Soils are made of living organisms (like invertebrates and microorganisms).
Soil has a multitude of different colours and textures.
Soils have structure.
Silt and sand are fun between our toes!
Puddles form in soil because it cannot drain away due to the soil porosity.
Soil properties influence land use
Humans have spent centuries studying the soil in order to discover ideal places to live, grow crops, raise animals and find minerals for industry. We know a lot about soil but like most things in nature, there is still much more to discover.
Here is some basic information about soil.
What are soils made of?
All soil is made up of:
- inorganic mineral particles (sand, silt, clay, stones and rock)
- organic matter (decaying plants and animals)
- living organisms (like invertebrates and microorganisms).
The combination of these components determine a soil’s properties.
Soils have texture
The mineral particles that make up soil are categorised into three groups according to their size – sand, silt and clay. The proportions of these three particles determine the soil’s texture.
The word texture refers to the tactile quality of something – how it feels. You can learn alot about a soil by rubbing it between your fingers or squeezing some of it in your palm.
Sand is the largest particle size. Sandy soils feel gritty. They tend to be free draining so they do not get waterlogged. Silts feel smooth – a little like flour. Clay particles are the smallest – really small compared to sand particles. By comparision, if a sand particle were the size of a netball, a clay particle would be the size of a 100’s and 1000’s sprinkle! Clay soils feel sticky and soils with a high percentage of clay tend to be poorly drained – that is, they hold onto water.
Soils have structure and porosity
Soil structure is the arrangement of soil particles. Clays, organic matter and materials excreted by soil organisms bind the soil particles together to form aggregates. Spaces, called pores, form between the aggregates. The pores allow air, water and organisms to move within the soil and provide spaces for roots to grow. Healthy soils have good structure and porosity – nutrients, air and water can circulate freely. Poor soil structure has coarse, firm clods, and few visible cracks or holes.
The way in which a soil is managed can affect porosity. Look around parks and playing fields. In places where people regularly walk or play, the soil may look different because it has become compacted. This compacted soil no longer has the same structure or porosity. Grass or other plants do not grow as well. When it rains, puddles form because the water is not able to drain away. Similar things happen to agricultural soils and can make the soil less productive.
Soils have colour
What colour is soil? Most people would automatically say brown, but soil colours range from black to red to white. Soil colour mostly comes from organic mater and iron. Soils with lots of organic matter are dark brown or black – think about the colour of compost Topsoil tends to be darker than the subsoil below it because it has more organic matter. Soils high in iron are orangish-brown to yellowish-brown. Soils with good drainage are usually even, single colours. Rusty spots and grey colours (sometimes even a light blue colour) are an indication of poor drainage.
Soil properties influence land use.
Have you ever wondered why some parts of the country are covered in dairy farms or why vegetables are grown in certain locations? Some of the variation is due to weather – warm growing conditions and ideal rainfall – but much of it is also due to the soils. For example, organic peat soils in the Waikato are ideal for growing blueberries. The stoney, gravelly soils in the Hawkes Bay are perfect for vineyards. Rich, volcanic soils in the Ohakune area produce abundant winter root crops, like carrots, parsnips and potatoes.
Soil properties determine how and where we put our buildings and roads. Some types of soils are prone to slips or subsidence so we avoid building on them or take special precautions to enhance safety.
Even sports are influenced by soils! Golf courses, rugby grounds, polo fields and football pitches often have dedicated soil and turf managers. If you are thinking of making your own cricket pitch at home, the Motty test is a DIY method to see if your clay loam soil is suitable for backyard cricket – or possibly even up to international standards!
The activity Milk, Meat, Wine and Wheat explores the link between soils and land use.
Check out these quick and simple tests to determine soil texture
Learn more about soil structure and porosity.