Porosity φ is the fraction of the total soil volume that is taken up by the pore space. Thus it is a single-value quantification of the amount of space available to fluid within a specific body of soil. Being simply a fraction of total volume, φ can range between 0 and 1, typically falling between 0.3 and 0.7 for soils. With the assumption that soil is a continuum, adopted here as in much of soil science litera-ture, porosity can be considered a function of position. Porosity or pore space refers to the volume of soil voids that can be filled by water and/or air. It is inversely related to bulk density. Porosity is calculated as a percentage of the soil volume:

Percentage Solid space =100 X Bulk density/Particle density.

Loose, porous soils have lower bulk densities and greater porosities than tightly packed soils. Porosity varies depending on particle size and aggregation. It is greater in clayey and organic soils than in sandy soils. A large number of small particles in a volume of soil produces a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.

Compaction decreases porosity as bulk density increases. If compaction increases bulk density from 1.3 to 1.5 g/cm3, porosity decreases from 50 percent to 43 percent. Aggregation also decreases porosity because more large pores are present as compared to single clay and silt particles that are associated with smaller pores.

Pores of all sizes and shapes combine to make up the total porosity of a soil. Porosity, however, does not tell us anything about the size of pores

The characterization of pore space is a vital and fruitful aspect of soil investigation. Liquid, solid and gas constituents of the soil govern the form and development of pores, whose character in turn profoundly influences the na-ture and behavior of the soil. Soil porosity is fairly well standardized in definition and measurement techniques. Pore size, however, is not obvious how to define, much less to measure. Yet it is central to topics like macropores, aggregation, fractures, soil matrix, and solute mobility. Pore size plays a key role in various proposed means of quanti-fying soil structure. It also has a major practi-cal role in the prediction of hydraulic proper-ties. New pore size concepts, measurement techniques, and relations to transport phenom-ena are likely to remain a major emphasis in the study of soil.

The air in the soil is similar in composition to that in the atmosphere with the exception of oxygen, carbon dioxide, and water vapor. In soil air as in the atmosphere, nitrogen gas (dinitrogen) comprises about 78%. In the atmosphere, oxygen comprises about 21% and carbon dioxide comprises about 0.36%. However, in the soil air, oxygen usually is replaced by carbon dioxide, so both range from about 0.4% to 21%.Oxygen is used by plant roots and soil microbes during respiration, and carbon dioxide is released. Thus, in the soil, the oxygen levels are generally less than atmospheric levels and the carbon dioxide levels are generally greater than atmospheric levels. Some factors that determine the extent of the difference between atmospheric and soil air constituents include depth in the soil profile, soil pore size distribution, and soil water content.]

• Depth: Oxygen levels generally decrease with depth in the soil profile due to slow diffusion rates of oxygen from the surface through the soil.

• Pore size distribution: Soils with large pores promote more rapid oxygen diffusion into and through the soil, and carbon dioxide movement out of the soil. Soils with small pores have slower oxygen diffusion into the soil and carbon dioxide diffusion out of the soil. Sandy soils generally have low total porosity but large individual pores. Clay soils generally have high total porosity but small individual pores.

• Aeration and drainage: Soils with large pores generally have good drainage (less water) and aeration, while soils with small pores generally have poor drainage and aeration. Thus, sands generally have good drainage, while clays have poor drainage and are more likely to become anaerobic (deprived of oxygen) as microbes use oxygen more rapidly than it is replenished through diffusion.

• Soils with more pores filled with water have less space available for air, thus become anaerobic more rapidly than drier soils.

• Water vapor: Soil air has a relative humidity very close to 100%. (Relative humidity is the amount of water vapor actually in the air relative to the amount the air could hold at that temperature.) This is much different than atmospheric air, which may vary in relative humidity between 5% and 100%, sometimes within 24 hours in semi-arid and arid regions. A Boy Scout Survival Kit applies this concept by providing a shovel, piece of plastic, and a cup. Dig a hole in the morning, place the cup in the bottom of the hole. Anchor the plastic with soil around the rim of the hole. Place a rock on the plastic above the cup. During the day, the plastic allows the soil to heat, evaporating water. At night, the plastic allows radiational cooling. As the air cools, water vapor condenses on the plastic and drips into the cup. This provides enough water for subsistence. If cacti or other succulents are available, place some of their vegetation in the hole to enhance the water provision.

