Factors affecting crop production have been broadly grouped into two categories: 1. Genetic/Internal Factors 2. Environmental or External Factors.
1. Genetic/Internal Factors:
Importance of genetics can be easily known with the development and introduction of new varieties & hybrids of cultivated crops and consequently upon there has been a big jump in the production & productivity of crops. With the increasing trend in the cultivation of new crop varieties and/or hybrids, the crop yields have increased many-folds.
These new crop varieties/hybrids are developed by adopting the principles of genetics and several techniques of plant breeding. Distinct characteristics are present in the certain variety of a crop. Plant morphology, response to pest & diseases, reaction to nutrition & environmental factors and yield of a crop variety are exhibited according to characteristics present in it.
Variety & Plant Nutrient Needs:
It is obvious that, the high crop yields produced with modem hybrids, varieties, lines will require more plant nutrients than was necessary for lower yields of past. Under low-fertility conditions a new high-yielding variety cannot develop its full yield potential. In fertile soils the same new variety will deplete the soil more rapidly, and eventually yields will decline if supplemental nutrients are not provided.
One of the first steps in a successful crop farming enterprise is the selection of hybrids or varieties that are genetically capable of producing high crop yields and of utilizing the fullest extent the supply of plant nutrients that will be made available to them.
Greater vegetative growth results from the use of these newer materials under high fertility condition and insects, diseases may be encouraged. If such pests are not controlled, crop may not perform satisfactorily.
It is necessary in poor soils that, adequate nutrients should be provided. Thus, recommendations for varieties or hybrids generally do not to be made on the basis of fertility level of the soil but rather on their ability to withstand insects, diseases or unfavourable conditions of moisture or temperature. It should be noted however that, one strategy for managing land area infertile because of low iron availability or an excess of chemical substances such as aluminum or soluble salts is to select a cultivar or species capable of tolerating the specific condition.
The genetic constitution of a given plant species limits the extent to which that plant may develop. No environmental conditions, no matter how favourable, can further extend these limits. It is imperative that, there be forward-moving plant-breeding programme that will produce new varieties or hybrids capable of achieving maximum yields under specific conditions.
There are promising indications that, it will be possible to breed widely divergent species and create new crop varieties by using innovative methods which go beyond those of conventional plant breeding. Some of the promising techniques for genetic manipulation or molecular genetics include in vitro techniques for asexual approaches, broad crosses between crop species, single-cell culture, anther & pollen culture and somatic hybridization. Also, genetic fortification offers a great challenge and opportunity for improving the nutritive value of the world’s feed grains by increasing protein content and amino acid distribution.
2. Environmental/External Factors:
Environment is defined as the aggregate of all the external conditions and influences affecting the life and development of an organism. Among the environmental factors known to influence plant growth, the following are probably the most important. These are temperature, moisture supply, solar energy, composition of the atmosphere, soil structure & composition of soil air, soil reaction, biotic factors, supply of mineral nutrient elements and growth restricting substances etc.
Many environmental factors do not behave independently. An example is the inverse relation that exists between soil air & soil moisture or between the content of oxygen and carbon dioxide in the soil atmosphere. As the soil moisture increases, the soil air decreases, and as the carbon dioxide content of the soil air increases, the oxygen content decreases. Another example is the relation between the diffusion rate of oxygen in the soil and soil temperature.
I. Temperature:
Temperature is the measure of the intensity of heat. Physicists consider that, the temperature of our universe ranges from a low of – 273°C to a high of several million degrees near the center of the Sun. In terms of biological life as we know it, this is unbelievably wide range.
The limit of survival of those living organisms on this planet has generally been reported to be between – 35°C and + 75°C. The range of growth for most agricultural plants, however, is usually much narrower; perhaps between 15 and 40°C. At temperature much below or above these limits, growth decreases rapidly.
Optimum temperatures for plant growth are dynamic since they change with the species & varieties, duration of exposure, age of the plant, stage of development etc. Temperatures directly affect the plant functions of photosynthesis, respiration, cell-wall permeability, absorption of water & nutrients, transpiration, enzyme activities and protein coagulation.
