digging into dirt Soil

Learning objectives

To learn more about soil, and why it is important
To use this knowledge to understand human's policies and actions that may have an impact on soil health

Table of Contents

We often just think of soil as dirt, and many of us no longer even touch it on a daily basis. We take it for granted. But without soil, humans would not exist.

The soil is the great connector of our lives, the source and destination of all.
(Wendell Berry, 1977)

What is soil?

Soil is the sum of its abiotic and biotic parts -

mineral particles of different sizes and compositions derived from underlying rock
mineral particles brought in from outside influences e.g. wind, moving water, animals
gases and moisture adhering to the other components
decaying organic matter (humus) - sometimes shortened to OM
animals, plant roots, microbes and fungi that live among the minerals and humus

Soil particles are aggregated into clumps of varying sizes. These are often called peds. The spaces between them are filled with water and air.
A "standard" composition of soil is shown in the accompanying diagram.

Mineral composition of soil

Soil contains a number of chemical elements in varying concentrations.

Soil formation

Soil develops over long periods of time. Fundamentally, soil is derived from a so-called parent material, which consists of rocks and minerals that occur within a metre or so of the surface.

Five main interacting factors affect the formation of soil:

parent material—minerals forming the basis of soil
living organisms—influencing soil formation by their actions
climate—affecting the rate of weathering and organic decomposition
topography—grade of slope affecting drainage, erosion and deposition
time—influencing soil depth Under temperate conditions, it can take about 20,000 years to create one metre
of soil. (What does this mean in terms of renewability?)

Interactions between these factors produce an infinite variety of soils across the earth’s surface.

What does soil do?

Soil serves several purposes

a medium to anchor land plants in place
a source of necessary nutrients for land plant growth
a home for many species of organisms
a recycling service for organic matter
storage of forms of carbon, nitrogen and other elements
water storage and filtration

The abiotic components

Soil layers

Soil Texture

The solid mineral particles in soil (roughly 45%) are classified on the basis of size as sand, silt and clay.

Particle name	Particle Size in mm
Sand	0.1 - 1.0
Silt	0.002- 0.05
Clay	< 0.002

The relative proportions of these particles determines the texture of the soils. The soil texture table is used to relate the % of each particle size grouping to the designated texture.
Most (but not all) crop plants thrive in a loam soil because it retains moisture without getting too soggy and it has adequate mineral nutrition available.

comparative sizes of different soil particles

Other properties of soil

Soil colour

Colour is determined by the nature of minerals present. Officially the colour of soil is determined by comparison to colour swatches in a special book called a Munsell chart.

Density

Density is the measured by dividing the weight of a given amount of soil by its volume.

Porosity

Pore space is that part of the bulk volume that is not occupied by solid matter. It is open space occupied by air and/or water. This air space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients.

Consistency

Consistency is the ability of soil to stick together and resist fragmentation and deformation. It is of use in predicting cultivation problems and also the engineering of foundations of built structures. The consistency of dry soil ranges from loose to hard, whereas that of wet soil ranges from non-sticky to sticky.

pH

The pH scale is a measure of the acidity or alkalinity. It ranges from 0 most acidic to 14 most alkaline. 7 is regarded as neutral, neither acidic nor alkaline.

Most soils range between 3.5 to 9.5.