Porosity φ is the fraction of the total soil volume that is taken up by the pore space. Thus it is a single-value quantification of the amount of space available to fluid within a specific body of soil. Being simply a fraction of total volume, φ can range between 0 and 1, typically falling between 0.3 and 0.7 for soils. With the assumption that soil is a continuum, adopted here as in much of soil science litera-ture, porosity can be considered a function of position. Porosity or pore space refers to the volume of soil voids that can be filled by water and/or air. It is inversely related to bulk density. Porosity is calculated as a percentage of the soil volume:

Percentage Solid space =100 X Bulk density/Particle density.

Loose, porous soils have lower bulk densities and greater porosities than tightly packed soils. Porosity varies depending on particle size and aggregation. It is greater in clayey and organic soils than in sandy soils. A large number of small particles in a volume of soil produces a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.

Compaction decreases porosity as bulk density increases. If compaction increases bulk density from 1.3 to 1.5 g/cm3, porosity decreases from 50 percent to 43 percent. Aggregation also decreases porosity because more large pores are present as compared to single clay and silt particles that are associated with smaller pores.

Pores of all sizes and shapes combine to make up the total porosity of a soil. Porosity, however, does not tell us anything about the size of pores

The characterization of pore space is a vital and fruitful aspect of soil investigation. Liquid, solid and gas constituents of the soil govern the form and development of pores, whose character in turn profoundly influences the na-ture and behavior of the soil. Soil porosity is fairly well standardized in definition and measurement techniques. Pore size, however, is not obvious how to define, much less to measure. Yet it is central to topics like macropores, aggregation, fractures, soil matrix, and solute mobility. Pore size plays a key role in various proposed means of quanti-fying soil structure. It also has a major practi-cal role in the prediction of hydraulic proper-ties. New pore size concepts, measurement techniques, and relations to transport phenom-ena are likely to remain a major emphasis in the study of soil.

The air in the soil is similar in composition to that in the atmosphere with the exception of oxygen, carbon dioxide, and water vapor. In soil air as in the atmosphere, nitrogen gas (dinitrogen) comprises about 78%. In the atmosphere, oxygen comprises about 21% and carbon dioxide comprises about 0.36%. However, in the soil air, oxygen usually is replaced by carbon dioxide, so both range from about 0.4% to 21%.Oxygen is used by plant roots and soil microbes during respiration, and carbon dioxide is released. Thus, in the soil, the oxygen levels are generally less than atmospheric levels and the carbon dioxide levels are generally greater than atmospheric levels. Some factors that determine the extent of the difference between atmospheric and soil air constituents include depth in the soil profile, soil pore size distribution, and soil water content.]

• Depth: Oxygen levels generally decrease with depth in the soil profile due to slow diffusion rates of oxygen from the surface through the soil.

• Pore size distribution: Soils with large pores promote more rapid oxygen diffusion into and through the soil, and carbon dioxide movement out of the soil. Soils with small pores have slower oxygen diffusion into the soil and carbon dioxide diffusion out of the soil. Sandy soils generally have low total porosity but large individual pores. Clay soils generally have high total porosity but small individual pores.

• Aeration and drainage: Soils with large pores generally have good drainage (less water) and aeration, while soils with small pores generally have poor drainage and aeration. Thus, sands generally have good drainage, while clays have poor drainage and are more likely to become anaerobic (deprived of oxygen) as microbes use oxygen more rapidly than it is replenished through diffusion.

• Soils with more pores filled with water have less space available for air, thus become anaerobic more rapidly than drier soils.

• Water vapor: Soil air has a relative humidity very close to 100%. (Relative humidity is the amount of water vapor actually in the air relative to the amount the air could hold at that temperature.) This is much different than atmospheric air, which may vary in relative humidity between 5% and 100%, sometimes within 24 hours in semi-arid and arid regions. A Boy Scout Survival Kit applies this concept by providing a shovel, piece of plastic, and a cup. Dig a hole in the morning, place the cup in the bottom of the hole. Anchor the plastic with soil around the rim of the hole. Place a rock on the plastic above the cup. During the day, the plastic allows the soil to heat, evaporating water. At night, the plastic allows radiational cooling. As the air cools, water vapor condenses on the plastic and drips into the cup. This provides enough water for subsistence. If cacti or other succulents are available, place some of their vegetation in the hole to enhance the water provision.

Dear Portia Phohlo,

We generally in soil science talk about total Soil porosity and  is related to bulk density through this equation :

PT ( %) = (1 - BD/PD)* 100 where  BD : bulk density and PD : particle density ( 2.65 g/cm3).

this theoritical equation which give good prediction on the total porosity of a given soil, tells us , that there is an inverse relationship between total soil porosity and bulk density. for your question of a soil with high porosity , we will presume that the soil had also a low bulk density.

