration and infiltration in heavy soils.
• 6) It reduces loss of soil by water and wind erosion
• 7) It regulate soil temperature
• 8) It serves as an important source of certain plant of food element (N, P, S etc).
• 9) The buffering nature of the organic matter is considered to be advantageous in the residue management of pesticides, herbicide and other heavy metals.
• 10) Help in the suppression of diseases and pests by enhancing soil microbial biodiversity and activity.

Summary

Soil is a critical component of the earth’s biosphere, allowing us to produce food and fiber and to maintain environmental quality. The biological components of soil, perhaps more so than the physical and chemical components, can be either threatened or enhanced, depending upon how we use this resource. Biological indicators that may relate to changes in soil function include, soil microbial biomass carbon and nitrogen pools, potentially mineralizable carbon and nitrogen, the presence and activity of certain soil faunal groups, soil enzyme activity, presence of fecal-borne bacteria, and microbial diversity.

Biological properties of soil have major contribution to soil aggregation, structure as well as porosity, SOM decomposition and mineralization. Organic matter decomposition by soil organisms is vital for the functioning of an ecosystem because of its significant role in providing ecosystem services for plant growth and primary productivity. Organism activity is controlled by various soil conditions and may change by altering management practices. Since many soil properties are interrelated with one another, it is difficult to differentiate where one type of property dominates the behavior of the soil. Thus understanding and recognizing soil properties and their interrelation with one another is important for making sound decisions regarding soil use and management. Soil biota is mainly related to the mineralization of organic matter and retention of nutrients within ecosystem. It’s important to understand soil biological properties for soil management point of view but also for prevention and control of crop pests and diseases. To summarize the whole process in one line then these organisms if simply managed for their survival will work for the farmers benefit. Many advances have been made in the description of biological processes mediated by soil organisms, yet much remains to be able to fully understand the complexity of interactions between soil organisms and with changes in environmental conditions. The significant role of soil organisms in ecosystem productivity, environmental quality, and human and animal health needs to be valued.

Reference:

• Sylvia, D. M., J. J. Fuhrmann, P. G. Hartel, D. A. Zuberer. 1998. Principles and applications of soil microbiology. Prentice Hall. Upper Saddle River, New Jersey. 550 p.
• Paul E.A., Clark F.E. (1996). Soil Microbiology and Biochemistry. 340 pp. San Diego, CA, USA: Academic Press [Textbook devoted to the classical study of soil microbiology and its role in the environment].
• Paul E.A., Clark F.E. (1996). Soil Microbiology and Biochemistry. 340 pp. San Diego, CA, USA: Academic Press [Textbook devoted to the classical study of soil microbiology and its role in the environment].
• Paul E.A., Clark F.E. (1996). Soil Microbiology and Biochemistry. 340 pp. San Diego, CA, USA: Academic Press [Textbook devoted to the classical study of soil microbiology and its role in the environment]
• Tate, R.L. 1987. Soil organic matter: biological and ecological effects. New York, USA, John Wiley & Sons. 291 pp.
• Theng, B.K.G. 1987. Clay-humic interactions and soil aggregate stability. In P. Rengasamy, ed. Soil structure and aggregate stability, pp. 32-73. Proc. Institute of Irrigation and Salinity Research. Tatura, Australia.
• Tugel AJ, Lewandowski AM. 1999. Soil Biology Primer. Ames, IA.: NRCS Soil Quality Institute.

Web links:

• Soil Ecology Society, http://www.wcsu.ctstateu.edu/ses/conference.html
• Soil Science Society of America, http://www.soils.org with link to Division S-3 Soil Biology and Biochemistry
• Digital Learning Center for Microbial Ecology, http://commtechlab.msu.edu/sites/dlc-me