This influence is reflected in the growth of the plant. A plant’s capacity for growth of new photosynthetic area can substantially influence total photosynthesis and plant productivity. Therefore, the initiation and expansion rate of new leaves and duration of the various phases of plant development contribute a lot to crop productivity.
Temperature affects various vital plant processes, whose details are given below in brief:
i. Photosynthesis:
The effect of temperature on photosynthesis is complex and different with plants of various species as well as the carbon dioxide content of the atmosphere, the intensity of light and the duration of light of a given intensity. The consensus among physiologist is that if light is limiting, temperature has little effect on photosynthetic rate. If, however, carbon dioxide is limiting and light intensity is not enough, photosynthesis is increased by an increase in temperature.
ii. Respiration:
Respiration is also affected by change in temperature. It takes place more slowly at low temperatures and increases as the temperature rises. At very high temperatures, the rate of respiration is initially great but is not maintained. After a few hours at elevated temperature, respiration rates for at least some plants drop off rather rapidly.
For many crop plants of the temperate zone, the temperature optimum for photosynthesis is lower than that for respiration. This has been suggested as one reason for the higher yields of starchy crops, such as maize & potatoes, in cool climates as compared to the yields of these crops in warmer regions. It is possible that, under conditions of prolonged temperatures above the optimum, a plant may literally suffer from starvation simply because respiration is taking place more rapidly than photosynthesis.
iii. Transpiration:
The loss of water vapour from the stomata of leaves is influenced by temperature. Generally, the transpiration rates are low at low temperatures and increase with increasing temperatures. Under conditions of excessive transpiration, water loss may exceed the water intake by the plant and wilting soon follows.
iv. Water Absorption:
Low soil temperature may adversely affect the growth of plants by its effect on the absorption of water. If the soil temperatures are low, yet excessive transpiration is taking place and the plant may be injured because of tissue dehydration. The moisture supply of the soil may also be influenced to some extent by temperature, for unusually warm weather produces more rapid evaporation of water from the soil surface.
v. Mineral Element Absorption:
Temperature also affects mineral element absorption. In a number of plant species, the absorption of solutes by roots is restricted at low soil temperatures. This may be caused by lower respiratory activity or by reduced cell membrane permeability, both of which could affect uptake itself as well as the rate and extent of root permeability in the soil. Nutrient availability & movement to roots are also influenced by temperature.
vi. Soil Microbial Activities:
Temperature exerts its influence on plant growth indirectly by its effect on the microbial population of the soil. The activity of nitro bacteria and most of the heterotrophic organisms increase with a rise in temperature. Soil pH may change with temperature, which may in turn affect the plant growth. It has been observed that, soil pH increases in winter and decreases in summer.
This is generally considered to be related to the activities of microorganisms, since microbial activity is accompanied by the release of carbon dioxide which combines with water to form carbonic and other acids. In soils that are slightly acidic, this small change in pH may influence the availability of such micronutrients elements as manganese, zinc or iron.
Numerous studies on the direct relationship between yield or dry matter production and temperature have been made. This relation varies for different crop species and varieties. The impact of soil temperature may also be modified by a number of soil conditions, including amount of clay, moisture content and drainage.
vi. Composition of Soil Air:
Temperature may also alter the composition of the soil air. This happens because of the effect on microbial activities due to increase or decrease in temperature. When the activity of the micro-population is more, there will be higher partial pressure of the carbon dioxide of the soil atmosphere as the oxygen content decreases.
Under conditions restricting the diffusion of gases in to and out of the soil, a decrease in the O2 pressure resulting from such activity might influence the rate of respiration of the plant roots, hence, their ability to absorb nutrients.
II. Moisture:
The growth of many plants is proportional to the amount of water present. The plant growth is restricted both at very low and very high levels of soil moisture. Water is required by plants for the formation 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.