What is the ideal soil porosity?

Porosity serve three main function which are retention, storage, and drainage ( aeration) .  total porosity is related to only the storage capacity of the soil I.E : the maximum amount of water can a soil store at a given time. but the two other function ( retention and drainage) are related the shape, the size of the pore which are dependant on soil texture, soil aggregation , soil organic matter.

the general principle here is that soil rich in clay to a certain extend ( so high in clay can  tend to have internal drainage problems) , and organic matter function in optimal way.

Soil rich in biological activity  tend to have more total porosity than soil poor in biological activity , because soil fauna create new pore with high  volume , so they tend to increse the soil porosity.

Is soil porosity of greater than 60% bad for the soil?

so , if we think in term of functionnality,  one of the problem with High Porous soil  is hydromorphic problems ( high storage capacity)  , and if there is an impermeable substratum in the subsurface, they will have low drainage capacity and low aeration.

In general , soil rich in organic matter tend to have low density and high porosity.

is there such a thing as too high organic matter in soil? i.e >15%

at my knowledge , these kind of soil are called Histosol  ( organic soil) and i will attach a link here that discussed the formation of these soil.

i hope this will be helpful.

Kind regard

Louadj yacine

The answer from Dr. Baboo is exceptional.  I denote from your question the interest in good or bad for soils.  The combination of soils with water and air, provide for land based life and habitat.  Although as humans, we may favor some soil types and characteristics for our own benefits and desires, the diversity of soils provides a wide range in habitat and conditions that contributes to diversity of flora and fauna.  Bad soils then would be unproductive, barren and unable to support native life forms.  Bad things for soils would be factors that alter soil function and productivity, or its ability to recover from activities or events in a reasonable period.  Soil condition and function has some natural range of variability as it responds to various circumstances of wet, dry, wildfire, severe storms, vegetation, etc.  When activities or actions take a soil out of its range for normal function and resilience, it presents a loss in natural function that result in changes of various types in habitat, productivity, and uses.  These changes can be more commonly bad but sometimes good, depending on who might be rating the changes.  A farmer may rate change different from wildlife biologist, hydrologist, forester, landscape architect, environmentalist, or urban developer.  Some change in soils in land management is avoidable, temporary, can be mitigated, while other changes constitute alterations that are difficult to reverse and may persist for extended periods.  It may be most important to understand when changes are made that cannot be easily or naturally reversed.  Soil scientists experienced in land management are perhaps the most prepared to suggest what to expect from changes to soils, and ways to adjust activities and actions to limit change, offer suggestions for recognizing soil capability, mitigation or improvement to soil conditions that can help landowners and managers make more informed decisions.

Certain wetlands have high organic accumulations and associated habitats and scenery are appreciated by some, yet not by others.   

Dear Portia,

(1) Typical bulk density of sandy soil varies from 1.5 to 1.7 g/cm3. This calculates to a porosity between 0.43 and 0.36. Typical bulk density of clay soil varies from 1.1 to 1.3 g/cm3. This calculates to a porosity between 0.58 and 0.51.

(2) Soils with porosity more than 0.60 may be too friable and may fail to support necessary anchorage/ stands of crop plants.

(3) Generally fertile mineral soils with less than 5% organic matter provide ideal combination of mineral, organics, water and air for optimum growth of most crop plants. Soils containing more than 15% organic matter are termed organic soils. They may support growth of only limited categories of crop plants. 

Excellent reply on the physical properties of soil. Just one additional comment or two.

The horticultural component of this is clear from the science of watering plants [correctly]. The optimum nutrient and/or mineral uptake from plants in soils requires proper watering. Water can fit into soil only if there is porosity.  If the soil is too porous, watering actually will wash nutrient away from the plants into streams and lakes.  If the soil is too non-porous, neither water nor nutrients reach plants in sufficient amounts for optimum plant growth.  The optimum watering routine [info directly from my horticulturalist spouse] is water only when the soil is just beginning to feel dry and then to re-soak the soil.  The physical chemistry of this is that one wants water staying around long enough to leach out nutrients from the soil and NOT flow away so the nutrients can be absorbed.  Once nutrients and water are absorbed, repeat by re-watering.   Porosity needs to be correct in a reasonably narrow range or the horticultural component will be compromised.   If one uses only inorganic nutrients and does not worry about the environmental impact, then this does not matter so much.

