Soil Physical Properties

Just as all the soil’s properties can only be understood as a consequence of three conditions so each of these conditions must similarly be framed if to be successfully quantified. With the soil physical properties these three conditions are the three states of matter: the solid, liquid, and gaseous states. They are however not static states within the soil but change relative to one another in response to environmental, largely hydrological, changes.

There are various reasons why these states change but the most significant one is the period since the last precipitation event: the time since it last rained significantly. For the moisture state of a soil is one that changes constantly and one that strongly influences the soil physical properties . It is thus one of the most important conditions for the agronomist to know.

Expressed as fractions of the whole the soil physical properties are usually measured when the soil is at its maximum water holding capacity: its so called field capacity. Field capacity being the state of the sampling area following prolonged rain and after sufficient time for drainage has elapsed.

Whilst climate is the chief driver and regulator of the frequency and rate of precipitation events; it is the textural aspect of the soil physical properties, in particular the particle size distribution (PSD) that is the main determining factor in the fate and availability of water within the soil.

The size and arrangement of the soil particles, its texture, is what determines the porosity (the drainage and water retentive properties of soil) and through the chemically active clays and sequioxides, the availability of basic nutrients (chiefly Calcium, Magnesium & Potassium) is largely controlled.

Thus a good understanding of the mineral state and in particular the PSD of the mineral fraction is the first step in understanding and managing the soil physical properties. The simplest and most efficient method in the field is to use a hand analysis chart below. However hand analysis is a qualitative rather than a quantitative technique so whilst extremely useful for identifying soil boundaries and taxonomic similarities with other related soils it is only useful in an applied context where a degree of quantitative analysis has been performed on the soil physical properties.

hand textural chart by S Nortcliff and JS Lang from Rowell (1994)


Soil Physical Depth and Vertical Structure

Soil physical depth is one of those factors that is important only when it is absent. In general a plant needs a principle rooting zone of 50 cm, a penetrable subsoil of 1m and a deeper permeable layer leading down to the aquifer, but this is not always possible. The average depth of top soil across the entire planet is a smear of just 8cm depth and many soils lie on impermeable layers of rock or clay making them both prone to water logging and susceptible to drought.

Such thin soils are impoverished and whilst they are easily damaged and eroded by agriculture they are not necessarily unsuitable. By the nature of their limitation they respond well and are very suitable for the techniques advocated on this web-site.

Similarly these depths are arbitrary; the numbers have been chosen rather than calculated and relate to the arbitrary needs of a annual crop plant in a temperate climate. No government body scientific or agricultural committee has similarly chosen these numbers: only I have, and they have been chosen as much to aid general calculations as they have to reflect some ‘truth’. So whilst this may seem unscientific, it does relate very well with the practicalities of both working and managing agricultural land.

top soil depths : under 15cm, under 30cm, under 45 cm, over 50cm

rootable sub soil: nil, under 30cm, under 90 cm, over 100cm

permeable sub soil: nil, under 30cm, under 90cm, over 100cm

next: Determining Particle Size Distribution (PSD)

Free Cultural Works (CC BY-NC-SA) Malcolm McEwen (2011)

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