Soil Density and Porosity

Soil is composed of solid particles of different sizes (minerals and organic matter) often "glued" together into tiny aggregates by organic matter, mineral oxides and charged clay particles. The gaps between the particles link together into a meandering network of pores of various sizes. Through this pore space the soil exchanges water and air with the environment. The movement of air and water also allows for heat and nutrients to flow. The number and size of pores directly relates to soil organic matter content, texture and structure.

Bulk Density is the weight of a given volume of soil which includes the pore spaces. It can be easily measured by gently pressing a small cylinder into the soil, removing the core and weighing after\drying to remove the water contained in the pore space. An average value would be 1.3 g per cm³. Coarse textured soils will usually have a higher bulk density because they have less pore space than fine textured soils. Bulk density is an important property of soils since it affects how easily plant roots can penetrate the soil when they propagate. Why are carrots often bent and misshapen when we grow them in a back yards? To grow a straight carrot you need a low and even bulk density as the carrot grows downward through the soil. Real Density is the weight of a given volume of the soil solids only. It would be equivalent to the average density of the soil minerals and the organic matter. An average value is 2.65 g per cm³

Porosity or pore space of soils is calculated simply from the Bulk Density (Db) and Real or Particle density (Dp)

Porosity = 1 - Db/Dp

An example of a soil that has a Real Density of 2.65 g per cm³ and a Bulk Density of 1.3 g per cm³ and converting to percentages.

% Porosity = 100 - (1.3/2.65) x 100 = 50%

What affects porosity? Texture, structure and organic matter are all important in determining the overall soil porosity. Coarse textured sandy soils have larger pores but much less pore space than finer textured clay soils. Why? See if you can think of a demonstration to illustrate this property.

**Soil Texture, Bulk Density and Porosity**
Texture	Bulk Density	% Porosity
Sand	1.6	40
Loam	1.2	55
Clay	1.05	60

Lower horizons within the soil profile tend to have higher bulk densities than upper layers. Subsoils are generally more compacted because of the overlying weight of soil. They usually have less organic matter to build the more open granular structure. Often subsoils accumulate clays and iron oxides which have been eluviated from upper horizons. These clays are often trapped in larger pores thereby reducing the overall pore space.

Two types of pores can be classified in soils - macro and micro. Pore diameters larger than 0.06mm are called macropores and those less as micropores. Most macropores exist between aggregates and are active in rapid movement of soil air and water which allows soils to drain. The micropores that exist inside individual aggregates do not allow air and water to flow readily. In very dry soil some micropores still hold moisture but surface tension effects are so high that plant roots can not extract it.

Man's activities can have a great effect on porosity-for example the compaction of soils by the wheels of heavy machinery. Any management practice that reduces organic matter, such as continuous cropping will reduce the granular structure of the soil and lower the pore space. Some Saskatchewan soils contain less than half the organic matter they had when they were broken a hundred years ago. Good cropping practices that include deep rooted plants in the rotation (pasture, alfalfa) will tend improve porosity