However, even in industrial scale agriculture or green house scale agriculture, seems to me one wants to design a system that tends towards limiting waste and minimizing cost. If the soil is too porous, the % of nutrients absorbed in plants versus the amount applied to the field will  be less than desired.  If the soil is too non-porous, % nutrients absorbed will also be less than desired.   The cost of nutrients are not the only loss: the number of times per year/season can be less if soil porosity is in the proper range.

Best regards,

Walter.

There are few points to be kept in consideration

1. Total porosity equals macro plus micro

2. macro porosity for aeration and gravity movement of water for redistribution within the micro-pores for recharging, too high value like in case of sandu and loamy sand is not that good

3. micro-porosity is the moisture held for subsequent uptake by crop plants, too high value is at the cost of reduced macro-pores in case of heavy clay soils where aeration becomes a problem

4. Pore size distribution decides the moisture retention, release behaviour and transmission character, which are imp from view point of plant health

5. Loamy and sandy loam appears to be optimal in terms of balanced m acro- and micro-pores distribution

6. Various resource conservation / tillage methods are defined to deal with the situations

7. crops/cropping systems choices are defined for different production environments,

regards

Dear Portia Phohlo

You are correct that Porosity should be 50 % for a well productive soil. Very high porosity may lead to leaching of nutrients and other toxic substances those could reach GW and contaminate it. This concern is in addition to economic losses of fertilizer leaching and low harvests of crops. Some pests will survive better in porous soils and to adversely affect plants and also could become carrier of many diseases, control of these will require additional funds..

Replies from all the colleagues are excellent covering the question subject pertinently. Yes, I agree to Dr. Dulip and Dr. Ghafoor that porosity more than 50% is not good because it will cause extra drainage of water and provide less support. 

i endorse the explanation of kundu

Its very simple that the aeriation is most important for proper root growth and respiration as well as optimum microbial activities. Ultimately proper blend or optimum aeriation should be around 50%. If exeeds may lead to voltalization or leaching losses or media may not support roots. The composition of soil says optimum OM content should be around 5% for optimum growth of the plant. It is very difficult to maintain 15 % SOM. For an instance the tropical rain ever green forest at Westren ghats of india contains around 5% OM. 

You are right Dr. Chandrashekara.

Dear Portia whats your opinion.

ARe you trying to grow any plants in organic rich soils.

Whats your requirement

Yes, Anil Kumar, you are right.

Getting back to ideal soil by definition, 45% mineral matter, 5% organic matter, 25% soil moisture and 25% soil air make a given soil productive. So, 50% of pore space occupied by water and air can be considered as most desirable. All soil operations need to aim to maintain the optimum space to hold water and oxygen. 

Well said.  Since I am a physical chemist, the question that of course arises is this ideal % by weight or by volume?  If one thinks of the water and air as exchangeable / renewable components, sufficient volume from porosity must be present or correspondingly, one would be slowing / restricting the rate at which air, water, and nutrients exchange in soils.  Wise horticulturalists are always checking for what is missing / getting low / being used up and adding it.  Porosity which is nothing but space is as essential a component as the components added by weight. .   .

Very overwhelmed by the responses and very informative too. Thank you all for your input. Prof Reddy, is it correct to say then that an ideal soil will have field capacity of 25% and a total water holding capacity of 50%?

Good question Prof. Poria. Field capacity is with reference to the soil moisture content. Whereas, the rest of the space beyond 25% for water, is to be under the jurisdiction of air. So, total water holding capacity as 50% becomes a misnomer which means minus air (oxygen?). Of course, pore space is a dynamic entity with continuous change in the content of water and air. Coming back to the question, Is too much porosity bad for the soil? The answer lies in the question itself. Too much porosity implies less of solid phase consisting of mineral and organic matter, volume being constant. Naturally the soil can not be productive to support life.

Dr. Pholo, it makes total porosity as 75, it is too much. The highest good value is 50%.

Is there any relationship to ash-rich soils in this study?  I ask that question as volcanic ash-rich surface soils formed in volcanic ash tend to be silty in texture and have high porosity and low inherent nutrient status and low inherent nutrient holding capability (McDaniel and others 2005).  There is considerable studies on the relationship between machinery activities on ash-rich soils and what effects such activities may have.  The question often at hand is length of time that such activities may place the soils in what we term as a detrimentally compacted physical capability.  Which opens up another door as to what in essence is truly detrimentally compacted - the length of time something may be detrimentally compacted and do we get a rebound or recovery curve?  All these factors do not pertain directly to your questions above but in areas containing a high ash content (Pacific NW) this is often something we are dealing with.  Just some different frames of thought when we look at bulk density and the relationship to a long time period.  Does recovery or change vary and if so - why?

