Soil Bulk Density & Compaction & Porosity

Bulk density is the oven-dry mass of soil divided by the total volume it occupies, including the pore spaces — expressed in grams per cubic centimetre (g/cm³). Because it counts the air and water gaps, it's lower than the density of the solid mineral particles and tells you how tightly the soil is packed.

Bulk density = oven-dry soil mass ÷ core volume. Push a ring of known volume into the soil, dry the intact core in an oven, weigh it, and divide. For example a core ring of 100 cm³ holding 135 g of oven-dry soil gives a bulk density of 1.35 g/cm³.

It depends on texture, but as a guide, sandy soils run higher and clays lower. Above roughly 1.6 g/cm³ most roots struggle to push through, water and air movement slow, and yields suffer. Values around 1.1–1.4 g/cm³ usually indicate a healthy, well-structured soil for cropping.

Use a steel core ring or soil core sampler of a known internal volume. Drive it into the soil so it fills without compressing the core, trim the ends flush, and remove the intact sample. Dry the whole core at 105 °C until the mass is constant, then weigh — that oven-dry mass is what you enter.

Porosity is the share of soil volume that is pore space (air and water). It's estimated as porosity % = (1 − bulk density ÷ particle density) × 100, using a particle density of about 2.65 g/cm³ for mineral soils. A bulk density of 1.3 g/cm³ gives roughly 51% porosity — good room for roots, air and water.

Most soil minerals — chiefly quartz, feldspars and clays — average close to 2.65 g/cm³, so it's the standard value for porosity estimates. Soils very high in organic matter or iron oxides differ, but 2.65 is a reliable default for typical agricultural soils and the basis of the porosity formula here.

Compacted soil has fewer and smaller pores, so roots can't penetrate, water infiltrates and drains poorly, and oxygen for roots and microbes is limited. The result is shallow rooting, waterlogging or drought stress, and lower yields. Measuring bulk density flags compaction early so you can loosen the soil.

Avoid working or driving on wet soil, control traffic to set lanes, deep-rip or subsoil compacted layers when the soil is dry, add organic matter to build structure, and use cover crops with strong taproots to open the profile. Re-measure after a season to confirm the soil is loosening.

They're as accurate as your sampling. A clean, uncompressed core of a known volume and a properly oven-dried, accurately weighed sample give reliable bulk density and porosity. Take several cores across a field and average them, since soil varies — bulk density is best read as a trend, not a single spot value.