Test kits can be found at Arbico Organics in Oro Valley, nurseries and online. 

Susan Billings / Arizona Daily Star

Like building a house on a good foundation, starting a successful vegetable garden begins with the quality of the soil.

We gardeners know that, of course, but many of us have only a vague idea of what’s really in our soil. We throw in some compost or manure — maybe some soil sulfur — and call it good.

But how healthy is your soil, really? Are you missing any of the Big 3 nutrients — nitrogen, phosphorus or potassium? And what about pH — chances are your soil is alkaline, but why not test it to be sure?

With fall planting season right around the corner, now is a great time to test your soil. You’ll have a lot more success if you have the right balance of nutrients.

Why test your soil?

Just as people need real-food nutrition, plants need quality sources of nitrogen (N), phosphorus (P) and potassium (or potash — K) for optimum growth and health.

Soil fertility also depends on the amount of micronutrients — such as magnesium, iron and zinc — and soil microbes. Adding organic matter helps build fluffy soil that allows air and water to reach the root zone.

Our native desert soil is naturally alkaline, with a pH of about 8.0, says Janet Gagnon, a volunteer master gardener with the County Extension Service of the University of Arizona.

Gagnon, who moved to Tucson from Massachusetts, is a longtime gardener who had to relearn just about everything.

“I had to start from scratch to garden here.”

The pH level affects how well plants will take up nutrients from the soil. The ideal pH for veggies, fruits and trees varies from plant to plant.

In general, if your soil is too alkaline, which is common here in Tucson, your plants will have a hard time taking up the N-P-K that they need to grow, flower and fruit. Sometimes if you adjust the pH, the nutrients will become more available.

Alkaline soils, of 7.8 pH and above, can have iron, zinc and phosphorus deficiencies.

The basics:

  • N: Nitrogen is responsible for leaf growth and keeping leaves green. Too little nitrogen causes stunted growth and yellow leaves. Too much will promote abundant foliage with few flowers.
  • P: Plants need phosphorus for proper fruit and seed development, and it helps plants resist diseases.
  • K: Potassium promotes protein synthesis and improves the color and flavor of fruit. Plants that lack potassium will be stunted and lack good root systems. Leaves will be spotted, curled and appear dried out at the edges.

What you’ll need:

  • Glass jars
  • Baggies
  • Labels
  • A small shovel or trowel.

Grab a sheet of paper to keep notes. Gather some clear, clean jars for mixing soil and water. Take several samples from different parts of your yard but don’t mix them together. Soil from the same yard can vary quite a bit and might have different needs.

(If you're having a company do the testing for you, ask if you should take several samples and mix them.)

How it’s done:

Read the kit handout for exact instructions before beginning.

  • Step 1: Dig to 2-4 inches, depending on what type of plant you’re testing for (lawn, trees, veggies). Scoop about a cup of soil into a baggie and label each sample.
  • Step 2: Test for pH first, using just a bit of each sample.
  • Step 3: Mix 1 part soil with 5 parts distilled or bottled water in a clean jar. (Label the jars, too.) Shake or mix thoroughly. Let it sit until the soil settles.
  • Step 4: Transfer some of the solution to the “color comparator” and add powder from the capsule. Shake well. Wait a minute and view the results. The color indicator will show whether each sample is adequate, deficient or depleted.

That’s it!

Note: If you’re testing more than one spot — say front-yard flowerbeds, backyard vegetable garden and fruit tree areas — be sure to write down your results.

It's a good idea to test every time you get ready to start a new bed or if your plants seem to be struggling and you can't figure out why. If you amend your soil regularly, one round of testing should do it.

Next: What to do if your soil falls short