We’re often told to provide “well-draining” soil for our plants. But what is it? And how do we know if we have it? The simple test here will show you how to assess your garden’s drainage conditions and understand why it matters.
If you want to check your basic soil composition, this show how to check for sand, silt, and clay.
What Is Well-Draining Soil?
It’s one of the most common phrases on plant tags and growing instructions:
Needs well-draining soil
Grows best in well-drained soil
But what does it mean and how do we check for it?
Whether outdoors or indoors, in the ground or in potting mix, there is a moisture range that is just right, landing somewhere between too wet and too dry and different plants like different ranges.
Some plants like it dry, including many desert plants.
Some like it wet, like aquatic plants and plants growing in wetlands.
But most garden plants like something in between.
And that’s the “well-draining” sweet spot.
So what is it and how do we check for it?
- Why Soil Drainage Matters
- Where Garden Water Goes
- How Soil Texture Affects Drainage
- How to Check Your Soil Drainage
- Can I Change My Soil Drainage?
Why Soil Drainage Matters
Soil is composed of five ingredients: minerals, soil organic matter, living organisms, gas (air), and water.
A mix of minerals make up about half of this composition. These are particles of sand, silt, and clay, that, depending on the proportions, determine our soil texture.
The other half is pores, providing space for air and water, both things that plants need.
If there is too much air, there is not enough water which is needed for photosynthesis and the delivery of nutrients.
With too much water, there’s not enough air, which is a common cause of root rot and numerous other problems.
Lots of microorganisms need oxygen too so there can be problems for the whole ecosystem when oxygen levels are depleted.
It’s normal for the pore space in soil to fill with water, either from rain or other watering. And it’s fine for there to be excess at times for the short term. Most plants can deal with this and recover from it.
But when water sits for prolonged periods of time, or passes through too rapidly, plants will struggle.
And that’s basic soil drainage. It can mean the difference between successful growing or not.
Where Garden Water Goes
The movement of fluids through porous materials, whether in science or coffee-making, is known as “percolation”.
That’s what happens with water in your garden. It is gradually pulled down through the soil by gravity. As the water leaves the soil pores, the air returns.
In good conditions, the drained water ends up as groundwater, which is usually at a depth below plant roots.
When the top of this groundwater, the “water table,” is too high, water pools in the root zone.
Instead of temporary immersion, there is prolonged water exposure and plant roots cannot get air. This is the same thing that happens when we overwater our houseplants in containers. The water essentially suffocates the roots.
On the flip side, if you have really sandy soil and a lower water table, water may drain so rapidly from the root zone that they miss out on the benefits of water altogether.
When we say we need “well-draining” soil we mean not too fast and not too slow. Exposure is balanced and plant roots get the air and water they need.
How Soil Texture Affects Drainage
As a general rule, sandy soil will drain the fastest, often more quickly than we would like. Sand is the largest of the soil particles and sandy soils have large pores. Water just rushes through.
In our garden, we live on top of a sand plain so our soil is basically a golf course sand trap. An hour after a rain storm, you’d have no idea the soil had been watered.
As the smallest of the soil particles, clay has the smallest pores and most surface area for water to stick to.
Our previous garden had very heavy clay soil and we had to install in-ground drains or water would sit for days or weeks after a rainfall until it could evaporate.
Clay is also the one particle that usually has an electrical charge. While water doesn’t have a net charge, it does have positive and negative areas which makes the clay stick to it. All this makes it very good at holding water.
You can use this simple home test to assess your basic sand, silt, or clay soil composition.
How to Test Your Soil Drainage
Soil Percolation Test
This is the percolation or “perc” test. There are all sorts of variations but the gist is the same.
Here’s how you do it.
Dig a hole in your garden about 1-foot wide by 1-foot deep.
2Fill Hole With Water & Leave to Drain
Fill the hole with water and let it drain out (percolate).
This may take minutes, hours, or longer depending on what type of soil you have.
3Refill With Water
Once it has drained, fill the hole again with water. This does not have to be immediately after it drained for the first time but perhaps within a day or so and without any rainfall in between.
Measure How Long It Takes to Drain
This is where the real test starts.
We want to know how many minutes or hours it takes for the water to drain this second time.
Have a pen and paper or your phone’s note app ready.
With the hole filled once again, note the start time. Or use the stopwatch on your phone to measure the total drainage time.
In extremely sandy soil, you might want to check the water depth every 10 minutes or so.
In loam or clay soil, you could measure every hour or maybe two.
Ultimately, you want to know the total time it takes for all the water to drain from the hole.
Soil Drainage Rates
Use the Soil Percolation Test to check your garden soil
Fast-Draining Soil: Under 3 hours
Well-Draining Soil: half full (30 to 70 percent) after 3 hours
Slow-Draining Soil: 80 to 90 percent full after 3 hours
As discussed, fast-draining soil tends to be sandy and slow-draining soil tends to be predominantly clay.
Our sandy soil took just 45 minutes to drain. That’s way too fast for most garden plants.
In our previous clay-based garden, the same test would have taken days or weeks.
If you’re lucky, your soil is right in the middle.
Can I Change My Soil Drainage?
If your soil drainage test results are not great, it’s time to look at ways to improve conditions.
But can we really change our soil?
Whether it’s soil composition, pH levels, fertility (nutrition) or all of these things, when you think about the mass of soil in our gardens and how much would have to change to make a difference, it’s a huge undertaking. And you’re never done.
For years, popular advice said to add loads of sand to clay soil to improve the drainage.
When we tested this many years ago we ended up with just that: clay and sand. They never really aggregated and the problems remained the same.
One thing that does help is to add organic matter to your soil. This should be routine for just about any soil unless you have that rare instance where there is actually too much. A reliable soil test from an accredited lab can help evaluate it.
Organic mulches like wood chips and tree leaves also help both for soil health and drainage.
If things seem impossible, switching to raised beds with new, good quality soil may be the most effective and frugal option in the long run.
No matter what, it’s the current conditions—not what we want them to be—that plants respond to, so choose accordingly. Otherwise we’re just throwing good plants at bad soil.
Solutions to Soil Problems III. Drainage | Utah State University Yard & Garden Extension
~Melissa the Empress of Dirt ♛