What is Soil?
Soil means different things to different people. To a horticulturalist, farmer, or everyday gardener, it is a medium for plant growth. To a hydrologist, soil functions as a source of water supply and purification. To an engineer it is something on which to build upon or remove during the process of construction. Comprising a mixture of minerals, organic matter, gases, liquids, and organisms, soil is a confluence of materials that enables our very existence here on Earth.
Understanding how soils and landscapes have evolved is of great importance for not only predicting the behaviour of soils in the future – particularly under shifting climate regimes, but also for deciding which plants are suitable for the soil in your garden. Soil is the product of the complex interactions of several key components – known as the five soil forming factors. The most important of these being: parent material, time, climate, plants and animals, and topography.
Parent Material: The weathered mineral matter from which soil is formed is called the parent material; it is a major factor in influencing a newly forming soil. Parent material may be the underlying bedrock, or it can be a layer of loosely arranged deposits, as in a river valley or gravelly floodplain. As the parent material – be it sandstone, limestone, granite, basalt or any number of other rock types – is subject to physical changes (such as droplets of rain) and chemical changes (such as the carbonic acid in rain) over time, soil is formed.
Time: The nature of soil is actively influenced by the length of time in which soil-forming processes have been operating. In most parts of Australia, the rate of soil formation (pedogenesis) is very slow – occurring faster in sandier materials due largely to the lower surface areas of individual sand particles. As weathering processes continue, the influence of parent material on soil is dwarfed by other soil-forming factors. As a general rule, the older a soil (the longer it has been forming), the thicker it becomes, and the less it resembles its parent material.
Climate: An integral step in the formation of soil from rock involves the weathering of the rock into smaller and often chemically altered parts. Physical (mechanical) and chemical weathering greatly are influenced by climatic factors such as temperature and rainfall. For example, in hot and moist climates (such as in the tropics), a thicker layer of chemically weathered soil may form in the same amount of time as a thin mantle of mechanically weathered soil forms in a colder and drier climate. Rainfall also influences the extent to which materials are removed (leached) from the soil, which in turn impacts the fertility of the soil.
Plants and Animals: Plants and animals play a critical and often overlooked role in soil formation. By adding organic matter such as leaf-litter, and nutrients through the decay of animal remains, plants and animals not only enrich soils through the production of various organic acids, but serve to increase their water-holding capacity by providing an abundance of chemical exchange sites for the hydrogen and oxygen atoms that make up water molecules. Some microorganisms add nitrogen to the soil directly, whilst others act to mix the organic and mineral fractions, inducing a profusion of vigorous chemical reactions that enhance soil health. The burrows and holes produced by earthworms and other burrowing animals create channels for water and air flow, which is critical for allowing water to access the root-zone, and for enabling important biochemical processes such as gas-exchange – processes vital for happy and healthy plants.
Topography: Variation in topography – the arrangement of the natural and artificial physical features of an area – can lead to localised differences in soils. For example the length and gradient of a slope has a significant impact on the amount of erosion and water content in a soil. If your backyard is on a steep slope, it is less likely to retain soil and nutrients due to a combination of gravity and weathering processes accelerating erosion. If however, your backyard is located at the bottom of a slope, or in a floodplain, your garden soil is more likely to be thicker, deeper, darker, and more often than not poorly draining.
At the scientific level, soils in Australia are categorised using a framework called the Australian Soil Classification system. However, in terms of the soil in your garden, there are three different types (or textural classes) you really need to know about: sandy soils, loamy soils, and clay soils. Depending on where you live, your soil will be a combination of these. The ribbon test, which you can download as a PDF from the New South Wales Department of Primary Industries website is a particularly useful method for working out what soil you have in your garden, which in turn can help you determine what you need to do to improve your soil, and what plants are suitable for the soil in your garden.
Sandy Soil: Sand grains are larger than silt and clay particles. Sandy soils pose a number of challenges for the home gardener. First and foremost, sandy soil does not hold nutrients. This is because sand particles are chemically inert, and as such do not allow nutrients to bond with them, meaning that nutrients tend to leach out. Secondly, sandy soils do not hold water. Because sand particles are both large (allowing more space for water to percolate between them), and chemically inert (rendering them unable to hold the oxygen and hydrogen ions that make up water), their ability to retain moisture, which is essential for many plants to flourish, is limited.
Loamy Soil: Loam is soil composed mostly of sand, with a smaller amount of clay. The proportions of sand and clay in loams exhibit a great degree of variability. This results in different types of loam: sandy loam, silty loam, clay loam, sandy clay loam, and silty clay loam. Loams characteristically exhibit more nutrients, humus, and moisture than sandy soils, have better drainage and infiltration capacity than silty soils, and are easier to work than clay soils. Loamy soils sit in-between pure sand and heavy clay, and provide the ideal soil environment for the great majority of plant species.
Clay Soil: Clays are the smallest of the soil particles. Clays are fine-grained soils that contain traces of metal oxides and organic matter. Geologically, deposits dominated by clays are composed mostly of sheet-shaped silicate minerals (phyllosilicates), which contain variable amounts of water trapped within their mineral structures. Clays have a high water-holding capacity, but often this water is tightly-bound and unavailable to plants. Additionally, clay soils – unless improved – are susceptible to structural collapse when wet, and excessive hardness and impermeability when dry.
