Dear Dr. Shantappa Duttarganvi; Please go through the links which may helpyou. Thanks

http://soilquality.org.au/factsheets/water-availability

http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447039&topicorder=10&maxto=10

Saturation is the maximum water holding capacity! 

You can also work this out as the maximum porosity using bulk density and particle density (specific gravity) - check google for the equation.

But soil scientists also talk about field capacity, that has a text book definition related to time after rainfall (24-48 hrs) and 'free drainage'  (dry soil below the wetted layer).  It can be determined in the field, but is often determined with pressure plate apparatus in the laboratory (or even in a measuring cylinder). 

To some extent field capacity is academic, because some soils do not drain in 24-48 hrs (or at all), whilst it is not uncommon for soils to be wetter than field capacity in the field if rainfall is high enough intensity and quantity for long enough.  Agronomists often prefer to determine an 'upper drained limit' under field conditions.  Do a google search on this term.

Do be careful using 'field capacity' to regulate water in pot experiments or in the field.  Often if we try to maintain field capacity we end up with parts of the pot or soil profile being drier than FC and other parts cycling between saturation and something drier than this - think about how this happens.

Saturation capacity is defined as maximum water holding capacity of soil and field capacity is maximum water holding capacity of soil against gravity. If we only talk about water holding capacity of soil, it generally refers to water content between field capacity and permanent wilting point.

The concepts of 'field capacity' and 'wiling point' have been largely superseded in agronomy (though not in soil science), as neither is as precisely defined as they appear at first sight. Nowadays we refer to the upper drained limit (UDL) and the lower limit of extraction (LLE), both determined in the field. But as I said on my first post, soils can be wetter than the upper drained limit (or FC) at times, and this water is available to plants.  The LLE is crop-specific. The potentially available water capacity (PAWC) lies between the UDL and LLE.

Generally  field capacity is considered to be 50 percent of saturation and permanent wilting point is 50 percent of field capacity.

The simplest difference between saturation and water holding capacity is: during saturation all the pores including air-filled porosity are full of water and with maximum water holding capacity, air-filled porosity is full of air.  The above answers explain in more depth.

In addition to the excellent discussion above I would add that the suction reading of soils (suction required to draw water out of the soil) varies from essentially zero at saturation (no suction required to extract water), to around -8 kPa at maximum water holding capacity, at least in the soils I have experience with in Australia.  This is still quite a small number when compared to "permanent wilting point" at around -1500 kPa, but reflects the fact that the most available water in the soil at maximum water holding capacity is held by capillary forces between soil particles, whereas at saturation there is a proportion of water that sits in the larger pores spaces, and will drain under gravity if allowed.

In my modest opinion the problem is the wide meaning of "to hold" in English. In spanish there is a difference between "to contain" and "to retain". If I understand English properly, "to hold" (in terms of "to contain") is used to define the amount of water in soils when all the porosity is full of water (water holding capacity? I would not use this term here. I prefer the term "saturation"), zero suction reading at saturation or no suction required to extract water, as Mark A. Skewes pointed out.

Nevertheless, "to hold" is also "to retain", when the larger capillaries of soil are full of air ("air-filled porosity is full of air", as mentioned  by Bruno Pitton), so water drainage is slow although drainage still exists (water at -8 or -10 kPa), more or less equivalent to field capacity. Then, at about -33 kPa (or equivalent to 1000 g during 1 hour in a centrifuge) the drainage stops. There soil reached water holding capacity in terms of retention. I hope it helps.

There are many terms in soil water relations that seems to have slightly different interpretation depending on the discipline i.e. Soil Science, Civil Engineering, Petroleum Engineering etc.   Here is my take as a Biosystems Engineer (= Agricultural Engineer)!  We use the term "saturation" to refer to the entire pore space being filled with water including "vugular pores" in rocks.  However, the vugular pores can trap air during saturation leading to a lower saturation value called "effective saturation."  The effective saturation does not include air trapped in non-interconnected pores.  If you really want to attain "full" saturation, then all the pore space should be flushed with highly soluble gas such as carbon-dioxide or helium prior to slow saturation.  This will help completely saturate the sample because the trapped gases will quickly dissolve in the pore water and diffuse out of the pore space.

