Introduction to agricultural chemistry part b

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FINAL TERM LECTURES FOR SS-302 (INTRODUCTION TO AGRICULTURAL CHEMISTRY)

Session 11: Soil Water; Structure and Importance:

It is the water which is present in root zone area of plant and important for plant growth. It includes all

the water, which is supplied to plants either naturally or through irrigation. It is also known as soil

moisture, while ground water is the water present beneath the water table. There is no life without

water as it is the medium through which all the vital processes take place. The water in the oceans and

the rivers is an important means for the transportation between the cities and the countries. The

waterfalls are used in generating the electricity in many countries. Consumption of water is very high in

the entire world, where Asia is the largest continent in consuming the water for its agriculture. North

America is the largest continent in consuming the water for domestic and industrial uses.

Structure of Water: Water molecule is polar in nature due to uneven distribution of charge. The

hydrogen atom sides of water molecule are tended to act as electropositive, while oxygen atom side

acts as electronegative, although water molecule is neutral. The two hydrogen atoms are attached to

oxygen in a nearly V shaped arrangement with an angle of 104.5 between two hydrogen atoms. Due to

this, water molecule exerts an attraction for solid surfaces (called adhesion forces or adsorption) and to

one another (called cohesion forces) as well. Strong adhesion and cohesion forces make the water to

attach and hold with soil for its controlled movement and use in agriculture. The polar water molecules

are linked together by means of hydrogen bonding, which is responsible for some abnormal properties

of the water like surface tension etc.

Importance of Water: The availability of water in soil is essential for plant growth. It is also essential to

microorganisms that grow in soil and decompose organic matter. It is important in the weathering

process which involves the breakdown of rocks and mineral to form soil and release plant nutrients.

Water is the solvent that together with the dissolved nutrients make up the solution from which plants

absorb nutrients (mainly through the roots). Soil water can provide control over both soil air and soil

temperature (other two factors essential for plant growth). Water is generally held in the soil by micro

pores. The force of gravity causes water to move downward through the soil, particularly in the larger

pores. Soil water is important for four special reasons i.e., i) Presence of water is essential for life on our

planet for both fauna and flora. ii) It is universal solvent. iii) It is the medium from which all plant

nutrients are assimilated by plants iv) Water is necessary for weathering process and soil development.

Session 12: Water Quality and Management:

The parameters to determine the quality of irrigation water include EC, SAR and RSC. The criteria for

these three parameters are as under:

i. Electrical Conductivity: EC meter is used to determine the Electrical Conductivity of given

sample. If EC of irrigation water sample is <1150 µS/cm, water is fit for irrigation. If EC of

irrigation water sample is 1150-1450 µS/cm, water is marginal fit for irrigation and it can be

used by mixing it with good quality irrigation water or canal water. If EC of irrigation water

sample is >1450 µS/cm, water is unfit for irrigation and it may create the problem of salinity

in the soil.

ii. Sodium Adsorption Ratio (SAR): It can be calculated for given water sample by using the

equation. SAR = Na/(Ca+Mg/2)

1/2

. If SAR is <6, the water is fit for irrigation. If SAR is 6-10, it

is marginal fit and if SAR is >10, the water is unfit for irrigation. Such water may create the

problem of sodicity.

iii. Residual Sodium Carbonate (RSC): For the given water sample, this criteria can be

calculated by the equation; RSC = (CO

+ HCO

) – (Ca + Mg)

If RSC is <10 meq/l, water is fit for irrigation. If RSC is 10-15 meq/l, water is marginal fit and

if RSC of given water sample is >15 meq/l, water is unfit for irrigation. The use of such unfit

water will deteriorate the soil physical properties due to occurrence of sodicity problem in

the soil.

As far as the management of poor quality irrigation water is concerned, there is yet no solution of

irrigation water with high EC. However, the waters with more SAR and RSC can be utilized by putting

gypsum blocks in water course or by using recommended dose of concentrated sulfuric acid along with

irrigation water.

Session 13: Water Potential; Introduction and Classification:

The difference between the free energy of soil water and that of pure free water in a standard reference

state is known as soil water potential. The soil water potential can be expressed in bars, atmospheres or

kilopascals (kPa).

Soil Water Classification:

The most useful classification of water content in soil is the biological classification as it relates water to

plant growth. There is a definite relationship between water retention and its use by plants. These water

contents are classified as gravitational (drainage) water, field capacity, permanent wilting point and

plant-available water.

a. Gravitational Water: Water present in excess of field capacity or water held at a potential greater

than -33 kPa is called gravitational water. It is available as it moves through the plant roots if

adequate aeration is maintained. It is of limited use to plants as it is present in the soil only for short

periods of time. It can affect plant growth due to poor aeration.

b. Field Capacity: The contents of water remaining in a soil two to three days after having been

saturated with water and after the free water (gravitational water) has been allowed to drain away

is called field capacity. It is the percentage of soil water that is held at -10 to -33 kPa water potential

and is a measure of the greatest amount of water that a soil can store (or hold).

c. Permanent Wilting Point: It is the amount of water at which plants growing in that soil will wilt

and not recover when placed in a humid chamber. It is estimated at about -1500 kPa water potential

or less (more negative). Water is held so strongly that plants are not able to absorb it fast enough

for their, needs. However, microbes can utilize this water.

d. Plant Available Water: Plant-available water is defined as the weight percentage of total soil

water held with a water potential between -10 to -33 and -1500 kPa and is said to be usable by

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FINAL TERM LECTURES FOR SS-302 (INTRODUCTION TO AGRICULTURAL CHEMISTRY) Session 11: Soil Water; Structure and Importance: It is the water which is present in root zone area of plant and important for plant growth. It includes all the water, which is supplied to plants either naturally or through irrigation. It is also known as soil moisture, while ground water is the water present beneath the water table. There is no life without water as it is the medium through which all the vital processes take place. The water in the oceans and the rivers is an important means for the transportation between the cities and the countries. The waterfalls are used in generating the electricity in many countries. Consumption of water is very high in the entire world, where Asia is the largest continent in consuming the water for its agriculture. North America is the largest continent in consuming the water for domestic and industrial uses. Structure of Water: Water molecule is polar in nature due to uneven distribution of charge. The hydrogen atom sides of water molecule are tended to act as electropositive, while oxygen atom side acts as electronegative, although water molecule is neutral. The two hydrogen atoms are attached to oxygen in a nearly V shaped arrangement with an angle of 104.5 between two hydrogen atoms. Due to this, water molecule exerts an attraction for solid surfaces (called adhesion forces or adsorption) and to one another (called cohesion forces) as well. Strong adhesion a ...
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