Edaphic factors, the abiotic factors related to soil, include the physical and chemical composition of the soil formed in a particular area. The study of soils is called Pedology.
Soil is the weathered superficial layer of the Earth in which plants can grow. It is a complex composite mass consisting of soil constituents, soil water, soil air and soil organisms, etc.
Soil originates from rocks and develops gradually at different rates, depending upon the ecological and climatic conditions. Soil formation is initiated by the weathering process. Biological weathering takes place when organisms like bacteria, fungi, lichens and plants help in the breakdown of rocks through the production of acids and certain chemical substances.
Based on soil formation (pedogenesis), the soils are divided into
1. Residual soils –These are soils formed by weathering and pedogenesis of the rock.
2. Transported soils – These are transported by various agencies.
The important edaphic factors which affect vegetation are as follows:
1. Soil moisture: Plants absorbs rain water and moisture directly from the air
2. Soil water: Soil water is more important than any other ecological factors affecting the distribution of plants. Rain is the main source of soil water. Capillary water held between pore spaces of soil particles and angles between them is the most important form of water available to the plants.
3. Soil reactions: Soil may be acidic or alkaline or neutral in their reaction. pH value of the soil solution determines the availability of plant nutrients. The best pH range of the soil for cultivation of crop plants is 5.5 to 6.8.
4. Soil nutrients: Soil fertility and productivity is the ability of soil to provide all essential plant nutrients such as minerals and organic nutrients in the form of ions.
5. Soil temperature: Soil temperature of an area plays an important role in determining the geographical distribution of plants. Low temperature reduces use of water and solute absorption by roots.
6. Soil atmosphere: The spaces left between soil particles are called pore spaces which contains oxygen and carbon-di-oxide.
7. Soil organisms: Many organisms existing in the soil like bacteria, fungi, algae, protozoans, nematodes, insects, earthworms, etc. are called soil organisms.
Soil is commonly stratified into horizons at different depth. These layers differ in their physical, chemical and biological properties.
This succession of super-imposed horizons is called soil profile. Types of soil particles Based on the relative proportion of soil particles, four types of soil are recognized.
Loamy soil is ideal soil for cultivation. It consists of 70% sand and 30% clay or silt or both. It ensures good retention and proper drainage of water. The porosity of soil provides adequate aeration and allows the penetration of roots.
Based on the water retention, aeration and mineral contents of soil, the distribution of vegetation is divided into following types.
1. Halophytes: Plants living in saline soils
2. Psammophytes: Plants living in sandy soils
3. Lithophytes: Plants living on rocky surface
4. Chasmophytes: Plants living in rocky crevices
5. Cryptophytes: Plants living below the soil surface
6. Cryophytes: Plants living on surface of ice
7. Oxylophytes: Plants living in acidic soil
8. Calciphytes: Plants living in calcium rich alkaline soil.
Holard –Total soil water content Chresard –Water available to plants Echard – Water not available to plants