How to test soil

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will learn how to test soil health by measuring pH, phosphorus, potassium, and ammonia nitrogen levels to discover how nutrients support plant growth and ecosystem biodiversity. Watch the video above to see how it is done.

Soil testing materials & ingredients

For this activity:

• Soil samples collected from different locations (for example, backyard garden, near a creek, different suburbs, coastal soil)
• Scale
• Clear test tubes or small lidded containers
• Pipette or dropper
• Pure water or distilled water
• Soil testing solutions and color matching chart (for pH, phosphorus, potassium, and ammonia nitrogen)
• Data recording sheet and pen

Safety Note: Wash hands thoroughly after handling soil samples and chemical indicator solutions, and avoid touching your face or eyes during testing.

How to test soil health: Step-by-step instructions

Step 1: Extract the liquid soil sample

1. Place 1 to 2 grams of your collected soil into a test tube.
2. Add a dropper full of pure water, seal the test tube, and shake vigorously for one minute.
3. Set the test tube aside and allow it to rest until stratification occurs (when the soil settles to the bottom and leaves a clear liquid at the top).

Step 2: Test pH, phosphorus, and potassium levels

1. Transfer 1 to 2 mL of the clear liquid from the top of your stratified sample into a clean mini test tube.
2. Add 2 drops of your chosen test indicator (pH, phosphorus, or potassium).
3. Shake gently, wait one minute, and compare the liquid color against your result chart to read nutrient levels.

Step 3: Test ammonia nitrogen levels

1. Add 1 to 2 mL of nitrogen activator solution into a mini test tube.
2. Add 1 to 2 mL of your clear soil water and shake for one minute.
3. Add 2 drops of ammonia nitrogen test solution, wait 10 minutes, and read the final color against the chart.

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How soil testing works: The science explained

Soil testing measures critical chemical indicators that determine whether plants can successfully absorb nutrients from their environment:

• pH Level: Measures soil acidity or alkalinity. Highly acidic or alkaline soil locks away essential minerals, preventing plant roots from absorbing them. Most Australian plants prefer a neutral "Goldilocks" pH range.
• Phosphorus & Potassium: Phosphorus strengthens plant immune systems against disease, while potassium helps plants cope with weather stress like drought or heat. Ancient Australian soils are naturally low in potassium and phosphorus due to weathering over millions of years.
• Ammonia Nitrogen: Nitrogen forms the building blocks of proteins and amino acids necessary for cell growth, flowering, and fruit production. Low nitrogen leads to pale, yellowing leaves.

Science fair projects & taking it further: Experimenting with variables

• Compare regional soils: Collect soil samples from different environments (a forest, an urban garden, and a sandy beach) to compare how soil chemistry affects plant biodiversity.
• Natural pH indicators: Create a homemade pH indicator using boiled red cabbage juice to test soil acidity naturally without chemical kits.