Unit 1: Soil, Plant, Water and Atmospheric Relationship

-A vital resource for Agriculture

• Plant growth depends on the use of two important natural resources, soil and water. Soil provides the mechanical and nutrient support necessary for plant growth and Water is the major input for the growth and development of all types of plants.

• Soil provides food and fertilizers to the crops where as water mobilize the organism of plant growth and helps in introducing food and fertilizers to crops.

• Soil provides the room for water to be used by plants through the roots present in the same medium.

Water Retention by Soil

• The availability of water, its movement and its retention are governed by the properties of soil.

• The properties like bulk density, mechanical composition, hydraulic conductivity etc.

• The ability of soil to retain water is strongly related to particle size. Generally, water retention is inversely related to permeability.

• Water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil, so clays generally retain more water.

• Soil is said to be saturated after heavy rainfall/irrigation at its maximum retentive capacity, with change in the pressure head caused by saturating water.

• As head changes soil-water content also changes

• The graph representing the relationship between pressure head and water content is generally called the ‘soil-water retention curve’ or the ‘soil moisture characteristic’.

• Applying different pressure heads, step by step, and measuring the moisture content allows us to find a curve of pressure head, h, versus soil-water content θ.

• The pressure heads vary from 0 (for saturation) to -107 cm

Soil-Moisture Characteristics

• The water below WT called as ground water.

• Soil serves as the storage reservoir for water. Only the water stored in the root zone of a crop can be utilized by it for transpiration and build up plant tissues.

• When ample water is available in the root zone, plants can obtain their daily water requirements for proper growth and development

• As the plant continue to use water, the available supply diminishes, and unless more water is added, the plants stops growing and finally die.

• Before the stage is reached when crop growth is adversely affected, it is necessary to irrigate again.

• The amount of water to be applied in each irrigation and the frequency of irrigation are dependent on the properties of the soil and crop to be irrigated.

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.

• Soil moisture tension is not necessarily an indication of the moisture content of the soil nor the amount of water available for plant use at any particular tension

• These are dependent on the texture, structure and other characteristics of the soil and must be determined separately for each soil.

• Generally sandy soils drain almost completely at low tension, but fine textured clay still hold a considerable amount of moisture at such high tension that plant growing in soil may wilt.

• Moisture Extraction Curves (Moisture Characteristics Curves) which are plots of moisture content versus moisture tension, show the amount of moisture a given soil holds at various tensions

• The knowledge of the amount of water hold by the soil at various tensions is required in order to understand the amount of water that is available to plants.

• Field Capacity: (F.C.) the quantity of water which any soil can retain indefinitely against gravity

• It is divided into TWO parts1) Capillary Water : which is attached to the soil molecules by

surface tension against gravitational forces, and can be extracted by plants by capillarity.

2) Hygroscopic Moisture (Unavailable Water): which is attached to the soil molecules by loose chemical bonds and which can not be removed by capillarity, is not available to the plants.

F.C. = Wt. of water retained in a certain volume of soil x 100 Wt. of the same soil volume of dry soil

• Permanent Wilting Point Is that water content at which plant can no longer

extract sufficient water for its growth and wilts up.

It is evident that the water which is available to the plants is the difference of FC water and PWP water, this is called Available water/moisture.

• Consider 1 sq. m area of soil having ‘d’ meter depth. Volume of soil = 1 x d = ‘d’ cubic m Dry unit weight of soil is = γd KN/cub. mthen, wt. of d cubic meter soil is γd d KN

F.C. (F) = Wt. of water retained in unit area of soil γd d

Wt. of water retained in unit area of soil = γd d F KN/sq. m If γw = unit wt. of water per unit volume KN/cub. mthen , Volume of water stored in unit are of soil = γd d F KN/sq. m

γw KN/cub. M

Hence the depth of water stored in the root zone in filling the soil upto field capacity

= γd d F meters. γw

Soil/Land Irrigability Classification (LIC) • This classification system predicts how the land would appear

if irrigated and/or drained, including changes in water table, salinity and land shaping.

• The objective of LIC is to select lands for irrigation development, and to characterize their main management factors.

• This identifies arable lands that are suitable for irrigation and identifies the irrigable lands that will be actually irrigated within the arable lands

• Arable land may not be irrigated because of geographic constraints, such as unfeasible delivery of water, or an isolated or odd-shaped parcel

• LIC system has six irrigability classes

Factors Affecting profile water storageIf a pit is dug in the soil, at least 1 m deep, various layers, different in color and composition can be seen. These layers are called horizons. This succession of horizons is called the profile of the soil.

1. Total Porosity or Void Space2. Pore-size and Distribution and Connectivity (infiltration rate)3. Soil Water Pressure Potential or Energy Status of the Soil

Water4. Soil texture5. Soil depth6. Soil structure

• Determination of soil water content 1) By Oven drying Method2) By Pycnometer Method

1) By Oven drying MethodThe water content (w) of a soil sample is equal to the mass of water divided by the mass of solids.