We use the term "maximum water holding capacity" to refer to the volumetric water content of a sample when all the "gravitational water" (= water freely drained under gravity) has drained.  The draining of the gravitational water will depend on where the sample is located.  i.e. If the sample is isolated in a sample holder with a porous bottom it will hold more water compared to a sample collected from a soil profile with a water table below.  In the latter case, the depth to the water table will influence how much water will be retained in the soil sample collected.  In this case, the capillary forces are also helping to drain the water in addition to the gravity!  So the term "maximum water holding capacity" will depend on the sample location as well.  It is not a precise term.

The reason for the difference between saturation and maximum water holding capacity is that they are different concepts.  The first saturation is the water content when the soil is saturated (the water potential >= 0) this is not necessarily the same as the porosity of the soil (1 - bulk density/particle density) as air often remains trapped in the soil.  The trapped air amount is usually in the range of 0 - 10% and often an estimate of saturation is 0.95xporosity.

The maximum water holding capacity is a term that can mean the field capacity which is the water content at which drainage has become negligible.  This is the original definition.  There are many other estimates of field capacity including water contents at various matric potentials (1/3 bar, 0.2 m, 0.1 etc) or fluxes 1 to 0.1 mm/day.  In the field as evapotransration is occurring at the same time as drainage this will cause a potential gradient flux reversal compared to drainage and so I personally use 0.1 m as a matric potential and 1 mm/day as estimates.

Finally in the soil biological literature you will find the term maximum water holding capacity used as a reference water content.  This involves the drainage of a core to a constant water content.  It is a terrible concept as it is dependent on the core length (the matric potential at the bottom of the core will be equivalent to the length).  It was what got me involved in working with soil biologists 30 years ago (see Cook and Orchard, 2008 Soil Biology & Biochemistry, 40: 1013-1018).

Hope this is helpful.

i agree with the answers of the above authors

also reffer water holding capacity of vermi copmosts using mono and poly culture reactors which may be useful to you.

Maximum water holding capacity is a soil moisture condition after free drainage has taken place, while saturation occurs when all the pores are filled with moisture

I had posted my first answer to the question before I realised that I erroneously explained maximum water holding capacity as if it were field capacity. These two terms are not the same. Rather, maximum water holding capacity and saturation are used synonymously.

Dear Santappa,

It has been quite interesting discussion, and I got many information. To the best of my understanding, there should not be 'max' water holding capacity. It should be simply water holding capacity, which does reflect the amount of water retained by the soil at field capacity, when the downward movement of water is materially ceased (no drainage). It does not happen for long, and thus WHC is dynamic. To be precise and scientific, we may call WHC as the moisture content at -30kPa (for soils with 20% clay). However, in most of the soils, actual field capacity (in the field) is achieved anywhere between -10 and -20 kPa potentials.Also there are other issues as Peter Stuart points out.

Saturation is when all the pores in soil are filled with water and may be calculated as (1-BD/PD)*100; The value is not truely saturation percentage, as some pockets of entrapped air will not allow the soil to be fully saturated.    

Dear Shantappa,

The term 'maximum water holding capacity' is not a common terminology in water science but usually it refers to the amount of water that a specific soil can hold without loosing it by processes such as drainage. Someone express the maximum water holding capacity as Field Capacity (FC). Saturation occur when all the pores of the soil filled with water and adding more water to it cause ponding.

I do not totally agree with Dabashis because in irrigation literature, there is maximum water holding capacity which is also referred to as moisture content at saturation. Furthermore, the maximum water holding capacity and the field capacity are not the same. At saturation, all pores are filled with water. However, the field capacity is the moisture content at which moisture at saturation is allowed to drain gravitationally for one to three days. Thus, field capacity is less than maximum water holding capacity or saturation. 

Thank you all for your informative answers..

Thank you so much for interesting discussion. So If I want to map saturated soil, shall I consider moisture content >= soil porosity? or moisture content >= maximum water holding capacity? or moisture content >= field capacity? Thanks in advance for your kind answer.

The saturation and maximum water holding capacity are different levels of soil water content.  The saturation capacity is the level of water content when the soil is saturated and all pores are filled with water (in compact soil, few air often remains trapped in the soil). At saturation, some water is under the effect of gravity more than under attraction to soil particles. This amount of water is known as gravitational or free water.

After drainage of free water, the level of soil water content is the field capacity, which known also as the maximum water holding capacity. The field capacity is the best level of soil water content as there is much water available to roots of plants and a sufficient amount of air for respiration. As the water is lost from the soil by evaporation or absorption of roots, more air will replace water and the soil became more dry.

Saturation capacity %= field capacity % + Free water %