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How to Measure Holding Capacity of Soil - Tutorial
Holding capacity of soil is the percentage of water that a given water can hold without dripping, after being saturated.
Sand has a low holding capacity, because water tends to drain out of it very easily. This is because the particle surface area density is less as particle size increases and so water tension decreases. Comparably, the holding capacity of clay soils is greater because the particle size is very small, and the water binds to the particles.
The picture above shows our setup. We are also correlating the VWC with readings from our VH400 moisture sensors.
It's very easy to measure holding capacity. You need the following items:
In our pictures in this tutorial we're using an 8 inch cube of rockwool as our soil.
CalculationsNote that grams of water is the same as mililiters of water, so you can use them interchangibly.
Where Mw is the mass of the water in grams.
Mt is the total mass of the continer and wet soil in grams.
Ms is the total mass of the continer and dry soil in grams.
Note that grams of water is the same as mililiters of water, so you can use them interchangibly.
Holding_Capacity(VWC%) = Vw/Vt*100;
Where Vw is the volume of the water.
Where Vt is the total volume of the saturated soil. If you've filled the container to the top with saturated soil then this is the volume of the container.
ResultsFor our 8 inch rockwool cube we calculated a holding capacity of 45%. It's our experience that most soils have a holding capacity of less that 50%.