What does well-drained potting soil mean?
Did you ever wonder what the term well-drained soil means? Every time I look to see what type of soil is recommended for a plant, I see the term well-drained soil. What does well-drained soil mean?
Let’s take a minute to remember what potting soil is.
This is simply the medium in which you grow plants.
For this discussion, the term potting soil and potting mixture will be interchangeable. To learn the real difference between potting soil and potting mixture read Organic vs non organic potting mixtures here.
Potting soil is often a combination of organic and non-organic materials. Things like peat, Vermiculite, perlite, and compost.
A great potting soil will be light and fluffy. It will include a material that will hold some water and stay airy. Both air and water in the right amount are essential for healthy root growth.
According to the University of Vermont, the right consistency in container gardening is 50/50 air to material mixture.
What potting soil is not
Potting soil is NOT dirt. Although it is tempting to use dirt from outside to fill your containers, don’t!
Potting soil is much lighter and airier than even healthy garden soil. Garden soil will likely compact in container.
Why is well-drained potting soil so important?
When you grow plants in containers, the soil is where the plant gets the things they need to grow. The health of the soil will be a huge factor in determining the health of your plant.
What your plant’s roots need
Your plant’s roots need nutrients, air, and water to grow.
When we talk about well-drained we are talking about controlling the air and water that your plants need.
Your plant can be drowned
Believe it or not, you plant roots breath. They need a small amount of air in the spacing between the soil molecules to breathe. Without these small spaces, your plant will be drowned and die.
That is why the “light and fluffy” component is important. This type of density allows for good air.
If the soil is too dense watering will cause the potting soil to become muddy. The mud/water will fill the air pockets.
Other problems with poor drainage
Plants that are growing in muddy soil can also develop root problems. Root rot, and bad bacteria can be caused by overwatering.
Salts can build up in the soil. These salts can damage the root system. Watering with a good drainage rate will help wash these salts away.
Dry soil is also trouble
Plants do need water so you can not sacrifice watering to ensure air. Most plants need a light, consistent amount of water to thrive.
The amount and frequency of watering will depend greatly on many facts; type of plant, type of container, the temperature outside, etc.
A test to tell if your potting soil is well drained
Outside gardeners have a pretty standard test to tell if the garden soil is well-draining.
- Dig a hole about 12″x12″x12″
- Fill the hole with water and let drain
- Fill the hole a 2nd time
- Keep track of the drainage rate the 2nd time
- The water level should drop an inch an hour
- If this happens you have well-drained soil
Test for container potting soil
Water your container well, at least until the water comes out the holes
- wait a full day
- Feel the soil, including deep in the pot
- The soil should not be muddy and have a consistent moisture level throughout
- It is best if you test your potting soil in the container and location of your future plant
According to Bonnie, the water should flow from the top and come out the bottom within 10 seconds for it to be well-drained.
What about self-watering planters?
Self-watering planters or planters that water from the bottom up should have the same well-drained soil. Your roots will still need a good air mixture and not dense or muddy soil.
Your self-watering planters still need to be watered from the top regularly to help wash away harmful salts that can build up in the soil.
Do rocks on the bottom of a container help with drainage?
For years I was taught to put small rocks or clay pieces on the bottom of a pot before planting. This is supposed to help drainage.
But the opposite is true. Water molecules form a bond that will create a barrier whenever the molecules reach a different material. This barrier will slow down the drainage.
If your container has no drain holes and you can’t make any then rocks on the bottom will allow for some space for excess water
to go. Be very careful with these containers. It is hard to regulate water without holes.
Please check out the video showing the difference in drainage between different mediums.