Food from Rocks

The theme for Science Week 2021 is Food – Different by Design. So, we will explore the effect of rocks and soil on plant growth.

Do we really need soil?

Plants do not absolutely require soil to live. However, soil provides the necessities that plants need. These are:

  • Support
  • Nutrients
  • 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.

Plants can be grown hydroponically, with their roots in a solution that contains fertiliser. This system is often used for delicate lettuces and herbs.

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 amount of each component will vary in different types of soil. Compacted soils contain little pore space. (JasonHS 2020, Creative Commons 4.0)

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.

Different types of rocks produce different colours and textures of soil. The rocks here are (left to right) gneiss, limestone, sandstone and banded iron formation.

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.