Hi

If there is a soil compaction layer, the bulk density decreases with the soil depth from that layer. That may happen due to inadequate soil management in some agricultural systems. So, if the soil is compacted at a 15 cm depth, the bulk density will decrease with depth until it back to increase at some point.

See the figure on the link below as an example:

https://sdsoilhealthcoalition.org/technical-resources/physical-properties/bulk-density/

Hi

If there is a soil compaction layer, the bulk density decreases with the soil depth from that layer. That may happen due to inadequate soil management in some agricultural systems. So, if the soil is compacted at a 15 cm depth, the bulk density will decrease with depth until it back to increase at some point.

See the figure on the link below as an example:

https://sdsoilhealthcoalition.org/technical-resources/physical-properties/bulk-density/

May to be a compaction generated by machinery, in particular when an heavy harvester combine used with wet soil. but more information are needed to reply to this question

Bulk density is the weight of soil in a given volume. Soils with a bulk density higher than 1.6 g/cm3 tend to restrict root growth. Bulk density increases with compaction and tends to increase with depth.

Bulk density usually increases with depth in the soil profile. Generally the 0-15 cm bulk density will be lower than the 15-30 cm depth due to disturbance by tillage, macro and micro fauna, type of plants growing on the soil-especially grasses such under prairie vegetation, repeated wetting and drying or freezing and thawing (which is a type of disturbance), etc.

In my experience with several thousand soil profiles in the Great Plains of the U.S., the greatest increase in bulk density occurs in the surface 30 cm, often with the density still increasing somewhat below that depth depending on soil texture, soil layering, presence of rock fragments, parent material etc. Most of the bulk densities below 30 cm to 100 cm that I have seen in this region are usually in the range of 1.35 to 1.45 g/cm3.

Under long-term no-till management (usually > 5 years) the bulk density in the 0 -30 cm depth will attain a bulk density of 1.30-1.35. This in not necessarily due to soil compaction but to soil consolidation. But, soil water infiltration and permiability can be much higher than in lower bulk density, tilled soils because of permanent macropores created by root channels and small fauna like eartsworms and some insects.

Others things kept constant, bulk density usually increases with depth as organic matter content decreases.

All previous consideration are true! in 99% bulk density increase with depth, and this is the standard.

in 1% of cases first layer under heavy tillage, heavy machinery, poor soil management bad structure, low organic matter could show a bulk density greater that deeper layer. But absolutely, it happens only seldom and in particular conditions.

This is unlikely situation. However, bulk density can theoretically decrease if clay and organic matter contents increase and sand content decreases down the depth.

It rarely does but it is possible. Given a soil that is uniform in structure and has been compacted maybe by tractor or otherwise. The top soil in this case will have more mass per unit volume.

In the case of severe erosion, the light top material may be carried away, leaving a hard layer that may be densr than the undelying horizons.

Faunal channels and krotovina in the bottom horizon may result to lower bulkdensities compared to top horizons.

As I stated before, bulk density usually increases from the surface downward unless compacted by tractors or other equipment traveling over the surface. Yihenew and Marco also brought up an excellent point about the relationship with organic matter. If the surface has very low organic matter and has been pulverized by tillage so that essentially no natural soil aggregates exist, after rainfall ( or a good wetting)(and disregarding erosion) the soils could become very compacted, thereby, providing bulk densities in the surface layer higher than the layer below it. As mentioned before, this is very rare and a field in this situation would not be a very good one to conduct studies on, unless the study wao on the nature of the compaction.

Another situation that can be common is many parts of the world is the presence of small rock fragments in the layers below the surface. It is difficult to get good, reliable data using standard bulk density measurement techniques. In this situation, a large volume of soil needs to be collected from the layer in question. The fragments and soil need to be separated and mass of each separate determined. Since rock has a different density than the soil (but not necessarily different from soil particle density) because of its lack of porosity, bulk density can be estimated by the fractional mass due to the soil and rock components in a given volume of soil. This estimate may or may not be useful for the intended purposes for the bulk density determination.

Thank you so much for all good answer.

Important information

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Atilla Polat

Yes, it is possible for bulk density to decrease with soil depth. Such a trend can be observed in soils affected by intensive human activities where the surface soil horizon is more compacted than the subsurface horizons.

It has been reported that bulk density of the soil is higher with no tillage or conservation tillage upto a definite depth...beyond that depth, there is no or even little variation in bulk density irrespective of conventional or conservation tillage....but in the conventional tillage, bulk density decreases as volume of the soil particle increases as compared to mass of soil solids upto a definite period of time but after cessation of rainfall or irrigation events, bulk density increases...

Decrease or increase of the bulk density of soils with depth depends upon the sediment texture and compactness including the presence of the organic matter content, generally bulk density of soil increases with depth with respect to decrease of the content of the soil organic matters inclusive soil nutrients.