Where M1=mass of empty container with lid, M2= mass of the container with wet soil and lid M3= mass of the container with dry soil and lid

2) By Pycnometer MethodA Pycnometer is a glass jar of about 1 liter capacity, fitted with a brass conical cap by means of a screw type cover. The cap has a small hole of about 6mm diameter at its apex.

Where M1=mass of empty Pycnometer,M2= mass of the Pycnometer with wet soilM3= mass of the Pycnometer and soil, filled with water,

M4 = mass of Pycnometer filled with water only.

G= Specific gravity of solids

Determination of soil water depletion• Soil moisture content near the wilting point is not easily extractable to

the plant, hence the term readily available moisture is used to represent the fraction of the available moisture which can be easily extracted by the plants.

• The readily available moisture is 75% of available moisture. • Soil moisture can vary between the Field Capacity and Permanent

Wilting Point. However, depending upon the prevailing conditions, soil moisture can be allowed to depleted below the Field Capacity but not below the Permanent Wilting Point before the next irrigation is applied.

• Example 1:After how many days will you supply water to soil in order to ensure sufficient irrigation of the given crop if,

1. FC of soil = 28%2. PWP = 13%3. Dry density of soil = 1.3 gm/cc4. Effective depth of root zone = 70cm5. Daily consumptive use of water for the given crop = 12mm

Soil Water Potential• The effect of force on soil water may conveniently be described by

potential energy of soil-water in a particular force field. • Water present in an unsaturated porous medium such as soil is subject

to a variety of forces acting in different directions• Different force field results from the attraction of solid matrix for

water presence of solutes and the action of gravity and external gas pressure.

• The terrestrial gravitational field and the overburden loads due to the weight of soil layers overlying a nonrigid porous system tend to move the soil water in the vertical direction.

• Effect of force on soil water may be described by potential energy of soil water in a particular force field.

• The concept is however very useful in evaluating the energy status of water at any time and place in the soil-plant-atmosphere continuum.

Total Soil Water Potential:• Total soil water potential is the sum of potentials resulting from different

force fields.• It may be defined as the amount of work done by a unit quantity of water to

transport reversibly and isothermally an infinitesimal quantity of water form a pool of pure water at a specified elevation at atmospheric pressure to the point of soil water under consideration.

• Total Soil water potential Ѱsoil can be written as Ѱsoil= Ѱg + Ѱ

p(m) +Ѱo

Where, Gravitational Potential (Ѱg), Pressure Potential (Ѱp)Metric Potential (Ѱm), Osmotic Potential (Ѱo)

Gravitational Potential (Ѱg)• Soil water subjected to the gravity equal to its body weight that being the

product of the mass by the gravitational acceleration.• An amount of work that a unit quantity of water in an equilibrium soil water

system at an arbitrary level is capable of doing when it moves to another equilibrium identical in all respects except that it is at reference level.

• Assuming a point at a height Z above a reference level, the gravitational potential energy Eg is

Pressure Potential (Ѱp)• Pressure potential is defined as the amount of work that a unit quantity of

water in an equilibrium soil water system is capable of doing when it moves to another system identical in all respects, except that it is at a reference pressure.

• When the soil water is below the water table at a depth ‘h’ , it is at a hydrostatic pressure greater than atmospheric pressure and the pressure potential is positive. This is also referred as submerged potential.

• On the other hand when soil water is above the water table its pressure is less than atmospheric pressure and the pressure potential is negative. It is often referred as suction or tension.

• The hydrostatic pressure of water P with reference to atmospheric pressure is P=ρwgh

Metric Potential (Ѱm)• Metric potential is the negative pressure potential from the capillary forces

emanating form the soil matrix, it is sometimes called the capillary potential or soil water suction or metric suction.

• Metric potential may be defined as the amount of work that a unit quantity of water in equilibrium soil water system is capable of doing when it moves to another equilibrium system identical in all respect except there is no matrix present.

• Soil water in an unsaturated soil has no pressure potential but has only metric potential.

• Assuming that an infinitesimal volume of water dv with pressure deficit P, the matrix potential is

Ѱm =Pdv

Osmotic Potential (Ѱo)

• Osmotic potential may be defined as the amount of work that a unit quantity of water in equilibrium soil water system is capable of doing when it moves to another equilibrium system identical in all respect except there is no solution.

• Presence of the solutes in soil water affects its thermodynamic properties and lowers its potential energy.

• Osmotic potential is also termed as solute potential Ѱo= -П• П=The osmotic pressure due to dissolved salts and solutes

Hydraulic Head• It is the elevation with respect to a standard datum at which water

stands in a riser pipe or manometer connected to the point in question in the soil.

• This will include elevation head, pressure head and velocity head.

• For non-turbulent flow of water in the soil the velocity head is negligible.

