Q. My question is about soil samples: When is the best time to retrieve a sample, where should you take your sample, how should a sample be retrieved? I have six raised beds that are used for tomatoes and cucumbers in the summer, and collards and lettuce in the cooler months. The beds are filled with compost that was purchased by the truckload from a local nursery. I top the beds off each year with compost from my own compost pile (of course, I mix it in). I have heard that periodically you should do a soil sample. So how do I do it with multiple beds? — Sandra Leach, Norfolk
A. First, let me briefly emphasize the importance of media testing. (I use the term “media” because, as you have described, mineral soil is not a major component of your growing mix.) Regardless of the medium, testing can help you apply nutrients at the right time and efficiently, avoiding the overapplication that can harm the environment. It is most important to know the pH: It influences the availability of those nutrients and the activities of beneficial soil organisms.
It sounds like you have a good system. Now, what about those pesky soil samples? Don’t be intimidated. The procedure is fairly straightforward. The references I provide here will provide detailed, but simple, instructions with pictures.
The consensus of numerous university extension services is that your garden’s media should be tested very two to three years. Technically, the time frame really depends on the type and percentage of your mineral component, as well as your crops and the way you rotate them from bed to bed. But let’s keep it simple, and just make two to three years a habit. Always keep notes in your garden journal so you remember what you’ve done and when.
The typical garden soil test will sample for phosphorus, potassium, calcium and magnesium, as well as the pH. Again, pH is the most important. If the test recommends adjusting the pH (likely through liming), it will take several months for any corrections to take effect. Thus, testing should be done and corrections made well before the next planting cycle.
When to take samples? The easy answer: any time. But ideally, sample for your raised beds in late summer or fall, after crops have been removed. Obviously, what your crop rotations are, and what you are growing, will determine when you can sample. This is where good record keeping will help keep you straight regarding what crops you’ve rotated in which beds, what beds have been tested, and what applications have been made.
How many beds to sample? Doing them all in every cycle is a little more expensive, but I think it would make keeping track easier. If you amortize, that is $20 a year well spent.
Test kits are available at your local extension office and some garden centers. For more information, check out the Virginia Tech extension publication “Soil Sampling for the Home Gardener” at bit.ly/2cXpGuJ; for North Carolina residents, the state extension’s soil testing kit page is at bit.ly/3EI4mWX.
In North Carolina, testing for samples submitted between April and November is free, and $4 otherwise. In Virginia, the fee is $10. Either way, this is one of the best gardening deals around to help ensure your gardening success.
So there you have it. Rotate your crops and sample every bed every two to three years. Be proactive; don’t wait for the spirit to move you. You will be rewarded for your efforts.
Send questions to wkspen@gmail.com
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