What is soil?
The main source of information for this learning topic is:
Handreck, K.(2001). Gardening Down Under: A Guide To Healthier Soils and Plants. CSIRO Publishing.
To begin to regenerate our local soils, we must first understand what it is we are working with.
In this section we aim to provide a platform for widespread knowledge and understanding of soil. When we understand what soil is, we can begin to understand why it is important to life on earth.
Soil is a natural living body composed of animal, mineral and organic components differentiated into horizons of variable depth which differ in structure, physical makeup, chemical properties, composition and biological characteristics. Anyone that has ever dug a hole or played in soil, knows that soil changes from one place to another.
Soil Components:
Handreck (2001) states that although every soil has the same types of components, soils look and behave differently from one another because:
They have different proportions of the main components.
Soils are grouped together in different ways.
There are many different types of minerals in the inorganic fraction, and each soil has different proportions of them.
Soils consist of smaller and larger particles arranged in widely different proportions.
They are formed in a specific way, unique to their environment.
The 5 main components of soil are:
Mineral particles - containing small particles of rock and other minerals that have been produced from these rocks by weathering.
Organic materials - humus and the dead and decaying parts of plants and soil animals.
Water - the ‘soil solution’, in which nutrient elements for plants are dissolved.
Air - which fills the spaces between soil particles not filled by soil solution.
Living organisms - ranging in size from small animals to viruses.
Soil Profile:
The soil beneath our feet has formed over millions and millions of years, as it forms, it creates what is referred to as a “Profile”. If we imagine a piece of soil cut like a cake, we can see that the soil profile is made of horizons of variable depths and types.
The soil profile is made up of distinct horizons. Each different horizon within a soil profile contains different characteristics and properties. Each of these different sections are made up of different mineral and organic particles. The following image displays a typical vertical cross section of a soil profile.
O - The organic horizon ‘O’ is a layer of organic material.
A - The Topsoil ‘A’ contains a relatively high organic matter content. This horizon contains the maximal amount of biological activity within the soil profile.
B - Subsoil ‘B’ contains a composition of altered and distinct material, strong colours with increased material density.
C - ‘C’ horizon is composed of weathered parent material.
R - ‘R’ horizon is known as regolith, commonly consists of parent rock
Physical Components of Soil:
Sand, silt and clay are the main physical components of a mineral soil. The composition of these physical components is always different. Loams and Humus are materials composed of a high amount of organic matter. Soil texture refers to the way soil feels. It is about how rough or smooth a specific substance presents. The texture relates to the size and proportions of the particles which are present.
Sand has the largest particles and can occur on its own. The large particle size means organic content is usually very low and moisture holding capacity is poor.
Silt has a much finer particle size but on its own may still be low in organic material. Its water holding capacity is better than sand but is still far from desirable/
Clay has the finest particle. The very small particles produce a sticky texture when wet. Clay is blocky and hard when dry.
Whilst these can occur on their own, most often soils are a mixture of both mineral and organic elements. Such mixtures are called loams.
Humus is important to all life forms inhabiting the soil. It is the most fine organic material size. It is the result of decomposed plant and animal matter.
Handreck (2001) illustrates soil particle size and the respective names as shown by table 1. These characteristics and proportions affect the overall structure of the soil.
Table 1. Name and size of soil particles
Soil Structure:
Soil structure is the term used to describe the way soil particles are arranged together. The mineral particles and humus are grouped together into crumbs or aggregates. These are arranged together so that there are spaces called pores between them. This arrangement give soil its characteristic structure. Good structure is an essential aspect for excellent plant growth.
Good structure is important:
Good structure enables plant roots to grow easily through the soil in search of water and nutrients. Plants in such soils use a minimum amount of energy in pushing their roots through the soil and so are able to use most of it for shoot growth.
Good structure allows for ample supplies of oxygen from the air to percolate down to roots
Good structure allows water that falls on soil to quickly soak in
Good structure allows rapid drainage of excess water
Poor structure is:
Inclined to remain waterlogged for extended periods during rainy seasons.
Soil surface prone to collapse during rain, with the collapsed material forming a dense crust. This crust will prevent water from penetrating deeper into the soil profile.
Clods or cloddy structure gives a very rough seedbed, where germination levels are very low.
Commonly difficult to dig.
Soil, Plant health and Testing:
In situations where soils are showing signs of poor structure and plants are displaying evidence of poor growth, pest and disease, the first step in management is to perform a soil test. The test results will determine the specific techniques required for restoration of soil health.
Essentially, soil restoration is the process of improving the structure, microbial life, nutrient density and overall carbon levels within the soil. Soil restoration and regenerative practices revolve around the use of organic inputs. The type of organic input, the amount required and the frequency in which it is applied are all situation specific. These variables are dependent on the long-term requirements of the soil and its specific history.
Regenerative gardening practices nurture soil health by promoting fertility, improving soil structure, sequestering carbon, fostering biodiversity and conserving water. By implementing these practices, we can create resilient and sustainable ecosystems that benefit both the environment and human wellbeing.
When selecting plants for your garden, it is important to consider the soil type, texture and structure. The soil test investigation will provide peace of mind when choosing plant species for your specific garden area. It will determine whether or not a plant is likely to settle comfortably where you want to put it. Resilient and sustainable ecosystems are those that utilise plant species that appropriately suit the conditions within those specific environments, otherwise known as microclimates.
More information on soil testing, organic inputs and how to treat specific soil types can be found by locating these specific learning topics.
