Importance

Soil porosity, and particularly macroporosity (or large pores), influences the movement of air and water in the soil. Soils with good structure have a high porosity between and within aggregates, but soils with poor structure have restricted drainage and aeration.

Poor aeration leads to the build-up of carbon dioxide, methane and sulphide gases, and reduces the ability of plants to take up water and nutrients, particularly nitrogen (N), phosphorus (P), potassium (K) and sulphur (S). Plants can only utilize S and N in the oxygenated sulphate (SO42-), nitrate (NO3-) and ammonium (NH4+) forms. Therefore, plants require aerated soils for the efficient uptake and utilization of S and N. The number, activity and biodiversity of micro-organisms and earthworms are also greatest in well aerated soils and they are able to decompose and cycle organic matter and nutrients more efficiently.

Assessment

• Remove a spade slice of soil (about 100 mm wide, 150 mm long and 200 mm deep) from the side of the hole and break it in half.
• Examine the exposed fresh face of the sample for soil porosity by comparing against the three photographs in Figure 5. Look for the spaces, gaps, holes, cracks and fissures between and within soil aggregates and clods.
• Examine also the porosity of a number of the large clods. This provides important additional information as to the porosity of the individual clods (the intra-aggregate porosity).

Good
Moderate
Poor

Scoring

Good condition: Score 2

Soils have many macropores between and within aggregates associated with good soil structure.

Moderate condition: Score 1

Soil macropores between and within aggregates have declined significantly but are present upon close examination of clods, showing a moderate amount of compaction.

Poor condition: Score 0

No soil macropores are visually apparent within compact, massive structureless clods. The clod surface is smooth with few cracks or holes, and can have sharp angles.

References

http://adlib.everysite.co.uk/adlib/defra/content.aspx?id=000HK277ZX.0HDECKKQLJIF9JD

ftp://ftp.fao.org/docrep/fao/010/i0007e/i0007e01.pdf