Fig. 2: Soil Texture Pyramid
Poor Soil versus Healthy Soil
A poor soil can mean any number of things: an overly sandy soil, a hard clayey soil, a hydrophobic soil, a nutrient-poor soil, or a combination of the above and more. Poor soils exhibit a number of key characteristics: they can be too acidic or alkaline; lack organic matter (decomposed plant and animal debris); display an inability to hold nutrients and moisture (a common problem with sandy soils); suffer from water-logging and physical impermeability (overly clayey soil); and/or exhibit water repellency (hydrophobicity). A healthy soil by contrast is a soil that exhibits a combination of the following attributes: good structural qualities (a mixture of particle sizes and porosity for air and water movement); good soil texture (a composite of sand, silt and clay particles); relatively high cation exchange capacity (the ability of a soil to hold nutrients); resilience to compaction; the presence of soil organisms; and plenty of dark, rich organic matter. If your soil lacks these characteristics, don’t feel dispirited as there are plenty of ways to transform your undesirable patch into a rich and healthy soil environment.
Improving Your Soil
In terms of improving your soil, the first thing you need to do is identify the reasons why your soil is poor. Once you have identified why your soil is poor, it is much easier to find appropriate solutions. Below is a list of common soil problems, and how to ameliorate them:
Too Acidic: Add lime or wood ash, and mix in.
Too Alkaline: Add sulfur, organic matter (mulches and compost), or sphagnum peat.
Heavy Clay: Despite being much maligned, clay is actually a critical component of good soil health. Because clay particles are negatively charged, they have the capacity to hold onto positively charged elements (cations) such as ammonium, potassium, calcium, magnesium and other trace elements. However, too much clay can render your soil water-logged and unworkable, so amending it is often necessary. Mixing in a combination of gypsum (which replaces sodium cations on clay particles with larger calcium cations, making the soil more porous) and organic matter in the form of grass clippings, compost, and manures, will increase permeability, nutrient availability for plants, and the general workability of your soil. It is important not to work clay soils in wet conditions as they have a tendency to lose their structure, so make sure when you’re digging in your gypsum and organic matter, you are doing it on a dry sunny day!
Sandy Soil: Sandy soil is problematic for home gardeners because it does not hold water or nutrients very well. The best way to improve sandy soils is to add organic matter in the form of compost or manure and to mulch over the soil in order to reduce evaporation and curtail temperature extremes. Because sandy soil is prone to nutrient loss through leaching, you may have to mix organic matter into your soil at once a year, or even more! The best way of ensuring your plants flourish in sandy soil environments is to use plants which that have evolved in similar conditions. Indeed whilst many herbs will flourish in a sandier soil – particularly Mediterranean herbs such as Thyme and Oregano, other plants – such as vegetables and many ornamental species, may struggle in these nutrient-starved, water-limited soil environments.
Nitrogen-fixing Plants: Nitrogen is vital for plants because it is a major component of chlorophyll – the compound used by plants to convert sunlight to carbohydrates. It is also plays an integral role in building proteins. Some plants ‘fix’ nitrogen, meaning they convert atmospheric nitrogen into a form which plants can use. In your veggie garden, try planting a winter crop of legumes – such as clover or peas. When spring comes along, mix these plants into the soil, and as they decompose, they’ll raise the total nitrogen in the soil, ensuring your spring and summer veggie crops get all the nitrogen they need to produce those big lush and tasty green leaves!
Physical and Biological Infrastructure: An often overlooked element in the creation and retention of a healthy soil environment is what I term physical and biological infrastructure. Physical infrastructure in the garden, such as retaining walls and landscape rocks, in addition to biological infrastructure, most notably the role of plant roots and foliage in mitigating erosion, is critical for maintaining a healthy soil. On steep slopes, retaining walls and landscape rocks can assist in reducing the loss of topsoil to lower parts of the landscape during heavy rainfall events. In terms of biological infrastructure, plant roots act to reduce the loss of soil through erosion by binding soil particles together, whilst foliage assists by reducing the physical impact of raindrops on the soil surface, and in the case of groundcovers, by providing an extra layer of protection against both wind and rain.
Mulch: Another way to keep your soil healthy and happy is to apply mulch. Mulch, in the form of wood chips, sugar-cane, pea-straw, or stones and gravels, perform a number of crucial roles in maintaining soil health. Mulches reduce erosion, insulate the soil from temperature extremes, and in the case of pea-straw mulch, add important nutrients, such as nitrogen, to the soil. It is important to remember when using wood chip mulch (or sawdust) to be especially careful not to mix the mulch into the soil, as this facilitates a phenomenon called ‘nitrogen draw-down’, where microorganisms, fungi, and bacteria use the nitrogen already in the soil to breakdown the mulch matter, reducing the amount of nitrogen in the soil that can be used by plants for growth.
Fig. 3: Gypsum is a widely used product available at any good nursery.
1 New South Wales Department of Primary Industries – Determining soil texture using the ribboning technique: http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/164615/determining_soil_texture_using_-ribboning_technique.pdf