eChapters

Under natural habitats microorganisms usually grow in mixed populations containing several species. This is really a problem for microbiologists because a single type of microorganism can not be studied adequately in a mixed culture. In laboratories these populations can be separated into pure cultures. The pure cultures contain only one type of organism and are thus suitable for studying the cultural, morphological and biochemical characters of an individual species. In the present days there are several ways of developing pure cultures in laboratories. Some of which are as follows:

Microorganisms, being ubiquitous, are found in soil, air, water, food, sewage, on body surfaces as well as on degrading and decomposing substances. Their survival and continued growth mostly depend on an adequate supply of nutrients and favourable growth environments. The culture media (nutrients) consisting of chemicals support the growth of microorganisms in a given condition. They are devised in such a way that the organism should get all the nutritional requirements. These are usually prepared in laboratory by weighing and dispensing the individual ingredients. Generally, the common media contain both inorganic and organic nutrients; however, for the cultivation of large number of microorganisms, specialized media are required.

Basically the culture media are of three types: (1) Natural, (2) Semi synthetic and (3) Synthetic.

Microbiology is a branch of science dealing with the study of organisms which are too small to be seen clearly by unaided eye. Roughly the organisms with a diameter of 0.1mm or less, which can not be seen by the human eyes unaided, come under the domain of microorganisms.

Thus, the laboratory work dealing with the microorganisms needs special care to avoid infection and infusion for maintaining good health. In fact all microorganisms should be treated as potential pathogens. Thus, for working with microorganisms aseptic techniques must be followed by students as well as by the laboratory assistants and teachers.

Fill in the Blanks

1.The highest amount of rain is received in India by _______ Monsoon

2.Country with largest irrigated area in the world is _______

3.The major source of irrigation water in Andhra Pradesh is _______

4.The annual water budget of India is about _______ million ha meters

5.Ultimate irrigation potential in India is _______ m. ha.

6.The portion of the total rainfall that is retained in the soil directly for evapotranspiration is _______ M ha m

7.The first irrigation commission was constituted in the year _______

17.1 Rice

Rice culture at present dominates irrigated agriculture. About 64% of the irrigation water resources in Andhra Pradesh are used for cultivation of rice. An adequate water supply is one of the most important factors in rice production. In many parts of India including Andhra Pradesh rice plants suffer from either too much or too little water because of irregular rainfall and land topography.

Percolation losses in rice fields

Percolation losses are a function of the local soil and topographic conditions. Therefore, at any time the amount of rainfall or irrigation water entering the soil becomes greater than its water holding capacity, losses by downward movement of free water (vertical percolation) will occur. Percolation is often defined as the movement of water through saturated soils due to gravity, hydrostatic pressure or both.

Thus where the soil is heavy and the water table is close to the soil surface, percolation losses are low, about 1 – 2 mm/day. On the other hand, where the soil is light and the water table is deep, percolation losses may be high, about 8 – 15 mm/day, or more. About 50 to 60% of applied water to rice is lost by deep percolation. The percolation losses can be reduced by adopting following agronomic practices:

16.1 Water logging

For optimum growth and yield of field crops, proper balance between soil air and soil moisture is quite essential. Except rice many of the cultivated plants cannot withstand excess water in the soil. The ideal condition is that moisture and air occupy the pore spaces in equal proportions. When the soil contains excess water than that can be accommodated in the pore spaces it is said the field is water logged.

Causes of water logging

1. Excessive use of water when the water is available in abundance or cheaply due to the belief that more water contributes better yield.

2. Improper selection of irrigation methods

3. Percolation and seepage from lands canals and reservoir located at nearby elevated places

4. Improper lay out and lack of outlets

5. Presence of impervious layer with profile impeding percolation

6. Upward rise of water from shallow ground water table or aquifer.

When rocks and minerals under go weathering process large quantities of soluble salts are formed. In humid regions these salts are washed down to the ground water and to the sea. But in arid and semi arid regions they accumulate in the soil. Excessive irrigation and poor water management are the two chief causes of water logging and salt accumulation. An accumulation of salts in soil leads to unfavourable soil water-air relationship and effect the crop production.

