In home gardens, over-application of fertilizer is common. There is not really a financial penalty involved when the garden plot is measured in square feet, not acres. So fertilizer is not usually in short supply.
As a rule gardeners either: don't fertilize nearly enough or grossly over-fertilize their garden. Few hit the Goldilocks zone.
That said, understanding what your soil needs is a good way to improve your garden. In California, there is no free soil test facility; soil tests are done by commercial laboratories.
Soil tests are seldom helpful in heavily amended garden beds. There are usually ample nutrients and all the lab does is confirm that. If there is a problem, a simple pH or salinity test will suffice. We can do those locally for free by appointment. Contact our helpline.
One situation that soil test can benefit is new garden beds. When there hasn't been much done to the soil, it can be helpful to find out what's needed.
Home orchards especially benefit from soil testing before planting.
How to Collect Soil Samples
Soil to be tested should consist of multiple samples taken from all over an area that will be managed the same way. While it's possible to analyze samples from each bed or small section, it is hard to tell whether the results are accurate, or due only to sampling error.
The quality of your results is related to the quality of the sample submitted. The samples should represent the average of your garden. If 20% of your garden is different from the rest, make 20% of the samples from that area. Unless the areas are large, comparisons between "good" and "bad" areas in your yard aren't often helpful since we can't tell if the issue is sample error or not, and it may not even be possible to treat that area differently — especially in home lawns.
Examples of what to get samples from:
- 10 to 15 random spots throughout the vegetable garden
- 10 to 15 spots in the home orchard
- 15+ samples around the lawn or pasture
- Sample blueberries separately (5 is probably enough)
- 10 to 15 samples from your home vineyard or cane fruits
Before getting a soil test done, find out more about your soil. In many cases, the answer you seek has already been figured out. If you haven't done anything ill-advised like adding wood ash, we've found that NRCS's soil survey data will be close to your actual soil conditions.
As a service to you we provide this list compiled by the Contra Costa Master Gardeners of soil test labs. No endorsement is implied. All charge for their services.
If all you need is soil pH and salinity, we can provide that service. There is currently no charge for pH or salinity. Contact our help line at firstname.lastname@example.org to arrange a test. Please follow the recommended sampling procedure.