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Chapter: 11 th 12th std standard Bio Botany plant tree Biology Higher secondary school College Notes

Organisms : Environment : Edaphic Factors

They include the factors related to the soil. e.g. soil composition, organic matter, soil water, soil air, soil organisms etc.

Organisms  : Environmental factors


All living organisms, including human beings live in some sort of abiotic component of environment, that contains matter and energy. Various environmental factors can be divided into following two groups:


1. Climatic factors 2. Edaphic factors 3. Biotic Factors. The former two factors are abiotic components that contain matter and energy.

I. Climatic Factors


These are related to the aerial environment of the organisms e.g. light, precipitation, temperature, atmospheric humidity, wind, etc.

II. Edaphic Factors


They include the factors related to the soil. e.g. soil composition, organic matter, soil water, soil air, soil organisms etc.

III Edaphic Factors

i) Soil


Soil is the upper weathered and humus (organic matter) containing layer of the earth, which sustains plant life and contains numerous living organisms along with their dead remains. Soil provides water, mineral salts and anchorage to plants. The characteristics of soil such as its constitution, origin, temperature range, water holding capacity, aeration, minerals, etc. determine flora and fauna of a particular place.


A productive, well aggregated soil is composed of mineral matter (derived from parent rocks as a result of weathering), organic matter, water and air.

ii) Mineral Matter


The physical attributes of the soil are due to the size of the soil particles. The different particles which are present in the soil vary in their size and depending on this as the soils have been classified into sandy soils (sand with poor representation of silt and clay), loam soils (fine sand with well representation of silt and clay), silt soils (more silt than sand and clay) and clay soils (soils with high percentage of clay).


Sandy soils are porous and hence well aerated but they have very little water holding capacity and are chemically inert. Clay soils have a greater capacity of retaining water and are rich in nutritive salts. They are, however badly aerated. The loam soils are ideally suited for plant growth because they possess appreciable porosity or aeration, sufficient nutritive salts and good water retaining capacity.

iii) Organic Matter


The organic matter (humus) is highly important for all types of soils because it increases both aeration and hydration. It maintains the structure of the soil and also provides inorganic salts and some growth promoting substances to the soil.


iv)  Soil Water


Soil water is of paramount importance in the physiology of plants. It occurs in various forms, such as gravitational, capillary, hygroscopic and combined water. Rain is the principal source of water for the soil. Water which flows down due to the force of gravity is known as gravitational water. The gravitational water is not available to the plants. However, it is a big soil water reservoir and is trapped out through tube wells.


A certain amount of rain water is retained within the intercellular spaces of the soil particles in the form of a capillary network. It is called capillary water and is used by the plants. Some water molecules form a thin sheet of water around soil particles. It is calledhygroscopic water (water of imbibition). The hygroscopic water is also not absorbed by the plants. The water, which is bound up in chemicals is called combined water or crystalline water. (e.g. MgSo4.7H2O). It is not available to plants.


The total water present in the soil is called as field capacity. Addition of water beyond field capacity causes water logging. It excludes soil air and thus inhibits plant growth. The soils that have poor water holding capacity, cannot afford luxuriant vegetation. In such soils, the plants generally show wilting of their leaves.

v) Soil Air


It is essential for the growth of root and micro-organisms. A badly aerated or water-logged soil will have more of carbon dioxide and lesser amount of oxygen.


vi) pH (Hydrogen ion concentration).


Most organisms thrive in an optimal pH range, pH of soil and water has a strong influence on the distribution of organisms. Some plants and aquatic animals require acidic conditions, others need neutral or alkaline conditions.


vii)  Mineral elements.


A number of minerals are essential for normal growth of organisms. Their availability and concentration control the distribution of microbes, plants, and animals. Deficiency or absence of anyone, results in abnormal growth. Excess of mineral is equally harmfull. Plants growing in nitrogen deficient soils have developed special adaptations for obtaining it. For example, leguminous plants harbour nitrogen fixing bacteria in root nodules and the insectivorous plants have devices to trap insects and absorb nitrogen from their bodies. The salts of calcium, magnesium and phosphorus are most important for aquatic forms. Salinity of soil and water greatly affects the distribution of organism..

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