The growth and general performance of any plant depend on an adequate supply of environmental factors including the sixteen elements currently known to be essential.
What is Growth?
Growth can be defined as the “progressive development of an organism”. It is expressed in terms of dry matter weight, length, height, and diameter. There are two major factors affecting plant growth; namely, the genetic factor and the environmental factors.
Factors Affecting Plant Growth and Development
Genetic Factor: concerns primarily the inherent capability of a given crop to give high yield and desirable characteristics.
Where such desirable characteristics are lacking, plant breeders could develop a given crop to give the desired character(s).
Where this is successful, the ability of the “hybrid” to exhibit its characters depends majorly on environmental factors.
Environmental Factors: can be defined as the aggregates of all external conditions and influences that affect the growth, development, and life of an organism.
Unless both the genetic and the environmental factors are brought under control, the desired yield or characters of an organism may not manifest itself.
The major environmental factors affecting the growth development and life of plants are as follows:
– Sunlight (radiant energy)
– Availability of oxygen/carbon dioxide
– Adequate supply of soil moisture
– Favorable temperature
– Favorable soil reaction (acidity and alkalinity)
– Absence of toxic substances
– Favorable root environment and
– Adequate supply of plant nutrients.
Among these factors, the one which is least optimum will determine the level of crop production. This is the principle of limiting factors which states that the level of crop production cannot be greater than that allowed by the most limiting of the essential plant growth factors.
Sunlight is very necessary for the supply of radiant energy needed for photosynthesis. Where there is inadequate sunlight the process of photosynthesis will be disturbed and thus the overall growth and yield of the crop will be affected.
It has been estimated that less than 2% of the radiant energy is used for photosynthesis while about 45% is used for evapotranspiration while the rest is re-radiated into space.
2. Availability of Oxygen and Carbon Dioxide
Roots have openings called lenticels that permit gas exchange. Oxygen diffuses into the root cells and is used for respiration, whereas carbon dioxide diffuses into the soil.
Respiration releases energy that the plant needs for the synthesis and translocation of organic compounds and the active accumulation of nutrient ions against a concentration gradient.
Oxygen is usually inadequate in water-logged soils. Differences exist between plants in their ability to tolerate low oxygen levels in standing water because they have morphological structures that permit internal diffusion of atmospheric oxygen down into the root tissue. Others like tomatoes may be wilted or killed by saturating the soil with water for just a day.
The wilting is believed to result from a decrease in permeability of the root cells to water, a result of a disturbance of metabolic processes due to an oxygen deficiency.
Aerobic microorganisms, bacteria, actinomycetes, and fungi utilize oxygen from the soil atmosphere and are primarily responsible for the conversion of nutrients in organic matter into soluble forms (mineralization) that plants can use. Under anaerobic conditions, many important processes will be hampered while the activities of important soil micro-organisms will be retarded greatly.
On the other hand, the carbon dioxide (CO2) concentration in the soil environment is higher than in the atmosphere because of the respiration of plants and microorganisms. CO2 is not limiting under normal conditions.
3. Adequate Supply of Soil Moisture
Water and oxygen are inversely related as regards utilization in soil. Roots need oxygen for their respiration process in order to obtain energy for the extraction of nutrients and water from the soil.
It has been estimated that about 500gm of water is needed to produce 1gm of dry plant material. About 5% of this water becomes an integral part of this plant. The remainder is lost through the stomata of leaves during the absorption of CO2.
Atmospheric conditions such as relative humidity and temperature determine how much water is available for plant use.
Since the growth of virtually all economic crop plants will be curtailed when a shortage of water occurs, even though it may be temporary and the plants are in danger of dying, the ability of the soil to hold water against the force of gravity becomes very important unless rainfall or irrigation is frequent.
It is equally important that optimum water be made available for plants as excess could cause a lot of damage to the plant system. As stated earlier, water and oxygen are inversely related.
A good soil, therefore, is that which allows 50% pore space (macro and micro) and 50% solid particles. The macro-spaces hold air while the micro-spaces hold water.
Good soil is that which has 25% micro-pores (H20) and 25% macro-pores (02). A high level of water leads to a low level of 02 which affects plant respiration and its related benefits while 02 signifies the low amount of water and its attendant consequences.
Water is needed in plants for photosynthesis, turgidity, evapotranspiration, nutrient uptake in an aqueous medium, trans-location of food, chemical reactions in plants as well as leaching of anions and cations from the soil-plant system.
Inadequate availability of water, therefore, leads to non-completion of vegetative and reproductive stages of crops. Moisture equally affects the activities of soil micro-organisms.
The absence of water leads to the development of resistant strains of micro-organisms or they may even enter a dormant stage. Most micro-organisms are less active at the wilting point for plants (i.e. less water for mobility).
Optimum moisture level for higher plants (moisture potential of between – 0.1 to 1 bar) is usually considered best for micro- organisms. Anaerobes, which comprise only a small portion of soil microbes, are hindered by free oxygen gas and grow best at saturation conditions.
4. Favorable Temperature
Temperature can be defined as the degree of coldness or hotness of an object. Usually, there is an optimum temperature or otherwise known as the “comfort zone” for most crops and micro-organisms.
The optimum temperature for plant growth ranges from 150C to 400C. Microbial activity in soil accelerates as the temperature rises to a maximum of 400C.
This is the Q10 theory. That is, as the temperature rises from 0 to 10 to 20 etc. the microbial activity increases twice up to a maximum by which their activities will be inhibited by high temperatures.
