Do we really need soil?
Plants do not absolutely require soil to live. However, soil provides the necessities that plants need. These are:
- Water and oxygen around the roots
Soil provides a solid base for plant roots. This support is particularly important for large plants like trees.
Plants make sugars with photosynthesis, but they need small amounts of nutrients like nitrogen, phosphorus and potassium to grow. These nutrients are vital for creating proteins that control reactions in the cells and DNA that contains the instructions for the cell.
Water is absorbed through plant roots in the soil. These cells also need oxygen. Fortunately, both water and oxygen can move through the tiny spaces (pores) in soil.
When plants are grown without soil, they have a careful balance of nutrients in the water and special containers to provide support. This works well for high value small plants like lettuce, herbs and strawberries. Hydroponics is not feasible for trees or large-scale crops like rice and wheat.
What is soil?
Soil brings together the geosphere (rock), biosphere (humus/organic material), atmosphere (oxygen) and hydrosphere (water). The solid portion of soil is a mixture of weathered rock and humus. Humus is decomposing organic material and soil organisms which usually makes up less than 10% of the soil. Both the weathered rock and the humus provide nutrients for plant growth.
The size of the weathered rock particles in soil affects its texture. Sandy soil has many quartz particles. It drains quickly and retains few nutrients. Clays are able to hold nutrients and water on their surface. Wet clay soils stick together and swell when wet. You can test soil texture easily at home.
The effect of parent rock on soil
Rocks are broken down by physical, chemical and biological weathering. Weathering breaks the rock into smaller pieces and alters the chemistry of the rock. Minerals like quartz and zircon are very tough and resist chemical weathering. Minerals like halite (salt) dissolve easily in water.
As minerals undergo chemical weathering, they release nutrients that plants need for growth. The minerals in the parent rock determine which minerals are present and what type of particles (sand or clay) are produced. Both nutrients and mineral particles affect the growth of plants in soil.
Food from rocks?
Without soils, we would not be able to grow food efficiently enough to feed the world. So, next time you eat a meal, think about how rocks help us by supporting and feeding the plants we depend upon.
- Take a close look at soil components with the Introduction to Soils activity and compare the amount of humus in soils
- Try making your own soil and exploring the effect of soil components on plant growth as shown in our video Soil and Plants – the Basics.
- Advocate for soil conservation by creating a Save our Soil poster
- Match soils with their parent rock in this student worksheet.
- Grow plants in different types of soil to see the effect of parent rock as shown in Soil and Plants – the Effect of Parent Rocks.
- Investigate soil components with these activities, which are also shown in the Soil Components video.
- Would food in space be an answer to food security? Explore the idea of an International Space Farm with this STEM project.
- The 2021 Science Week teaching resource book has a wide variety of activities and ideas to explore.