Plant water stress results when the extractable water in the root zone is insufficient to meet the plant’s transpirational demands. If the latter is excessive, symptoms of moisture stress may appear even though the extractable water supply is present at a level otherwise considered adequate.
Variations in soil waiter deficits are mainly responsible for year-to-year fluctuation in crop yields. The various physiological processes in plants are affected differently by water stress. For example, leaf elongation is more sensitive to soil water deficit than the other processes.
Yield is not the only plant property affected by soil moisture. Protein content of grain is frequently influenced by the degree of available water. Soil moisture level also has a pronounced effect on the uptake of plant nutrients. Low levels of extractable water in the root zone retard nutrient availability by impairing each of the three processes involved in nutrient uptake. These processes are diffusion, mass flow and root interception & contact exchange. As a general rule, there is an increase in nutrient uptake when extractable water is high rather than low.
The tendency for plant nutrients to be taken up more readily as supplies of extractable moisture increase has a favourable effect on water use efficiency of plants. Water use efficiency is the amount of dry matter that can be produced from a given quantity of water. It is also described as the weight of dry matter derived from a hectare-centimeter of water.
Placement of fertilizer nutrients is an important consideration in cropping situations where upper portions of the root zone are subject to rapid and prolonged drying. Fertilizers placed at depths in the root zone where soil is moist will be more effective. In arid & semi-arid regions, where leaching is not a problem, improved distribution of fertilizer nutrients in the root zone can result from occasional heavy surface dressing.
Soil moisture level also influences plant growth indirectly by its effect on the behavior of soil microorganisms. At extremely low or extremely high moisture levels the activity of organisms responsible for the transformation of nutrients in to plant-available forms is inhibited, with the result that plants may not be adequately supplied with the essential plant food elements. Activity of plant pathogens in the soil can also be affected by soil moisture.
III. Solar Energy:
Solar energy is a significant factor in plant growth & development. The quality, intensity and duration of light are all important. Clear-day radiation is a useful indicator of the amount of solar energy available for physiological processes within plants. Most plants are generally able to make good growth at light intensities of less than full daylight.
Changes in light intensity caused by shading can exert considerable influence on crop growth. With high plant populations, light penetration to lower positions in the plant canopy may be inadequate for bottom leaves to carry on photosynthesis. Shading of crops can also occur when two different species are grown in a mixture, such as cereals & pulses.
Photoperiodism:
Even though light quality & intensity may be of limited significance from the standpoint of field grown crops, the duration of light period is important. The behavior of the plant in relation to day length is termed Photoperiodism. On the basis of their reaction to the photoperiod, plants have been classified as short-day, long-day and intermediate or day-neutral.
Short-day plants are those that will flower only when the photoperiod is short or shorter than some critical period of time. If the exposure to light is longer than this critical period, the plant will develop vegetatively without completing their reproductive cycle. Tobacco & coleus are examples of these categories of plants.
Long-day plants are those that will bloom only if the period of time during which they are exposed to light is long or longer than some critical period. If the plants are exposed to light for period shorter than this critical time, they develop only vegetatively. Wheat, berseem, Lucerne etc. are the members of this group.
Plants that flower and complete their reproductive cycle over a wide range of day lengths are classed as intermediate or day-neutral. Cotton, buckwheat, sunflower, pumpkin etc. are the examples of this category.
IV. Composition of Atmosphere:
Carbon is required for plant growth and except for water, is the most abundant material within plants and other living things. The major source of carbon for plant is carbon dioxide gas in the atmosphere. It is taken into their leaves and through photosynthetic activity is chemically bound in organic molecules.
Plants do not always obtain sufficient carbon dioxide since its concentration in the atmosphere is usually only about 0.03% by volume (300 ppm). Consequently, concentration of atmospheric carbon dioxide is a dominant factor as a rate determinant in photosynthesis. Many economic plants respond to elevated levels of carbon dioxide by increased growth & productivity.
Plants that fix carbon dioxide through the C3 enzyme route (ribulose 1,5- diphophate carboxylase) have a greater potential for response to increased atmospheric levels of carbon dioxide than do C4 plants which utilize this source of carbon via phosphoenolpyruvate carboxylase.