The following are the main causes which leads to development of salty soils (salinity or alkalinity)

Whatever may be the source of irrigation water viz., river, canal, tank, open well or tube well, some soluble salts are always dissolved in it. The main soluble constituent in water are Ca, Mg, Na and K as cations and chloride, sulphate bicarbonate and carbonate as anions. However ions of other elements such as lithium, silicon, bromine, iodine, copper, cobalt, fluorine, boron, titanium, vanadium, barium, arsenic, antimony, beryllium, chromium, manganese, lead, selenium phosphate and organic matter are also present. Among the soluble constituents, calcium, sodium, sulphate, bicarbonate and boron are important in determining the quality of irrigation water and its suitability for irrigation purposes. However other factors such as soil texture, permeability, drainage, type of crop etc., are equally important in determining the suitability of irrigation water.

The following are the most common problems that result from using poor quality water.

An efficient irrigation system implies effective transfer of water from the source to the filled with minimum possible loss. The objective of the efficiency concept is to identify the nature of water loss and to decide the type of improvements in the system.

Evaluation of performance in terms of efficiency is prerequisite for proper use of irrigation water.

Irrigation Methods

Surface

Sub-surface

Pressurized irrigation

12.1 Surface Irrigation

Surface irrigation method refers to the manner or plan of water application by gravity flow to the cultivated land wetting either the entire field (uncontrolled flooding) or part of the field (furrows, basins, border strips). Most irrigated areas have characteristic land features and differ from those in other areas. Hence, for efficient application of water it is important to select such method of irrigation, which fits one’s own land.

The adoption of a certain method need not be necessarily based on convention or that followed in the adjoining farm. The factors, which determine the suitable method of irrigation are local conditions (soil type - its permeability & water storage capacity; land topography, climate, water availability & water quality), crop type, type of technology, previous experience with irrigation, required labour inputs etc. Good yield of crops can be obtained from irrigated land only if the water is applied judiciously to meet the needs of the plant, but not to cause waste and damage. Irrigation water is applied to cultivated land by the following surface methods of irrigation:

Allocation of the water receipt including anticipated within the crop period and its detailed account of expenditure for efficient and profitable farm management is called as water budgeting.

Water budgeting may be for an irrigation system planned by irrigation engineers; may be for a canal or for an area (block) or may be for a farm according to the need and plan by responsible persons who plan the irrigation efficiency.

10.1 Evapotranspiration (ET = Evaporation + Transpiration)

Evaporation is a diffusive process by which water from natural surfaces, such as free water surface, bare soil, from live or dead vegetation foliage (intercepted water, dewfall, guttation etc) is lost in the form of vapour to the atmosphere. It is one of the basic components of hydrologic cycle.

Likewise transpiration is a process by which water is lost in the form of vapour through plant surfaces, particularly leaves. In this process water is essentially absorbed by the plant roots due to water potential gradients and it moves upward through the stem and is ultimately lost into the atmosphere through numerous minute stomata in the plant leaves. It is basically an evaporation process. However, unlike evaporation from a water or soil surface, plant structure and stomatal behaviour operating in conjunction with the physical principles governing evaporation modify transpiration.

Thus, evapotranspiration is a combined loss of water from the soil (evaporation) and plant (transpiration) surfaces to the atmosphere through vaporization of liquid water, and is expressed in depth per unit time (for example mm/ day). Quantification of evapotranspiration is required in the context of many issues:

Crop water requirement is the water required by the plants for its survival, growth, development and to produce economic parts. This requirement is applied either naturally by precipitation or artificially by irrigation. Hence the crop water requirement includes all losses like:

• Transpiration loss through leaves (T)

• Evaporation loss through soil surface in cropped area (E)

• Amount of weather used by plants (WP) for its metabolic activities which is estimated as less than 1% of the total water absorption. These three components cannot be separated so easily. Hence the ET loss is taken as crop water use or crop water consumptive use.

