Testing Soil Quality by Growing Radishes

Have you ever heard of the phrase "canary in a coalmine?” Coal miners used to take canaries into coal mines where they acted as early-warning signals for toxic gases or fumes. If the birds became sick, or died, miners would have  quality of the air they were breathing.  You can do something similar to test the quality of your garden’s soil by growing radishes. While growing radishes to test your garden soil will not give you any insight into toxins located in soil, this experiment can give you insight into what is lacking in your garden’s soil thus saving you time, money and a lot of heartache before you begin planting.

'French Breakfast' radishes 90 days after planting seeds in generic potting soil from Home Depot last year. No wonder the French are so thin-- look at the tiny vegetables they eat!

I first learned about growing radishes in the garden to act as living soil testers from reading The Bonsai Workshop by Herb L. Gustafson when I grew bonsai in the late '90s. Bonsai grow in small pots with minimal amounts of soil; because of this, much consideration goes into to the quality of the soil the plant material is growing in.

Using radishes to test your garden soil is easy. All you need is a handful of inexpensive radish seeds, a container garden, raised bed or plot of land that has soil you are unsure of. Maybe you have inherited the soil in your community garden plot, or maybe you cannot remember when you last amended the soil in your container garden. Whatever the reason, this trick will give you some insight into what is in your garden’s soil.

1. Plant the radish seeds in the soil you want to know more about by following the directions on the seed packet your radish seeds came in.

2. Water your seeds and keep them moist until they sprout.

3. Once you have seedlings continue to tend to them as if you were going to eat them, but do not fertilize!

4. Observe your radish plants carefully as they grow.  Watch for pests or diseases and take notes. You are playing a garden detective and your observations here will be important.

5. Within 30 days your radishes should be ready to harvest.

If your radishes matured and you harvested a healthy crop, your garden soil is good to go and you are ready to plant. Although, it would not hurt to freshen up the soil by added compost or choosing a fertilizing regiment that is appropriate to your needs. If your radishes fail to produce a vegetable root but grew a lush, healthy top that means that the soil lacks phosphorous and potassium. Yellow, unhealthy-looking (chlorotic) leaves tell you that iron, sulfur, nitrogen and other trace minerals are lacking in your soil. You will have to amend for these deficiencies as needed.

Here's an example of a radish I grew in Happy Frog potting soil.

Do you see the difference between this radish and the radish I grew up in generic potting soil? It was the same type of radish. From the same seed source, even the seeds came from the same seed packet. The difference was in the quality of potting soil I used.

Radishes are great to use in this experiment because the seeds are cheap, sprout quickly and are sown directly into the soil early enough in the garden season that you still have time to do something about the condition of your soil before the gardening season gets going. In the time that it took the 'French Breakfast' radishes to get to size pictured above, I could've had three harvests if the soil had been any good. Testing your garden soil by growing radishes is not an alternative to testing your garden soil for the presence of toxins. Contact your nearest Cooperative Extension office for information on how to test your soil for toxins. I often say that bonsai taught me everything I need to know about gardening. If you can keep a plant alive and healthy in such a small pot then you can grow just about anything.