Hi Portia

good questions …

1) Is 50 % ideal … ?! Yes and No, I believe you talking about agriculture land use ?! The soil porosity depend i.a. on the soil texture and tillage system (ploughing minimum-tillage or no-tillage). So for my understanding this is important to do an interpretation about a soil porosity value of 50% or 40% and what would be good or ideal.

An other point is to determined the pore size distribution and not only look at the total of soil porosity (pore volume). The difference pores size have different function at the soil:

- the coarse pores = air filled pores and bring down the rain / drainage water

- the small coarse pores and medium pores are keeping the plant available water and

- the fine pores are holding the not plant available water but important for micro-organisms and for chemical processes.

A sandy soil can have a pore volume (= soil porosity) 46% (± 10%) and pore size distribution coarse pores = 30% (±10); medium pores = 7% (± 5) and fine pores = 5% (± 3).

In comparison with the sandy soil are silty loam can have a pore volume 47% (± 9%) and pore size distribution coarse pores = 15% (±10); medium pores = 15% (± 7) and fine pores = 15% (±5).

And at least a clayey soil can have a pore volume 50% (± 15%) and pore size distribution coarse pores = 8% (±5); medium pores = 10% (± 5) and fine pores = 35% (±10).

(s. Scheffer&Schachschabel 2010)

All three soil have ideal soil porosity or pore size distribution but it depend on the soil texture. With soil tillage it is possible to change the pore size distribution but it is not possible to change the pore size distribution of a sandy soil into the pore size distribution of a silty soil. The change is just a little bit.

If you ask what the best soil porosity or pore size distribution will be, so it is nearly 50% pore volume and pore size distribution of coarse pores = 15%; medium pores = 15% and fine pores = 15%. This you can mostly find at fields with a silty loam (in Germany) and the farmer do a mulch seeding and conventional plough seeding. But if you have only sandy soil you can reach 50% pore volume but the pore size distribution will be difference.

2) 60 % soil porosity (= pore volume) is quite much; John mention already the ash-rich soils … or soil with a high content of organic matter (like peaty soil up to 30% and more organic matter). And also over-loosened soil (after ploughing) without an reconsolidation of the soil after tillage.

You mention … the soil aggregation would be poor .. you mean that the soil aggregation is very big and with a lot of interconnecting cavities between the soil aggregate?

There is always a relationship between bulk density and soil porosity!

3) Yes and No … if you have a peaty soil it is naturally that the organic matter content is very high: 15% and more (for Germany).

But if you bring a lot of organic matter like compost, slurry into a soil and the organic matter content increased much higher than the natural level would be, that can turn into problem. Now there is a potentially risk of a high mineralization and the (released) nutrient can leached into the ground water, e.g. nitrate leaching.

With best regrads from Osnabrück / Germany, Stefan

I will go by Stefan Hinck explanation on porosity

Drs. Prem Baboo and William F. Hansen have explained the effects of excessive soil porosity and its related information very correctly and precisely.

Suppose a soil has porosity levels which is not normal to a given soil, how can we alter this through management?. Addition of organic matter, deep tillage in heavy soils or passing rollars in coarse soils, altering crops and crop rotations, ammending with tank silt etc are routine recommendations. Is there any other means of altering soil porosity?

i dont know anything i need help

Sudboy Masamba,

No one knows nothing and  some people need  help  less,  others for a time  need help more.   Learning is cumulative until it show up as redundant.  When what we are learning shows up as redundant,  actually we may have stopped learning [more] at all. .    .       It is very healthy to own when and where we doubt what we appear to know.  .    .      The ancient value of pi is  22/7,  which is 99.9576% of the modern answer.   Both work to find the circumference of a circle knowing its diameter.   Each generation gets to,  even needs to,   re-discover what others know in their own way  or all we have will be exactly what we already have /know.

I think we are moving away from discussion. I repeat my question for a possible answer from this group

Suppose a soil has porosity levels which is not normal to a given soil, how can we alter this through management?. Addition of organic matter, deep tillage in heavy soils or passing rollars in coarse soils, altering crops and crop rotations, ammending with tank silt etc are routine recommendations. Is there any other means of altering soil porosity?

Porosity can be thought of the opposite of density.  If soil to too porous, it needs to be made more dense; if it is too dense, it needs to be made more porous.  In terms of soil management,  agricultural expertise and horticultural expertise is what is needed.  Easy to see more that one solution can work.  The science part is simple:  do half of the soil one way, the other half a second way and report in a peer reviewed journal your results.