Soil temperature where it is far from optimum could be modified through mulching, shading, and appropriate tillage operations. Temperature is considered important because it affects the process of photosynthesis, respiration, cell-wall permeability, absorption of water and nutrients, transpiration, and enzyme activity.
5. Favorable Soil Reaction (Soil Acidity + Alkalinity)
The majority of soil micro-organisms that influence organic matter decomposition and nutrient availability (fixation) grow best at pH 6.5 – 7, which is the pH of microbial cytoplasm (the cell material).
Bacteria and actinomycetes are usually less tolerant of acid soil conditions than are fungi, and very few grow well at pH less than 5 except the sulfur-oxidizing bacteria (the Thiobacillus spp) which produce sulphuric acid.
Most crops do well at a pH of 6.5 – 7.0. Extreme acidity and alkalinity affect nutrient availability uptake by plants. At low pH, for instance, Mn, A ﺎ could be present in excess and thus become toxic for plant growth and development.
At a very high pH, there may be a preponderance of Ca++ and Na+ both of which affect nutrient availability and maintenance of suitable soil structure. High and low pH, therefore, is detrimental to plant growth and development.
6. Favorable Root Environment
The root environment or otherwise known as the Rhizosphere must be favorable for plant growth and development. A favorable root environment is that which has enough pore space in which roots can function.
Oxygen must be available for root respiration. There must be an absence of frangipanes and it should provide enough anchorage to hold to plants. In addition, there must be an absence of toxic substances which affects the normal growth of the plant.
7. Absence of Toxic Substances
Toxic substances that affect plant growth and development are many and vary as follows:
- Under reducing conditions and low pH, we have toxic levels of A13+, Mn++, and Fe2+, below pH of 5.2, there is a preponderance of A13+ and Mn2+.
- Plants differ in their tolerance to the availability of Mn2+ and A13+. Most often it affects plant growth adversely.
- The decomposition of organic matter yields CH4 gas instead of N, P, and S. It is important to note that under reducing conditions there is less availability of 02 mainly due to water-log conditions.
We have occasionally toxic levels of boron, selenium, and sodium as well as high levels of salts namely calcium sulphate (CaS04), Magnesium sulphate (MgS04), and potassium chloride (KCL).
Soils high in these elements and salts are usually characterized by high pH and high electrical conductivity that inhibit plant growth and development as well as the structural stability of soils. Unless these elements/salts are drastically controlled it affects crop productivity.
Copper (Cu) may be present at toxic levels owing to the application of fungicides such as the Bordeaux mixture.
Herbicides could be toxic because of their specificity to certain crops e.g. atrazine applied to maize may adversely affect soybean.
Soil pollution resulting from oil spillage, industrial wastes, and sewage sludge can also affect plant growth adversely.
Physical Condition of the Soil: The soil must be loose enough to allow free root penetration. If the soil is compacted, it will be difficult for the root to penetrate; as such, even if other conditions are optimal, crop productivity will be retarded.
High bulk density, frangipane (any hard layer), indurated or concretionary layer could prevent root penetration and thus root development.
8. Availability of Plant Nutrients
At least there are about 18 elements currently considered necessary for the growth of vascular plants.
Carbon, Hydrogen, and oxygen obtained from air and water combine in photosynthetic reactions to produce carbohydrates needed for plant growth and development.
These elements which make up to 93% of the dry matter are referred to as structural elements. Carbon constitutes about 45% followed by oxygen (43%) and Hydrogen (6%).
The remaining nutrient elements (other than CHO) are obtained largely from the soil. Of these, we have primary or major nutrient elements followed by secondary elements then the micro-nutrients otherwise known as trace elements.
Nitrogen, phosphorus, and potassium are the elements referred to as primary/major elements because they are required in large quantities by plants.
These elements (NPK) have to be supplied into the soil for plant growth because they are most deficient and because they are needed in relatively greater amounts.
In plant, Nitrogen constitutes about 1 – 6%, phosphorus (0.1-0.5%) and potassium (1-4%). Calcium, magnesium, and sulfur are referred to as secondary nutrients which have to be supplied but not as much as the primary elements.
In plant, Calcium ranges from 0.1 to 1%, Magnesium 0.1 – 0.5% and sulphur 0.1 – 0.5%. Then we have the micro-nutrients which are the nutrients required in considerably smaller quantities.
The micro-nutrients include Iron (Fe), Manganese (Mn), aluminum (A13+), Copper (Cu), Zinc (Zn), Boron (B), Molybdenum (Mo), and chlorine (Cl). Other elements present in plants but not essential and which could be substituted with other nutrient elements include Cobolt (Co), sodium (Na), and silicon (Si).
Most of the nutrients exist in mineral and organic matter and as such are insoluble and unavailable to plants. They only become available through mineral weathering and organic matter decomposition. The nutrients are absorbed from the soil into the plant through diffusion, ion exchange, and carrier hypothesis. They are either absorbed from the soil solution or from colloid surfaces as cations and anions. Cations are positively charged, and anions are negatively charged.
These nutrients are required by plants for their normal growth and development. Their deficiency in soil has to be supplied through fertilizer application.
Given that the genetic and environmental factors are favorable, the plant can exhibit its qualities which are usually manifested in luxuriant growth and high yield from the crops. Effort has to be made to ensure that both factors are maintained so as to achieve optimum crop production.