Soil compaction is basically reduction of the larger pore size distribution, which means reduction in gravity pores through which water moves down. 

In case of sandy and loamy sand soils the extent of macropores needs to be reduced, for which we carry out compaction

The condition of compaction is not irreversible, and with time the near restoration may take place due to buffering action

Through compaction, we may alter soil moisture retention, release and transmission characteristics, as well alter soil mechanical impedance. All these changes are important for effecting crop growth

If we wish to achieve maximum compaction, the term proctor moisture ontent is used i.e. the moisture content at which bulk density of the soil is maximum.

Soil compaction, basic aim is to reduce the soil permeability, through which soil water balance components, soil aeration components, soil mechanical impedance gets altered, which may or may not alter the crop yield depending uon the soil texture, organic carbon content, 

regards

Dear Dr. Ren,

I think out of the four listed properties, penetration resistance is the most sensitive property of soil compaction level. Farzaneh et al. (2012) prepared soil compaction map of their farm based on penetration resistance (cone index) values of soil at different depths. Given below is the reference of their work.

Ref: Farzaneh B, Almassi M, Sadeghi M and Minaei S (2012) Assessment of Soil Compaction Bulk Density Indices and Cone Index in Different Moistures and Depths for Application in Precise Tillage. World Applied Sciences Journal 20 (12): 1704-1712.

Hope you find this input interesting and useful.

I think bulk density(BD) would be the most sensitive property. Due to use of heavy agricultural implements most of soil of the N-W parts of the Indo-Gangetic alluvial plains , BD has increased so much that caused a remarkable reduction in saturated hydraulic conductivity in non-saline and non-sodic soils. You may refer a paper by Tiwari et al. 2014. Current Science, Vol. 107, No. 9, pp. 1431-1439.

Soil compaction is the reduction of soil volume by external factors, it mainly reduces the large pores.I partially agreed with the Dr. Kundus observation that penetration resistant is a very important factor because the density equal to 1.7g/cubic centimeter restricting root growth. I also partially agree with Dr. Pal because temporal variability of soil hydraulic properties effect a lot on soil quality. But I think, the most harmful effect of compaction is the deterioration of soil structure and very adverse effect on soil microbial population as well as nutrient transformation due to less oxygen infiltration.

interesting discussion

Nice discussion really. I think , particle size distribution will dictate  both, the intensity and magnitude of soil  compaction , since it will distinctively affect the pore volume of the soil, hence moisture content , a major driving force to participate in  soil compaction...

I agree with most of the comments made before. However I would like to introduce the importance of the depth where compaction takes place. If it is very shallow, the most important problem it's probably the fact that roots can not go down (as J. C. Tarafdar has said). But it is possible to break it more or less easily with tilling techniques. But when compaction takes place deeper, perhaps the roots do not need to go so deep but Ks reduction could lead to flood (that make imposible to roots breathe) or runoff situations (loosing your most fertile soil).

I do not think that only one principal aspect could be remarked, but diferent aspect for each kind of soil, crop or climatic region. 

Jose , good point of  further discussion , you have added. Are we talking of a  soil compaction easily warded off  by tilling or a compaction  having adverse effect on plant growth etc.. 

Porosity of soil I feel will have the greatest impact on soil compaction, soil moisture also plays an important role along with soil texture. 

Soil Cohesion properties.

I believe that the most affected land ownership is the permeability of the same: are affected water availability penetration of roots, gas exchange and nutrient availabilityI 

Compliments Dr. Kalra for good reply. For maximum compaction, the term proctor moisture content is used. Compaction will reduce the soil permeability, through which water balance and aeration components, soil mechanical impedance gets changed, and impact varies with the soil texture and organic carbon available into it. But in case of conservation agriculture in one or the other way we allow soil compaction to prevail through zero tillage. It may or may not affect yield.

Size of the soil aggregates , which is again a function of both soil texture and structure , could display an  eventual effect on soil compaction ..?

Dear Lidong

The most sensitive properties to soil compaction is air-filled porosity (macroporosity).

Increasing in soil bulk density is related to decreasing in soil porosity especially decreasing in macroporosity.

