This article throws light upon the nine main environmental factors that influence plant growth. The factors are: 1. Temperature 2. Moisture Supply 3. Radiant Energy 4. Composition of the Atmosphere 5. Soil Structure and Composition of Soil Air 6. Soil Reaction 7. Biotic Factors 8. Supply of Nutrient Elements 9. Absence of Growth Inhibiting Substances.
Environmental Factor # 1. Temperature:
Temperature is a measure of the intensity of heat the limit of survival of living organisms has generally been reported to be between -35Â°C and 75Â°C. The range of growth for most agricultural crops is between 15Â°C and 40Â°C. At temperatures much below or above these limits growth decreases rapidly.
Optimum temperatures for plant growth are dynamic since they change with the species and varieties, duration of exposure, age of the plant, stage of development etc. The important plant metabolic processes like photosynthesis, respiration, evapotranspiration etc.. are influenced by the temperature.
Besides these, temperature influences the absorption of water and nutrients and also influences the microbial activity which ultimately affects the plant growth.
Environmental Factor # 2. Moisture Supply:
The growth of various plants is proportional to the amounts of water present, for growth is restricted both at very low and very high soil moisture regimes. Water is essential to plants for the manufacture of carbohydrates, to maintain hydration of protoplasm, and as a vehicle for the translocation of foods and mineral elements.
Internal moisture stress causes reduction both in cell division and cell elongation, hence in growth. In addition to these, the various physiological processes in plants are also affected by water stress.
Soil moisture regime also has a pronounced effect on the uptake of plant nutrients. Low moisture regimes in the root zone reduce nutrient availability by impairing each of the three major processes (diffusion, mass flow and root interception and contact exchange) involved in nutrient uptake.
As a general rule, there is an increase in nutrient uptake when soil moisture regime is high. The plant growth is also affected by soil moisture regimes through an indirect influence on soil micro-organisms and various soil pathogens causing various diseases.
Environmental Factor # 3. Radiant Energy:
Radiant energy is a significant factor in plant growth and development. It has three components namely, quality, intensity and duration of light. All these components of radiant energy significantly influence various plant physiological processes and hence the growth of plants.
However, the intensity of light is relatively more important for good growth of plants than that of full daylight. Changes in light intensity caused by shading can exert considerable influence on crop growth. The intensity of light has marked effects on the uptake of phosphate and potassium. It was also observed that oxygen uptake by the roots increased with increasing light intensity.
Even though the quality and intensity of light may be of limited significance from the stand point of most field crops, the duration of the light period is important. The behaviour of the plant in relation to day length is termed photoperiodism.
On the basis of their reaction to the photoperiod, plants are classified as short day (those plants that will flower only when the photoperiod is as short or shorter than some critical period of time e.g. tobacco), long day (those plants that will bloom only if the period of time during which they are exposed to light is as long or longer than some critical period, e.g. grains) indeterminate (those plants that flower and complete their reproductive cycle over a wide range of day lengths e.g. cotton).
Environmental Factor # 4. Composition of the Atmosphere:
Carbon is required for plant growth and is the most abundant material within plants and other living organisms. The main source of carbon for plant is CO2 gas in the atmosphere. It is taken into their leaves and through photosynthetic activity is chemically bound in organic molecules.
The concentration of CO2 in the atmosphere is usually only about 0.03 per cent by volume (300 ppm). Carbon dioxide is continually being returned to the atmosphere as a product of respiration of animals and plants. Microbial decomposition of organic residues is an important source of CO2 gas. It is reported that as CO2 levels of the atmosphere are increased, photosynthesis becomes more sensitive to temperature.
Environmental Factor # 5. Soil Structure and Composition of Soil Air:
Soil structure has a profound influence on plant growth especially on root and top growth of plants. The structure of soil also determines the bulk density of a soil. As a rule, the higher the bulk density, the more compact the soil, the more poorly defined soil structure and the smaller the amount of pore space and therefore, the plant growth is restricted.
High bulk densities inhibit the emergence of seedlings and also offer increased mechanical resistance to root penetration. Bulk density also influences the rate of diffusion of oxygen into the soil pore spaces and root respiration which in turn affect the plant growth to a great extent. The oxygen supply at the root absorbing surface is critical.
Hence not only is the gross oxygen level of the soil air important, but also the rate at which oxygen diffuses through the soil to maintain an adequate partial pressure at the root surface. So it may be concluded that good soil structure and aeration are imperative for maximum yields of most agricultural crops (excepting rice) and the limiting effect is an inadequate root oxygen supply which can influence the plant growth.
Environmental Factor # 6. Soil Reaction:
Soil reaction (acidity or alkalinity) affect plant growth and nutrition through the influence of various physico-chemical, chemical and biological properties of soil. The availability of phosphorus is low in acid soils high in Fe and Al. On the other hand the availability of Mn is also low in soils having high pH values and high organic matter content.
A decline in the availability of Mo results from a decrease in soil pH. Very frequently it is observed that the concentration of Mn and Al is so high in acid soils that cause toxicity to plants. High soil pH (pH > 8.0) will favour the conversion of water soluble phosphorus into less soluble forms resulting lower availability to plants.
Besides nutritional aspects, some soil borne diseases are influenced by soil reaction. Potato scab, root of tobacco etc. are favoured by neutral to alkaline soil conditions and these diseases can be controlled by lowering the soil pH (acidic soil reaction).
Environmental Factor # 7. Biotic Factors:
Various biotic factors affect plant growth and nutrition and also pose a potential threat of reduced crop yields. Heavier fertilization may encourage greater vegetative growth and better environmental conditions for certain disease organisms. The nutrient imbalances in soils may also be a reason for increased incidence of disease.
Sometimes certain pests may impose an added fertilizer requirement. Viruses and nematodes attack the roots of certain crops and reduce nutrient and water absorption and thereby reduce the growth of plants.
Weeds are other serious factors that affect plant growth to a great extent through the competition for moisture, nutrients, sunlight and other biochemical interference or allelopathy. Weeds are known to produce and release harmful substances into the root environment.
Environmental Factor # 8. Supply of Nutrient Elements:
About 5-10 per cent of the dry weight of plants is composed of the nutrient elements nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, boron, copper, zinc, iron, manganese, molybdenum etc. Soil is the main source of these essential nutrient elements as well as other elements that are beneficial for plant growth.
Environmental Factor # 9. Absence of Growth Inhibiting Substances:
The growth and development of plants can be restricted or inhibited by toxic substances like higher concentrations of nutrient elements (Fe, Al and Mn) and certain organic acids (lactic acid, butyric acid, propionic acid etc.).
Besides these, waste materials from mines and metallurgical operations, sewage systems, pesticides, animal and poultry enterprises, garbage collections, paper mills etc. also produce toxic substances in soils which finally affect plant growth and nutrition.