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In this video, I’ll show you how I determined the *amount* of water that is held in a volume of soil or sphagnum moss.
To summarize: most planting media hold no more than a third the volume of water compared to the volume of planting medium. But even if you are sure to pour in only this amount, the moisture must be used up by the plant working – the plant will only work in the right light.
Step 1: find the dry weight of soil
Step 2: fully moisten the soil – watering it in a container WITH drainage holes so that any excess water can drain away.
Step 3: find the weight of the saturated soil
Step 4: subtract the dry weight from the saturated weight; now you have the weight of water being held in the saturated soil.
Step 5: pour out this weight of water into an identically shaped container (obviously, this container can’t have holes)
Step 6: observe the volume of water compared to the volume of soil
Here are some water volumes that I’ve found for different types of soils:
Pure peat moss: 1/2
Regular potting soil (peat moss + perlite): just less than 1/2 down to 1/3
Sphagnum moss: 1/3
Cactus soil (peat moss + coarse sand + perlite): around 1/4
Orchid medium (bark chips): around 1/4 because there are lots of air spaces
Why volume? It’s easier to visually compare volumes (water to soil ratio) than to compare weights. In your everyday life, you pour out liquids into differently shaped containers so you’re more familiar with how water would fill them up.
Sphagnum moss (the good kind):
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