What’s in soil? When you pick up a handful of soil, what do you see?
All soil is made up of inorganic mineral particles, organic matter (including living things), air and water.
Inorganic mineral particles
Inorganic mineral particles make up more than half the volume of soil. These particles come from rocks – the parent material that formed the soil. Soil mineral particles are sorted into three groups based on their size – sand, silt and clay. Sand particles are the biggest and clay particles are the smallest. If you rub soil between your fingers, the sand particles make it feel gritty. Silt soils feel smooth and floury. A slick or sticky feel comes from clay.
Organic matter (living and non-living)
Unless there is an earthworm or a slater in your handful of soil, it appears to be dead. Actually, soil is home to an amazing amount of life. Some living things are big so we can see them, but most aren’t. Soil is full of life – billions of bacteria, fungi and other microorganisms. Scientists think there is more life in one teaspoon of healthy soil than there are people living on Earth!
The other component of organic matter is humus. It comes from dead plants and animals and the waste products of living things. When we add compost to the soil, we are adding humus.
Organic matter makes up a small part of the soil, but it plays a really important role. The living organisms recycle nutrients. Humus stores the nutrients and water for plants. Organic matter makes it easier to work the soil.
Air and water
Believe it or not, air and water can often make up about half the volume of soil! Air and water are in small spaces called pores between soil particles. Plants and soil animals need the air and water to live and grow.
Different combinations result in different soils
Soil scientists group soils according to the types of mineral particles and organic matter they contain. Different amounts and different combinations give soils special properties.
Soil properties help us decide how to use the soil. Look at a cricket ground, for example. The cricket pitch is made from clay soil. Dry clay has a hard surface, ideal for bowling and batting. The surrounding field will have a different type of soil. This provides a safer playing surface for fielders and better drainage if it rains.
Soil properties also help us decide where to put roads and buildings and where to grow different types of crops.
Nature of Science
Soil scientists use observation and measurement to determine the components in soil. The observation can be as simple as rubbing the soil to feel if it is sandy or sticky. More precise tests involve measuring the percentage of mineral particles.
What makes up soil? uses an interactive or paper-based graphic organiser to explore student ideas about the components of soil.
Visit Te Ara to learn more about New Zealand soils.
If you want to make a 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!
See NIWA's Soil Portal to help you get a better appreciation of soils.