Carbon dioxide is continuously being returned to the atmosphere as a product of respiration of animals & plants. Microbial decomposition of organic residues is an important source of this gas. One of the beneficial effects of FYM and crop residues on crop yields may be the carbon dioxide that is released into the air trapped in and below dense plant canopies.
Although, the normal concentration of carbon dioxide in the atmosphere is approximately 0.03%, the concentration may range from one-half to several times this value. Within a thick plant canopy on a still day, the carbon dioxide concentration may become considerably low during day-light hours when there is high rate of photosynthesis. Similarly, in a dense forest, the content may drop considerably. Under green house conditions the carbon dioxide level may drop appreciably below the average concentration in outside the air.
For more than 60 years, it has been realized that the crop may grow more rapidly if they are supplied with supplementary carbon dioxide. About 0.1% (1000 ppm) is generally thought to be ideal. At lower concentrations a full response to enrichment is not obtained, and higher concentrations usually do not produce any further increase in yield. Use of supplementary carbon dioxide seems to have greatest potential in green houses.
It is more practical to maintain higher than normal carbon dioxide concentrations within the green houses than open field situations, where wind action usually results in natural mixing of air. However, in densely planted crops which restrict air movement, it may be possible to raise carbon dioxide concentration through microbial break down of fresh sources of organic matter.
V. Soil Structure & Composition of Soil Air:
The structure of soils, particularly those containing appreciable quantities of silt & clay, has a good influence on both the root & shoot growth of plants. Soil structure to a great extent determines the bulk density of a soil. As a rule, higher the bulk density, the more compact the soil, the more poorly defined the structure and smaller the amount of pore space. This is quite frequently reflected in restricted plant growth. Pore space is occupied with air & water, the amount is being inversely related to the amount of water.
High bulk densities inhibit the emergence of seedling. High bulk densities offer increased mechanical resistance to root penetration. They almost certainly influence the rate of diffusion of oxygen in to the soil pores and root respiration is directly related to continuing and adequate supply of this gas.
Under field conditions oxygen diffusion in to the soil is determined largely by the moisture level of the soil if bulk density is not a limiting factor. On well drained soils with good structure, oxygen content is not likely to retard plant growth except for possible periodic flooding, during which it may become a serious consideration.
The oxygen supply at the root absorbing surface is critical. Hence, not only 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.
Agricultural plants differ widely in their sensitivity to soil oxygen supply. Paddy is grown under conditions of complete soil submergence. Tobacco is so sensitive to poor soil aeration that, flooding a field for only a few hours causes serious damage or complete loss of the crop.
Some pasture species are more tolerant to poorly aerated conditions than others; hence, species adapted to the conditions peculiar to the area should be selected. Good soil structure & aeration are imperative for maximum yields of most agricultural crops.
VI. Soil Reaction:
Soil reaction (soil acidity, pH) may affect plant development by its influence on the availability of certain elements required for growth. Examples are the increased sorption and precipitation of phosphates in acid soils high in iron & aluminum and of manganese in high organic matter soil with high pH values.
A decline in the availability of molybdenum results from a decrease in soil pH. Acid mineral soils are frequently high in soluble aluminum and manganese; and excessive amount of these elements are toxic to plants. The solubility of Fe3+ is low in well-aerated acid soils.
When ammonical nitrogen fertilizers are left on the surface of the soil with pH values greater than 7, ammonia may be lost by volatilization. If losses are large, the expected responses to the applied nitrogen will not be obtained. High soil pH values of about 7.5 to 8.0 and above will favour the conversion of water-soluble fertilizer phosphorus in to less soluble forms of lower availability to crops.
Certain soil borne diseases are influenced by soil pH. Potato scab, pox of sweet potatoes and black rot of tobacco are favoured by neutral to alkaline conditions. These diseases can be almost completely controlled by lowering the soil pH to 5.5 or less.