You can read my papers that you can download those in researchgate.

Dear all, thanks for all your valuable information and very interesting discuss. It seems that we can hardly say which one is the most sensitive factor to soil compaction but those properties are really indicated soil compaction levels.

i think that soil texture and structure could effect the agregat size and thus will affect the porosity , permiabillity ...

Hi Lidong,

good questions … which one is the most sensitive (soil) property in relation to the level of compaction … that not easy. I believe …

As you know, the soil properties are correlated. Thus, a negative relationship exists between pore volume and bulk density, or a positive relationship between soil penetration resistance and bulk density.

If the soil be driven, e.g. with a tractor, a mechanical load is applied to the soil. And if this mechanical load is greater than the support forces of the soil, the soil will be compacted. In generally the wide/large coarse pores will be compressed at first. If this mechanical load is smaller than the support forces of the soil, the soil will carry the tractor just with a little effect of compaction.

We may have different soil conditions (e.g. plowed or no-till field, sand or loam, etc.) or different mechanical soil loads (wheel loads, contact surface pressure, etc.). This must be considered. A plowed soil has to be re-compacted. But a plowed soil is very sensitive to mechanical stress and tends to harmful soil compaction when this soil is (be) driven with heavy machinery. A soil of a no-till field has usually a higher soil support forces and can carry a higher mechanical strength. But is the mechanical load to high so also this soil will be harmful compacted. This are 2 examples for level of compaction. A plowed soil has only a very little soil compaction and a no-till soil is compacted but is not harmful compacted.

Which is the most sensitive soil property … ? That depends on, what you are looking for. If you are looking for water movement or drainage capacity so the wide/large coarse pores are the only one, which can drain the water into the soil. If you are looking for plant production all your mention soil properties (Ks, soil penetration, air permeability and the shape of soil water characteristic curve etc) are important. Please consider that different soil texture have different soil pore volume characteristic etc (s. also: https://www.researchgate.net/post/Is_too_much_soil_porosity_bad_for_the_soil).

If you do a soil compaction, e.g. a re-compaction of a plowed soil, the pore volume especially the coarse pore will be reduced and the bulk density will increase and also the penetration resistance increased. And also the other soil process will be affected. And this will also happened, if you drive with (to) heavy machinery on a field. But now the soil compaction is harmful, that mean the wide coarse pores nearly all destroyed, the bulk density is high … and the plant roots can not penetrated the soil. Without root growth no water and nutrient uptake and this mean no plant growth ...

For my option, look for the whole system soil and take your reference to your research question.

With best regards from Osnabrück, Stefan

Hi Stefan Hinck, thanks so much for your detail explanation. I agree with you useful information. 

I agree with the suggestion of Dr.Kundu .One can measure soil penetration resistence with cone penetrometer which is commonly used and so results can easily be compared and interpreted.Of course various versions of penetromters are being used  from hand held to tractor mounted.

Soil compaction is compacting soil according to ASTM standards to obtain maximum dry density by adding optimum moisture content to achieve minimum settlement, shrinkage, frost, reduce permeability and to increase soil strength and bearing capacity.

Apart from use of cone penetrometer to measure penetration resistence, a good measure of bulk density is important in compacted soils to get an idea of the macroporosity .Bulk density can an indicative of ease of root penetration, water movement and soil strength.Conventionally intact soil clads have been used for measuring bulk density of soils.Two interesting scanning methods are available to measure soil bulk density nondestructively and fast. Scientists at the University of California- Riverside used an automated desktop three dimentional scanner to measure bulk density of soil clods (results published in November -December 2008 issue of Soil Science Society of America).The soil macropore system was also analysed with the help of image analysis,comparing images from computed-tomography(CT) scanning with images obtained from thin sections. Through three-dimentional analysis of the CT scans, boundaries of compacted soil layers could be identified to the nearest 1mm.

The characterization of soil macroporosity with CT scanning. By Grevers et al .

Can.J.Soil Sci.69:629-637(Aug.1989) provides the details. More recent literature is also available on this aspect.

Bulk density, soil texture is most sensitive soil properties for soil compaction.