5.5: Porosity

Porosity (percentage of total pore space) is the volume of the soil occupied by pores.

\[ \%P = \frac{V_p}{V_t} \times100 \nonumber \]

Where %P is percent porosity; Vp is volume of pores; Vt is volume total (solids plus pores). The total volume of pores is usually greater in a well-structured fine-textured (clayey) soil than in a coarse-textured (sandy) soil. However, the individual pores tend to be larger in sandy soils than in clayey soils (which have many more micro pores). But why can a soil which has larger pores have less total pore space than a soil which has smaller pores?

Although a soil’s total porosity is important, its pore size distribution is equally important. Individual pores can be categorized as macro pores and micro pores. The large (macro) pores drain quickly of excess water and allow free movement of air and water. These macro-pores promote soil aeration (free movement of gases) and enhance water infiltration, percolation and drainage. Soils containing a large proportion of macro pores are usually very sandy and tend to retain only a limited amount of water. The small (micro) pores retain much more water, and consequently drain slowly and have restricted air and water movement. Restricted soil aeration causes reduced plant growth because roots need oxygen to conduct the process of root respiration in every cell. Also, many microorganisms require oxygen - therefore, some biological and chemical reactions are inhibited by poor aeration.

Aggregation, or the clustering of the soil particles into aggregates, creates larger macro pores between the peds, which enhance aeration and root penetration. Within each aggregate exist smaller micro pores that function primarily to retain water. A balance between macropores and micropores is desirable for most agricultural situations to provide both adequate aeration and optimum water retention for crop growth.

If the bulk density and the particle density of a soil are known, then the porosity can be calculated by using the following relationship:

Since, 100 % total soil volume = % solid volume + % pore volume then, % total pore volume = 100% total soil volume - % solid volume

\[ \%P = \%V_t-\%V_s \nonumber \]

This last equation can be used to determine the porosity, if one recognizes the % solid volume can be obtained by dividing the bulk density by the particle density and multiplying the quantity by 100. The resulting equation becomes:

\[ \%P = 100\%- \left( \frac{D_b}{D_p} \times100 \right) \nonumber \]

Note, the units of bulk density and particle density must be the same for them to be able to cancel in the calculation. The term in brackets, i.e. [bulk density / particle density x 100] represents the percentage of the soil volume occupied by the soil solids. When the volume of soil solids is subtracted from 100%, the difference is the percentage of the soil volume occupied by pores, or the total porosity or total pore space.