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Vermicompost can reduce the need to water your plants by 30%

Two students from the University of Witwatersrand conducted a study to determine the benefits of adding vermicompost and traditional compost to 3 types of soils. The benefits on the water holding capacity of sandy and loam soils are incredible! They conducted the experiment in search of a solution to the water shortage and soil degradation issues in South Africa and won a national competition in the process!

South Africa is a water scares country and the 30th driest country in the world. Our water demand is expected to out grow supply by 2030 according to Adam, F (2021). The water scarcity plus poor-quality soils result in an environment susceptible to drought.

What is water holding capacity?

It is the ability of soil to hold water – the more porous the soil in the case of sandy soils have the lowest water holding capacity as more water is lost via the spaces between the soil particles when it rains. Clay soils, on the other hand, have a higher water holding capacity due to the large surface area between the pores that trap and absorb more water when it rains and reduces the amount of water lost as runoff. Loam soils have properties in between clay and sandy soils.

The shortage of water in South Africa causes many soils to dry up and erode, the decrease in soil quality reduces the ability of the soil to filter and hold water. When it rains, less water is retained by the soil and instability in terms of water availability and soil quality are seen. Most soils in South Africa are made up of a mixture of clay, loam, and sand.


Vermicompost is an underestimated resource that is fairly simple to obtain. Earthworms consume garden waste, food waste, and cow dung (organic matter) and excrete it as castings or vermicast. Vermicast in the form of compost increases the biological and physical properties of soils:

  • Pathogen suppression

  • Nutrient quantity and delivery

  • Water holding capacity

  • Rich in nutrients

  • Increased microorganism populations

  • Higher plant yields

Testing to see how vermicompost and compost benefit soils water holding capacity

The experiment below was done to understand how adding vermicompost and compost effects the water holding capacity of three different types of soils – clay, loam, and sand.

  1. 100g of each soil type were oven dried at 105 degrees Celsius.

  2. Vermicompost from The Compost Kitchen and traditional compost were used as packaged and not oven dried.

  3. Clay, loam and sandy soil samples were kept separate and never mixed together.

The soil samples were mixed as follows to determine the water holding capacity of three types of soils when using 10% of vermicompost (VC) or traditional compost (C).

Experiment 1 - 0% mix (control) made up of 100g of soil.

Experiment 2 - 90g of soil and 10g VC = 10% VC mix. Experiment 3 - 90g soil and 10g C = 10% C mix.

To test the water holding capacity of the different mixtures each sample was placed in a separate funnel, lined with filter paper. The funnel was positioned over a measuring cylinder to collect excess water. 200ml of water was slowly poured over each soil mix to saturate the soil.

After 2 hours of percolation, the saturated soils were weighed to calculate how much water the different soils absorbed when they had vermicompost or compost added. The ‘Percolation Method’ uses the following equation to calculate the water holding capacity of the soil. The calculation is as follows:

WHC = saturated mass of the soil/dry mass of the soil (100g) x 100%


The biggest result was the impact that 10% vermicompost had on sandy and loam soil – it increased the WHC by 31.59% and 10.4% respectively. The addition of traditional compost also increased the WHC by up to 27.39%. The reason is because the organic matter in the vermicompost and compost fills up the porous spaces in between the sand and loam particles and acts as a sponge – holding onto the water instead of it being lost as runoff.

This study showed that vermicompost is effective at increasing the WHC of sandy and loam soils found in South Africa by up to 30%. Using vermicompost in your garden will mean you have to water your garden 30% less!

More than 15% (18 million hectares) of the land area in South Africa consists of sandy soils and 2.6 million hectares of that are used for cultivating crops despite their poor water holding capacity. Imagine the positive impacts vermicompost could have for our farmers? It could increase the water holding capacity and nutrients of the soils resulting in an increased crop yield all while saving on irrigation costs.

Soils are essential for life. Their ability to store water and nutrients is vital in supporting plant life and therefore facilitating food production. Without healthy soils and water, our farmers are unable to produce food!

To turn your food waste into a product that will have positive effects on South African soils join the recycling movement. By using vermicompost in your garden you can help repair the soil quality and save water! Contact The Compost Kitchen to find out how you can make your own vermicompost and join the movement.

Written by Sarah Oxley MSc (University of Wits)

The experiment was conducted by Liam Macgregor & Ashley Randall supervised by Dr Precious Biyela from the University of Witwatersrand.

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