What is it?
Bulk density is the weight of soil in a specified volume. As a soil quality indicator, the soil weight is measured after oven drying and the weight and volume of any stones are subtracted. The measure is then more correctly called ‘fine earth dry bulk density’. Bulk density provides a measure of how loose or compacted the soil is (the higher the bulk density, the greater the degree of soil compaction). Compaction is caused by either animal treading, the impact of heavy machinery, cultivation, or the loss of soil organic matter and subsequent desiccation, or some combination of these factors. Compaction reduces the porosity and stability of soil aggregates. The consequences may include reduced supply of air to plant roots, increased resistance to penetration that may limit root extension and germination, and reduced capacity of the soil to store water that is available to plants. Reduced water entry into the soil may increase water runoff over the soil surface. Compaction will increase soil strength, requiring increased tillage power (Shepherd 1992). Low bulk density can also be a problem as loose soils may be subject to increased risk of erosion.
Bulk density is also an important measurement for interpreting other soil health indicators because it is used to convert concentration values into weight per unit of soil volume. For example, per cent organic carbon may be converted into weight of carbon per unit of soil volume (necessary for calculation of carbon storage). The exploitation of soil by plants relates more to characteristics of soil volume than soil weight, so that volume measures may have more ecological relevance than weight measures.
How to interpret it
(Note: “very compact” and “very loose” are not shown in some soil type/land use combinations)
Very compact - The supply of air to plant roots is severely reduced and increased
resistance to penetration will very likely limit root extension and germination.
The capacity of the soil to store water that is available to plants is also reduced.
Impeded water entry into the soil may increase water runoff over the soil surface.
High soil strength will require increased tillage power.
Compact - The supply of air to plant roots is reduced and increased resistance to
penetration may limit root extension and germination. Reduced water entry into the
soil may increase water runoff over the soil surface. Increased soil strength will
require increased tillage power.
Adequate - Bulk density is normal for the soil type. Soils that have low secondary
oxides and have either low soil organic matter (Pallic Soils, Semiarid Soils and
Recent Soils) or high clay content (Ultic Soils) have normal levels of bulk density
that are relatively high. Soils with either high soil organic matter contents (Organic
Soils and Podzols) or allophanic soil material (Allophanic Soils and Pumice Soils)
have normal levels of bulk density that are relatively low.
Loose - Porosity is high and soil strength low. There is a risk of erosion is if
the soil is exposed to wind or running water.
Very loose - Porosity is very high and soil strength very low. The risk of erosion
is high if the soil is exposed to wind or running water.
How to improve it
Compacted soil can decrease root growth and result in greater runoff from precipitation. Amelioration of a compacted soil involves the reduction of any mechanical impact and restoration of vigorous vegetation cover. Growing plants add organic matter to the soil through root growth and decomposition of litter, and stimulate increased burrowing activity of the soil fauna. This has the effect of ‘puffing up’ the soil volume and increasing porosity and aggregate stability. Clayey soils are particularly vulnerable to compaction and similar to macroporosity, limiting heavy vehicle traffic, tillage, and intensive grazing on clayey soils when wet soils can also help to minimise compaction. If necessary, tillage or ripping may break up compacted layers.
Loose soils are likely to be more susceptible to erosion and may be compacted by rolling. Establishment of vegetation (particularly on slopes) or a cover crop can also help to create good soil structure and minimise surface erosion.
Technical details
The soil is sampled by pressing a cylindrical steel core of known volume into the soil. The sample is extruded from the core and dried in an oven at 105°C until the weight remains constant (taking about 12–14 hours). The sample is then weighed. (Gradwell &Birrell 1979). The results are reported in units of t/m3. When a sample contains stones, they have the effect of increasing the bulk density value and need to be discounted by measuring or estimating the volume of stones and subtracting their weight and volume from the total sample.
Reference material
Blake, G.R.; Hartge, K.H. 1986: Bulk density. In: Klute, A. ed. Methods
of soil analysis, Part1, Physical and mineralogical methods.2nd edn. American Society
of Agronomy, and Soil Science Society of America, Madison, Wisconsin, USA. Pp. 363–375.
Gradwell, M.W.; Birrell, K.S. 1979: Methods for physical analysis of soils. DSIR,
New Zealand Soil Bureau Scientific Report 10C.
Jones, M.G. 1993: Effect of soil texture on critical bulk densities for root growth.
Soil Science Society of America Journal 47: 1208–1211.
Shepherd, T. G. 1992: Sustainable soil crop management and its economic implications
for grain growers. In: Henriques, P.R. ed. Proceedings, International
Conference on Sustainable Land Management, Napier 1991. Pp. 141–152.
Viehmeyer, F.J.; Hendrickson, A.H. 1948: Soil density and root penetration. Soil
Science 65: 280–288.