• Other application losses are conveyance loss, percolation loss, runoff loss, etc., (WL).

• The water required for special purposes (WSP) like puddling operation, ploughing operation, land preparation, leaching, requirement, for the purpose of weeding, for dissolving fertilizer and chemical, etc.

• Hence the water requirement is symbolically represented as:

8.1 Moisture extraction pattern

The moisture extraction pattern reveals about how the moisture is extracted and how much quantity is extracted at different depth level in the root zone. The moisture extraction patter shows the relative amount of moisture extracted from different depths within the crop root zone. The moisture extraction pattern of plant growing in a uniform soil without a restrictive layer and with adequate supply of available soil moisture throughout the zone is shown in Figure. 8.1.

The measurement of soil moisture is needed to determine when to irrigate and the amount of water needed when irrigating, to evaluate evapotranspiration, and to monitor soil matric potential. The soil moisture is measured in two ways- direct and indirect methods as follows:

The water contents expressed under certain standard conditions are commonly referred to as soil moisture constants. They are used as reference points for practical irrigation water management. The usage of these constants together with the energy status of soil water gives useful knowledge. These constants are briefly explained below:

6.1 Saturation Capacity

Saturation capacity refers to the condition of soil at which all the macro and micro pores are filled with water and the soil is at maximum water retention capacity”. The matric suction at this condition is essentially zero as the water is in equilibrium with free water. Excess water above saturation capacity of soil is lost from root zone as gravitational water.

5.1 Kinds of Soil Water

The following re the three main classes of soil water:

(i)  Hygroscopic water. Water held tightly to the surface of soil particles by adsorption forces.

(ii) Capillary water. Water held by forces of surface tension and continuous films around soil particles and in the capillary spaces.

(iii) Gravitational water. Water that moves freely in response to gravity and drains out of the soil.

4.1 Soil Moisture Tension

Soil moisture tension is a measure of the tenacity with which water is retained in the soil and shows the force per unit area that must be exerted to remove water from soil. The tenacity is measured in terms of the potential energy of water in the soil measured, usually with respect to free water. It is usually expressed in atmospheres, the average air pressure at sea level. Other pressure units like cm of water or cm or mm of mercury are also often used (1 atmosphere = 1036 cm of water or 76.39 cm of mercury). It is also sometimes expressed in bars (1 bar = 106 dynes / cm² = 1023 cm of water column. 1 millibar = 1/1000- bar).

Soil moisture tension is brought about at the smaller dimensions by surface tension (capillarity) and at the higher dimensions by adhesion. Buckingham (1907) introduced the concept of ‘capillary potential’ to define the energy with which water is held by soil. This term, however, does not apply over the entire moisture range. In a wet soil, as long as there is a continuous column of water, it might be called ‘hydrostatic potential’, in the intermediate range the term ‘capillary potential’ is appropriate. In the dry range the term ‘hygroscopic potential’ would be suitable. However, the term ‘soil moisture potential’, ‘soil moisture suction’ and ‘soil moisture tension’ are often used synonymously to cover the entire range of moisture (Khonke, 1968).

The soil is a heterogeneous, polyphasic viz., solid, liquid and gaseous, particulate, disperse and porous system (Fig. 3.1). The solid phase constitutes the soil matrix, the liquid phase consists of soil water, which always contains dissolved substances so that it should properly be called as soil solution and the gaseous phase is the soil atmosphere.

Plants and any from of living organisms cannot live without water, since water is the most important constituent about 80 to 90% of most plant cell.

2.1 Role of water in crop and crop production

(A) Physiological importance

•The plant system itself contains about 90% of water

•Amount of water varies in different parts of plant as follows

•Apical portion of root and shoot >90%

•Stem, leaves and fruits - 70 - 90%

•Woods - 50 - 60%

•Matured parts - 15 - 20%

•Freshly harvested grains - 15 - 20%