Soil: What it is and where it came from

It has always amazed me how many of us are ignorant of the earth we stand on.

Even gardeners are often confused about where their soil came from, what type it is and how it can affect the plants that they grow.  Many will follow standard instructions on adding sand to clay to help it drain, and lime to sweeten it, without really understanding why they're doing it or the effect these actions may have.

So, to try and increase our knowledge a bit, this is going to be the first of a series of posts designed to give the average Joe (or Wayne if you're an Aussie!), a good working knowledge of soil.




This info is based on my old soil science notes, which are now approaching 20 years of age, and updated knowledge from further readings (occasionally I'll link you to this, but remember, Google is your friend!) and time spent being taught by Chris Alenson.  If any of you are lucky enough to attend a lecture or course led by Chris, then I really recommend you sign up as soon as possible!

What is this dirt stuff?

Quite simply, dirt, or more correctly soil, is the outer layer of the Earth's crust.  It is loosely arranged and sits on top of a rock layer, which is often (but not always) the parent bedrock.

Soil is highly variable, even over a distance of a few metres or less, and you may have several different types or variations of soil even in your small garden.

Even with this variation however, there are 4 ingredients that all soils are made up of: mineral, organic matter, water and air.  In general, the average top soil is consists of around 46% minerals, 25% water, 25% air and 4% organic matter.    The average subsoil is mainly mineral in content, with around 3% consisting of the soil biota (living plant and animal life) and organic matter.

In healthy soils, there is a 5th component: the biota.

Minerals

These are the remains of parent rocks, that have broken down through the process of weathering, over many thousands of years.  Due to the amount of minerals in soil, the characteristics of a soil (it's 'texture') are heavily influenced by the parent rock.  

The 4 main particle types that make up the minerals are gravel, sand, silt and clay.  They vary in size from being over 2mm in diameter (gravel), to under 0.002mm in diameter (clay).

Clay

Soils are often described based on the amount of clay that they contain.  This is because clay particles have a negative electrical charge.  This charge allows them to hold and exchange nutrients, effectively creating a reserve of nutrients, and is the reason that clay soils are generally more fertile than sandy soils.

This electrical charge, and the flat structure of clay particles, also causes them to stick together.  As a result, the pore spaces (gaps between particles) in a clay soil are very small.  These small pores are very good at holding onto water, but not so good at letting in air.  Consequently, clay soils can become very dense and compacted, making them difficult to work.

Gravel, Sand & Silt

These particles do not have an electrical charge and are not very good at holding onto nutrients.  However, nor do they stick together and therefore have larger pore spaces between particles, allowing more air in.  Unfortunately, the larger pore spaces also mean that water drains away freely and this, coupled with the lack of nutrients, means that soils with a large proportion of these particles are generally low in fertility.

Organic Matter

This is possibly the most important part of a soil.  It consists of both living micro-organisms and decomposing organisms.  Organic matter provides great benefits to the soil at all stages of it's cycle.

The living organisms feed on organic matter and the products of plants.  They work to decompose organic matter further, allowing nutrients to be released for the uptake of plants and other organisms.  Some, decompose the organic matter into it's last, completely decomposed form, which is humus.

Humus consists of insoluble matter, humin and humic acids.  It is highly insoluble and often tightly bound to other particles, meaning it is not subject to further deterioration.  It is therefore relatively stable within the soil and provides a source of organic carbon, which greatly improves soil structure.

Compost that you make at home is not quite humus, as it can still be decomposed further.  It does however have an electrical charge, similar to clay, and can therefore attract and retain nutrients and water.

The Biota

Although this can be considered part of the organic matter, it refers more to the living plant and animal life in the soil.  It includes the following:

• microfauna & microflora (1-100 micrometres) - yeasts, bacteria, fungi, protozoa, roundworms and rotifers;
• mesofauna (100 micrometres to 2mm) - tardigrades, mites and springtails;
• macrofauna (2mm to 20mm) - earthworms, woodlice, beetles, centipedes, snails, slugs, ants, small spiders;
• megafauna (20mm and upwards) - anything that lives in the ground from larger spiders such as the Victorian Funnel-web, to rodents, rabbits, foxes, wombats etc. 

