Last week covered pore space, or the holes in soil. This week we look at the opposite of holes — the solids in soil.

Pore space is expressed as a percentage of soil volume, while solids are expressed as a mass per volume. This measurement is referred to as the soil’s bulk density, and is expressed as grams of dried* soil per cubic centimetre (g/cm³), or grams per millilitre (g/mL), as a millilitre’ is equal to a cubic centimetre.

(*The soil sample is first dried at 105°C in a special-purpose oven over several hours to constant mass to ensure all water has been removed.)

The solid parts of soil are its mineral and organic matter components. Mineral soils could have a bulk density from 1-2 g/mL while organic matter could range from 0.05-0.2 g/mL.

The lower density of organic matter compared to that of minerals means that as soils become richer in organic matter, their bulk densities decrease. The more organic matter, the more soil crumbs will develop, and the more pore spaces will form, and the lower the bulk density becomes again.

Bulk density is not an intrinsic property of a substance like density is. Density of, say, fresh, pure water is always 1,000 kg/m³ at 4°C — this is an intrinsic property of water and never changes. Bulk density, on the other hand, is a property of solids that exist in powdered, granular, or other particulate forms. As such, bulk density of a substance can change, usually from particulates settling further in a container or being compressed (compacted).

As soil solids are particulates, their (and the soil’s) bulk density will change if that soil is compacted, such as from heavy traffic.

You can actually do a very simple field test for compaction with nothing more than your palm and a sharp pencil! You need moist soil, so this is best done a day or two after rain or irrigation. You may even like to compare an area with a lot foot traffic and sparse, trampled grass, with one showing healthy plant growth nearby.

Place the pencil’s point on the soil surface, and, making sure that the pencil’s top is in the centre of your palm, try and push the pencil into the soil. If you can do this without your palm hurting, then plant roots too will be able to penetrate that soil. If your palm hurts before the pencil goes in a centimetre or so, the soil is compacted and it will need cultivation to break this up.

Soils with a bulk density of less than 1.5 g/mL are considered optimal for good air and water movement. Higher values indicate poorer soil structure due to compaction.

Soils compact more in the presence of water. This is because water acts as a lubricant to ’squish’ the soil particulates together. Dry soil particles will tend to form a powdery surface as they grind against each other under pressure.

Next week we’ll go over the implications of wet soil, compaction, and the effects this has on plant growth.