Knowing just how well your soil drains and percolates water is important for you to have a successful garden. In researching plants or trees to grow, you’ve probably come across the term “well drained soil”. I will show you a method to quantify how much your soil drains, and what the appropriate levels are step by step. And, how to address too much or too little drainage.
To determine and measure your soil drainage, dig a hole that is 1 foot wide by 1 foot deep. Fill it with water and let it drain overnight. Then, refill it with water the next morning. Measure the drop in water level every hour until empty. Then calculate the drainage rate.
Knowing the drainage of a location is very important if you are planning on building a vegetable garden. In fact, it is one of the key factors you should consider in locating your vegetable garden in the proper spot.
You will need the following to complete this soil drainage test, or percolation test.
- A shovel (I prefer a garden spade)
- A bucket or hose
- A tape measure or long ruler (more than 12″)
- Stopwatch or clock
- Time! You may be checking the water level hourly for several hours
Process to test garden soil drainage
Dig a hole approximately 12 inches diameter by 12 inches deep (30 cm diameter by 30 cm depth). Try to keep the sides of the hole vertical.
*Special TIP! – This is an excellent time to gather soil to perform the Soil Texture Analysis using a mason jar! If while digging you notice differences in the soil type, or different layers, you can easily segregate them for use in the soil texture analysis!
Fill the hole with water and let it drain over night. It will take about 5 gallons (20 L) of water to fill. This will saturate the surrounding soil, so that the real test will show true water drainage from your garden soil.
The next morning, fill the hole with water. Then, lay a board or stick across the hole so that it sits flat. Measure and record either the initial depth of water, or the distance from the stick to the water. Also, note the starting time.
Every hour, measure the distanced drained. You can do this by checking the distance from the bottom of the flat board to the water, or to the bottom of the hole. Just be consistent, and record your measurements. Write down the time, and either the amount of water (height) in the hole.
Calculate your drainage. Tally up your measurements into a sheet showing the amount of water drop over time.
See the image below for an example of results!
Interpreting the results
Soil that drains 1-3″ an hour (2.5-7.5 cm/hr) is considered to be good for a vegetable garden. This is considered a good mix of drainage and water retention. Soils that drain in this range should hold enough moisture for plants during hot summer days without risk of roots rotting.
Soil Drainage Rate Reference Table
|Drainage Rate (English)||Drainage Rate (Metric)||Result|
|<1″ per hour||<2.5 cm / hr||Drainage is too slow|
|1-3″ per hour||2.5-7.5 cm/hr||Good Drainage|
|>4″ per hour||> 10cm / hr||Drainage is too Fast|
Video Demonstration of Testing Soil Drainage / Percolation
What is too much soil drainage?
Soil that drains 4″ per hour (10 cm/hr) or more is too fast, and will likely harm plants during high heat or drought. You should amend with compost to increase the water holding capacity.
What is soil that drains too little, or not enough?
Soil that drains less than 1″ per hour has too little absorption, and is likely heavy clay. This is too low drainage for soil and can cause root rot.
How to fix drainage on soils that drain too fast, or too slow
Amending soils that drain too fast with compost or other organic matter will slow drainage. The organic matter will greatly promote moisture retention in sandy soils that drain water too fast.
And for soils that drain too slow, the same solution applies! For heavy clay or silt soils that drain water too slowly, add compost. Mixing compost in with clay, silt, or compacted soils will help break the bonds between the tiny particles. This will in turn feed and promote microorganisms to improve the structure.
*If you don’t make your own compost, you should start. It is really easy. See our guide here for how to make compost
As the microorganisms consume the organic matter, they will leave behind voids that will be taken up by air and water. This will allow roots to penetrate deeper as well as the soil to drain water more!
What about soil that has different drainage rates over time?
Soil will often drain at different rates over time. This is because the soil texture often changes with depth.
This is because the upper most layer is primarily organic matter, which will be very porous and drain quite quickly.
This will be followed by layer upon layer of soil that are identifiable as soil horizons. Each one will likely have different texture, or different organic matter content. Each horizon will drain at a different rate.
So, if your soil drains too quickly near the top, your plants can be at risk of drying out if those plants don’t produce roots that reach the deeper layers.
Additionally, if your soil drains quickly in the first 2 inches, but then doesn’t drain hardly at all in the bottom 2 inches, you could have a potential root rot situation if the plants roots reach that layer.
An example of different drainage rates
For example in my vegetable garden, I have 3 different layers. I have about 3-4″ of organic humus soil (which is mainly decomposed leaves). Next, I have about 3-4″ of sandy loam (67% sand, 27% Silt, 6% clay). Finally I have a bottom layer of heavily compacted sandy loam that is basically hardpan (68% sand, 30.5% silt, 1.6% clay). This compacted sandy loam drains less than 1″ per hour.
So, how do I classify my situation? Well, it’s complicated.
My vegetable garden is on a slight slope. It runs at a 5 degree downward slope. This aids in any drainage issues, as gravity will keep pulling water down the slope and out the deeper exposed soil layers. So, the quick draining upper layer will allow for saturation of my ‘poor draining’ layers, but won’t leave everything sopping wet.
Additionally, when I transplant vegetables, I always do a few things to improve the localized drainage. I dig my holes at least twice as deep and wide as the plant I am going to transplant. Then, I mix in compost with the soil to approximately 50/50 volume. This loosens the compacted soil.
How do I know my poor draining soil is ok?
Well, I have no issue growing large tomato and pepper plants. I have no problem growing and harvesting zucchini, cucumber, squash, lettuce, or peas. We generally have no problem gardening. Our harvests are more than satisfactory. And we do all this with what would normally be classified as ‘poor draining soil’.
So, take this information however you like it. Just because the big gardening websites and magazines tell you that you need to have well draining soil, like most things in life, the true answer is probably more complicated….
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