VII. Biotic Factors:
The many biotic factors that can limit plant growth present a constant hazard to farming operations and pose a potential threat of reduced crop yields. Heavy fertilization may encourage greater vegetative growth and better environmental conditions for certain disease organisms. The imbalance of nutrients available to plants may also be a reason for increased incidence of disease.
Certain pests may impose an added fertilizer requirement. Viruses & nematodes for example, attack the roots of certain crops and reduce absorption. It is then necessary to supply a greater concentration of nutrient elements in the soil to provide reasonable growth. These pests attack many species of crop plants and cause serious reduction in yield. Fortunately, they can be controlled by practices such as the production of virus-free seed, crop rotation and by chemical treatment of the soil.
Closely allied with disease is the problem of insects. Any infestation may seriously limit plant growth. If insects are not controlled, they may cause complete crop failures. Heavier fertilization may encourage certain insects, such as the cotton weevil, by greater vegetative growth. Definite advances have been made in breeding insect-resistant strains of certain crops, and great strides have been made in the development of insecticides over the last two decades.
Weeds are another serious deterrent to efficient crop production. They compete for moisture, nutrients, light etc. In addition to these competitive effects of weeds, crop growth may also be suppressed by biochemical interference or allelopathy. Weeds are known to produce and release harmful substances in to the root environment.
Chemical weed control is an established practice with most economic crops. Pre- emergence application of herbicides has the advantage of destroying weeds before they begin to impose competitive & alleopathic effects on crops. Serious losses of yields may occur in heavily weed infested fields.
VIII. Nutrient Elements:
About 5 to 10% of the dry weight of plants is composed of the nutrient elements; nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, copper, iron, manganese, molybdenum and zinc. Soil is the prime source of these essential nutrients as well as other elements that are beneficial for plant growth. If these nutrients are available in adequate quantities in soil, there shall be good crop growth and hence, good yields.
IX. Growth Restricting Substances:
Normal development of plants can be restricted or stopped completely by toxic substances. Almost all soil elements, even those essential and beneficial for plant growth, will become toxic to plants when they are present in higher concentrations in the root zone.
One of the most widespread toxicity problems involves excessive levels of soluble aluminum. Other elements that are potentially toxic are nickel, lead, mercury, cadmium, chromium, manganese, copper, zinc, selenium, arsenic, molybdenum, chloride, boron and fluorine. Other elements that pose potential toxicity problems include strontium, lithium, beryllium, vanadium, bismuth, iodine and tin.
Fortunately, toxicity problems associated with these various elements are few in most agricultural soils. They are most likely to occur in situations involving disposal of waste materials, such as those from mines and metallurgical operations, sewage system, animal & poultry enterprises, food processing industries, pulp and paper mills, garbage collection and others.
Organic compounds including phenol, cresols, hydrocarbons, substituted ureas and chlorinated hydrocarbon insecticides can be toxic if they are present in high concentrations. Such chemicals are not usually a problem at low concentrations since soil microorganisms can acquire the ability to decompose them.
X. Toxic Atmospheric Substances:
The quality of the atmosphere surrounding above ground parts of plants may under certain conditions influence growth. Certain gases, such as sulfur dioxide (SO2), carbon monoxide (CO) and hydrofluoric acid (HF), when released in to the air in sufficient quantities, are toxic to plants, although, the incidence of such cases are very rare.
Strong acids such as sulfuric, nitric and hydrochloric have lowered the pH of the rain and snow falling in some of the areas. Acid rain is often due to relatively high concentrations of sulfur dioxide and sulfates. Some of the effects that acid rain can have on plants and soil include increased leaching of inorganic nutrients and organic substances from foliage, accelerated cuticular erosion of leaves; leaf damage when pH values fall below 3.5; altered response to associated pathogens, symbiants and saprophytes; lowered germination, reduced availability of soil nitrogen; decreased respiration and increased leaching of nutrient ions from soils.
Injury to vegetation by fluorine released during the manufacture of metallic aluminum and the production of phosphatic fertilizers has been reported. Damage to crops, however, may not be as important as the toxicity to grazing livestock.