• Hydraulic head has dimension of length L.

Field Water Budget (Balance)• The field water Budget is an itemized statement of all gains, losses and

changes of storages of water occurring in a given field within specified boundaries during a specified period of time.

• The task of monitoring and controlling the field water balance is vital to the efficient management of water and soil.

• It is essential to evaluate the possible methods to minimize loss and maximize the gain and utilization of water which is often the limiting factor in crop production.

• Gains of water in the field are due to precipitation and irrigation .• Occasionally, there may be gains due to accumulation of runoff from higher

tracts of lands , or to capillary rise from below.• Losses of water include surface runoff from the field, deep percolation out of

the root zone, evaporation from soil surface and transpiration from crop canopy.

• The change in the storage of water in the field can occur in the soil as well as in the plant.

• Total change in the storage must be equal the difference between sum of all gains and the sum of all losses.

• Accordingly the water balance equation may be stated as follows.

• (Gains) - (Losses) = (Change in Storage) (P+I) - (R+D+E+T)= ΔS+ΔVWhere

P=Precipitation; I=Irrigation; R-Runoff; D=Drainage; E=Evaporation; T=Transpiration; ΔS=Change in soil water content of the root zone and ΔV = Change in plant water content.

• All these quantities are expressed in terms of units of depths (cm)

Capillary Rise• Capillary rise is a well known unsaturated soil phenomenon that describes the

movement of pore water from lower elevation to higher elevation driven by the hydraulic head gradient acting across the curved pore air/pore water interface.

• Soil pores are rarely uniform. The height of capillary rise will therefore depends on the largest opening that the water encounters. Once the soil is saturated with water, and then the water is allowed to drain away, some water is held by the smaller capillaries, even though the larger openings are freed of water.

• In a fine textured soil the rise is so slow that the plants seldom gain from the presence of ground water if its level is about 80cm or more below the roots.

• The capillary rise on the dry range of soil moisture content is much slower than in the wet range.

• Three fundamental physical characteristics related to capillary rise are of primary practical concern: (1) the maximum height of capillary rise, (2) the fluid storage capacity of capillary rise, and (3) the rate of capillary rise.

Water Requirement of Crops• Water requirement of crop refers to the amount of water required to

raise a successful crop in a given period.• It comprises of WR = E +T + IP + Wm + Wu + Ws

or WR = ET + Wm + Wu + Ws

or WR = CU + Wu + Ws

• Evapotranspiration (ET) and Consumptive Use (CU)• ET denotes the water transpired by crop plants and the water evaporated

from the soil or water surface in the crop field and the intercepted precipitation by crop in any specified period.

• Expressed in depth of water i.e. mm or cm• CU of water by crop refers to the ET together with water used for metabolic

activities by the crop plants.• ET = CU because water used by plants in metabolic activity is generally 1%

of ET value.

Potential Evapotranspiration (PET)• It denotes the highest rate of ET by a short and actively growing crop with abundant foliage

completely shading the ground surface and abundant soil water under a given climate.

Actual Crop Evapotranspiration (AET)• Refers to the rate of ET by a particular crop in a given period under prevailing soil water

and atmospheric conditions.• It involves crop factor called, crop coefficient (k).

Reference crop evapotranspiration (ETo)• The evapotranspiration rate from a reference surface, not short of water, is called the

reference crop evapotranspiration or reference evapotranspiration.

Factors Affecting ET:1) Climatic factors2) Growing season3) Crop characteristics4) Soil Characteristics5) Cultural Practices

• Methods of Estimating Evapotranspiration1) Direct Methods

a) Lysimeter methodb) Field Experimentation methodc) Soil water depletion methodd) Inflow- Outflow method

2) Pan Evaporimeter Methoda) USWB Class-A Pan Evaporimeterb) Sunken Screen Pan Evaporimeterc) Piche Atmometer

3) Empirical Methodsa) Blaney-Criddle Formulab) Thornthwaite formulac) Penman Formulad) Modified Penman Methode) radiation method

Blaney-Criddle Formula• It is developed in 1950, to estimate the CU, based on mean monthly

temperature, day light hours and locally developed crop coefficients.

• This method fives sufficiently accurate estimate of seasonal CU.• But in 1975, Doorenbos and Pruitt recommended following

formulas for ‘f’ factor to achieve more accurate results

f = p (0.46t + 8.13 ) ….t in oC OR f = 25.4 (p x t)/100 ….. In oF

Penman Formula (1948)• The ET are obtained by multiplying the estimated values of

evaporation by the crop coefficient (K). The evaporation calculated as below

Modified Penman Formula (1975)• Doorenbos and Pruitt proposed modified Penman method to evaluate

reference crop ET

• Doorenbos and Pruitt (1977) suggested the adjustment factor ‘C’ to determine the reference crop ET, from the unadjusted ET0

*

compensating day and night weather effects.