Air & Water

These are the non-solid elements of a soil and should be available fairly equally in a good agricultural soil, because both are essential for plant growth.  There are various factors which affect the percentage of air and water present (the type of soil, the climate and the season for example), but they should comprise around 30-50% of the soil.

It is obvious why water is necessary to plant growth, but many people are surprised to find that plant roots require air for respiration, just as the leaves of a plant do.

Where did it come from?

I've already partially answered that question by referring to 'parent bedrock' etc when talking about minerals above.  However, it is not only the rock that is a factor in soil formation, but also the physical features of an area, it's climate, the biological activity there and, of course, time.

Meet the Relos!

As mentioned, the composition of the parent material strongly influences the soil that is formed.

Rocks are a combination of minerals which formed millenia ago.  They can be classified into 3 broad groups:

1. Igneous - volcanic rock which solidified on the surface, such as granite and basalt.  These can make fertile soils (basalt tends to form clay soils and granite can form either clay or sandy soils);
2. Sedimentary - formed from particles of other rocks carried in air, ice and water, which form layers and then are subject to great pressure, creating rock.  Examples are chalk, sandstone, limestone and shale and these are generally formed from silica based minerals, which tend to have little in the way of nutrients.  They therefore make sandy, fairly infertile soils, but they are great for preserving information about the Earth's past and sure look pretty!
3. Metamorphic - these are actually either igneous or sedimentary rocks, which have been changed either by intense heat and / or pressure.  They are very hard so take even longer to weather, but can contain lots of nutrients and make a good soil.  Examples are slate and marble.

The bedrock directly below a soil is not necessarily the parent rock that the soil formed from!  Soils can form from materials transported from elsewhere, say by water down a river or even by wind, as in the Mallee and Wimmera areas of Australia.

Depending on the parent bedrock, the amount of weathering and time required to make a soil can vary greatly.  Some primary minerals, such as quartz, resist weathering and will remain in the soil as sand or gravel.  

Physical Features (or topography to you!)

The physical features, or topography, of a landscape refers to both natural and man-made elements which affect the layout of the land.  The topography, affects the rate at which water can runoff / or penetrate a landscape, it's drainage and the possibility of erosion.

In general, the position in a landscape can influence the depth of the soil and how it drains.  Soils up a slope will drain freely and are likely to be shallow because of erosion, with material being transported downhill.  Soil at the bottom of a slope, or in a depression, are consequently more likely to be deeper and more poorly drained.  They are, however, also likely to be more fertile because of accumulated organic matter.

Rocks, forests and breaks in slopes etc can all change the way in which water, and soil, moves downhill and therefore affect how a soil develops in that area.

Climate

Water is perhaps the main factor in rock weathering and it's availability can affect the rate of weathering.  In high temperature and high rainfall areas, rock may weather faster, whereas low temperatures and rainfall will reduce the rate at which rock weathers, but increase the rate at which organic matter accumulates on the surface.

Living Organisms

The accumulation of organic matter whilst a soil is forming can speed up the process, particularly once plants and animals have moved in.  In addition, human and animal activity changes landscapes and can therefore impact soil formation.

Our old friend, Time

It can take 500 years to grow just 1 inch of top soil and the older a soil is, the less nutrients it is likely to contain, whether through lack of replenishment or from leaching to deeper soils.  Clay particles also tend to leach from older soils, leaving them sandier and dryer.  The basalt that we mentioned previously as forming fertile clay soils, weathers to bauxite which, although a great source of aluminium ore, contains very little nutrients of use to plants!

Add a Dash of Weathering...

Weathering is the process by which rock is worn away, or broken down and has it's appearance and texture altered, by the atmosphere.

It can be a physical or mechanical process (e.g. the cracking caused by heating and cooling, subjection to ice, winds, rain and hail etc), a chemical process (e.g. the effects of exposure to constant moisture or weak acids) and biological (e.g. due to the actions of humans, animals or plant roots etc).

The rock eventually breaks down into secondary minerals (e.g. silicate clays), soluble ions and resistant minerals, such as quartz.

Put it all together...

1. parent rock starts to disintegrate
2. organic matter accumulates on the surface, facilitating disintegration
3. horizons form (more on that next time!)

And what do you get?

Soil that is capable of supporting plants and animal life :)