1. Soil, Plant and Water Relationships
Shri M. U. Kale
Assistant Professor
Deptt. of Irrigation and Drainage Engg.

2. The Hydrologic (Water) Cycle
Water is constantly moving through the atmosphere and, into and out of the soil.
Soil moisture is one portion of the cycle which can be controlled to the greatest extent by affecting the soil.

3. Physical Properties of Soil Influencing Irrigation
Soil is a three phase system –
Solid phase i.e. soil mineral & organic matter;
Liquid phase i.e. soil moisture
Gaseous phase i.e. soil air.
The main component of the soil phase is the soil particles,
The size and shape of which give rise to pore spaces of different geometry.
These pore spaces are filled in varying proportion with
Water (i.e.soil moisture) and
Air (gas i.e.soil air ).
The volume composition of soil system varies widely

4. Soil Properties Soil Texture
Soil texture refers to size distribution of particles making up the soil i.e. sand, silt and clay.
Soil texture affects water movement and storage.
The texture of soil is more or less constant and does not change with tillage.
Textural triangle: USDA Textural Classes
Coarse vs Fine,
Light vs Heavy
Loam soil – soil particulates (less or more) in equal proportion

5. USDA Textural Triangle
USDA classifications
Sand: – 2.0 mm
Silt: mm
Clay: <0.002 mm

6. Soil Structure
Soil structure refers to the arrangement of the particles forming aggregates within the soil mass.
It affects root penetration and water intake and movement.
Soil structure classification
1.Simple structure in which nature cleavage planes are absent or indistinct.
2.Compound structure in which natural cleavage planes are distinct.
a) Granular
b) Blocky
c) Prismatic
d) Columnar
e) Platy
f) Single grained

7. Soil Structures

8. Soil structure
Soil structure affects penetration of roots, air movement, crop production etc.
Soil structure can be changed with tillage practices.
Good soil structure could be obtained with addition of organic matter, growing legumes etc.
Cultivated fine textured soil when either too wet or too dry is likely to destroy good soil structure.
Irrigating with water containing large amount of sodium causes undesirable structure by dispersing soil particles.
Granular structure is most favourable for crop production.
Platy structure is least water transmitting.

9. Volume and Mass Relationship of Soil Constituents
Ms = mass of solids,
Mw = mass of liquids,
Ma = mass of gases-negligible
Mt = Ms + Mw = Total mass
Vs = volume of solids,
Vw = volume of liquids,
Va = volume of gases,
Vp = Vw + Va = Volume of pore space
Vt = Vs + Vw + Va = Total volume

10. Soil Density Soil density is it’s mass per unit volume.
Soil density - particle density and bulk density.
Particle density – It is also known as True density.
It is the ratio of the dry solid soil particles to its volume.
Particle Density (s) =
 = soil particle density, g/cm3
Ms = mass of dry soil, g
Vs = volume of solids, cm3
Typical values: g/cm3.
Most mineral soils have a particle density of approximately 2.65g/cm3.
This value is often used as a standard particle density in soil calculations.

11. Bulk Density (b)
Bulk density is the density of soil as it exists in its place.
It is mass of dry soil per unit bulk volume, the volume being determined before drying.
b = soil bulk density, g/cm3
Ms= mass of dry soil, g
Vc = volume of soil sample, cm3
Typical values: g/cm3

12. Porosity
Porosity (n) is ratio of volume of pores to total soil volume.
Porosity =
Porosity (n) defines only relative pore volume of soil and not pore size distribution.
n = 1 –
Typical values: 30-60%
n = 30 % - for compacted soils
n = 60 % - for loose soils.
Aeration porosity / aeration capacity / non-capillary porosity is the ratio of volume filled with air to total soil volume (under specified moisture tension).

13. Void ratio
It is defined as the ratio of the volume of voids to the volume of solids.
e =
Void ratio index is used when soil is used as construction material.

14. Soil Wetness
The term soil wetness refers to the relative water content in the soil
Mass Wetness-
It is the ratio of mass of water to the mass of soil.
It is often referred to as the gravimetric water content
Mass Wetness=
Volume Wetness –
It is the ratio of volume of water to the total soil volume.

15. Soil Wetness Degree of saturation s = 0 % - for dry soil condition
Degree of saturation refers to volume of water content in the soil relative to volume of pores.
s = 0 % for dry soil condition
s = 100 % - for saturated soil condition
In soil mechanics the term water saturation or degree of saturation, Sw is used, defined as
where ϕ = Vv / VT = porosity
Vv = volume of void or pore space.
Values of Sw can range from 0 (dry) to 1 (saturated).
In reality, Sw never reaches 0 or 1 - these are idealizations for engineering use

16. Capillary Pores-
It contain the water which remains after most of the free drainage in the soil completed.
Capillary Porosity-
It is the percentage of pore space that may be occupied by capillary water.
Non-Capillary Pores-
It contain large pores which do not hold water tightly by capillarity.
Non-Capillary Porosity-
It is the percentage of pore space filled with air after the soil has drained to field capacity.

17. Soil Consistency -
It describes the characteristics of the soil at various moisture contents when influenced by the physical forces of cohesion and adhesion.
Soil consistency varies with
Texture,
Structure,
Organic matter,
Percentage of colloidal material,
Shape and type of clay mineral.
Soil Profile -
It is the vertical section through the soil mass. Significant changes in the texture and structure with depth are observed in most soils.

18. Water Relations of Soil
The mineral and organic compounds of soil from a solid matrix.
Interstices of solid matrix consist of irregularly shaped pores with a geometry defined by boundaries of matrix.
Pore space is partly filled with soil air and partly with soil water.
Soil moisture is the important ingredients.

19. Kinds of Soil Water Hygroscopic Water
It is the water held by soil particles by means of adhesion forces. It is so strongly held by the soil particles, that it is not available to the plants.
Capillary Water
It is the water held in pore spaces due to capillarity. It is held by cohesive forces greater than gravity and is available to plants.
Gravitational Water
It is the water which flows freely due to gravitational forces.

20. Cohesion –
It is the attraction of two similar molecules (water to water)
This force makes possible a marked thickness of films of water established by hydration until they attained microscopic size
Adhesion –
It is the attraction of two different molecules (water to soil)

21. Soil moisture constants
Saturation capacity
Field capacity
Moisture equivalent
Permanent wilting percentage
Wilting range
Ultimate wilting
Available water

22. Saturation Capacity
A soil where all pores are completely filled with water, then that soil is said to be at saturation capacity.
The tension of water at saturation capacity is almost zero and it is equal to free water surface.

23. Field Capacity
The field capacity of soil is the moisture content after drainage of gravitational water has become very slow and the moisture content has become relatively stable.
The terms field capacity, field–carrying capacity, normal moisture capacity and capillary capacity are synonymous.
The soil moisture tension at field capacity varies from soil to soil, but it generally ranges from 1/10 to 1/3 atm.

24. Moisture Equivalent:
It is defined as the amount of water retained by a sample of initially saturated soil material after being subjected to a centrifugal force of 1000 times that of gravity for a definite period of time, usually half an hour.
Permanent Wilting Percentage:
It also known as permanent wilting point or wilting coefficient.
It is the soil moisture content at which plants can no longer obtain enough moisture to meet transpiration requirements and remain wilted unless water is added to soil.

25. Wilting Range:
It is the range of soil – moisture content through which plants undergo progressive degrees of permanent or irreversible wilting, from wilting of the oldest leaves to complete wilting of all leaves.
Ultimate wilting:
The moisture content at which the wilting is complete and the plants die is called the ultimate wilting.

26. Available Water:
Soil moisture between field capacity and
permanent wilting point is referred as
available water.
It is moisture readily available for plant use.
In general, fine textured soil have a wide
range of water available between field
capacity and permanent wilting point than
coarse textured soil.

27. Kinds of soil water

28. Some important relationships
Hygroscopic water (%) = Hygroscopic coefficient
Capillary water (%) = Field capacity – Hygroscopic coefficient
Available water (%) = Field capacity – wilting point
Unavailable water (%) = Wilting point
Gravitational water (%) = Water content – Field capacity

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