How to Take a Soil Test in Your Tomato Garden

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Updated 2.7.2024

A soil test is an analysis that determines your garden soil’s nutrient content and pH.

Testing allows you to get a good idea of how well the soil will perform when you place your precious tomato seedlings in its holes. In other words, testing “predicts” how your plants would grow in your particular garden with your particular soil if you did nothing to it.

Once you get the test results, you can amend the soil by adding appropriate nutrients which are deficient.

Testing lets you make a much more educated guess about what to work into the soil to make it more tomato-friendly and nutritious.

A good way to understand soil tests is to ask the usual investigative questions: who, what, where, when, why, and how. These frequently asked questions will help you get started.

Tomato Growing Tip #14: you can check your soil with a do it yourself soil test. With Tomato Dirt

Who should test their soil?

Everyone who has a garden.

What do I need?

A kit. Choose between:

Testing with do-it-yourself test kit. The advantage to a kit is convenience – you can test quickly on your own. Many manufacturers offer a variety of soil test kitsYou can also use an electronic soil meter to get pH readings and water content. Some electronic meters also check nutrient levels. An electronic meter is a good investment if you’d like to check pH and water levels regularly throughout the season.

Testing through a soil testing lab. Use a local lab if possible, because they will be familiar with soil content in your area. Most charge a small fee to provide you with a soil analysis from your sample. Check with your local cooperative extension service to find out about their soil testing services, obtain a pre-paid soil sample mailer, and get instructions. 

Which is better?

It depends what you need. Do-it-yourself kits give you a general idea about your soil’s three major nutrient levels and pH. They are quicker and inexpensive. Rapitest produces one of the most popular home soil test kits.

Other manufacturers offer soil test kits with varying features. Lab tests provide more data than simply pH and the 3 major nutrient readings. They are conducted by professionals and are considered to be more thorough.

What will a test tell me?

Do-it-yourself kits test mostly for three major soil nutrients – nitrogen (N), phosphorus (P), and potassium (K) – and check the soil pH level.

Most lab tests provide three categories of information: 

1. Sample’s major nutrients: nitrogen (N), phosphorus (P), and potassium (K)

2. Sample’s secondary nutrients (sulphur, calcium, magnesium) and minor nutrients (iron, manganese, copper, zinc, boron, molybdenum, aluminum)

3. Sample’s pH level.  Many lab tests include recommendations for sodium and lime levels. Some offer recommendations for water saturation, too. (You can also get a soil pH kit.)

Where should I take samples?

Just one sample from one hole in the soil will only test that specific bit of soil. To get the best overall reading of your garden plot’s soil, take a composite sample. (See below, HOW should I take a soil sample?)

When should I test my soil?

  • When you start a new tomato garden
  • When tomatoes aren’t performing well
  • When tomatoes, their stems, or their leaves exhibit unusual coloration
  • When tomatoes demonstrate abnormal growth
  • Every 3-5 years, even if plants are producing, to check for changes in soil composition

When getting ready to test your soil ...

If possible, conduct the test a few months before growing season so you have time to get results and correct the soil.

If you’ve added lime or fertilizer to your garden, wait 6-8 weeks before testing for most accurate results. Otherwise, nutrient content and pH may be skewed because of the addition of recent amendments.

Why test?

Two reasons:

  1. To know what amendments to add to the soil
  2. To know how much correction to make. Testing allows you to add fertilizers in an environmentally-friendly way plus save money. For instance, tomatoes grow best in slightly acidic soil with a pH level between 6.0 and 7.0 – optimum is between 6.5 and 7.0. Testing provides pH level results. When pH is too high (over 7.0), you’ll add sulfur … but naturally, you’d add less sulfur if your soil measures 7.1 than if it measures 7.7. Likewise, if results show a low pH, you will add lime proportionally to raise it. But why add excess lime (or sulfur) if it’s not needed? You’ll just waste money. Testing not only helps you know how to correct the soil, but prevents you from making unnecessary adjustments.

How should I take a composite soil sample?

A composite sample is an “average” of the soil in your entire garden. So you need to get an average!

  • Select a clean bucket in which to collect samples. If the bucket has previously used for fertilizer or chemicals, wash it thoroughly first.
  • Remove vegetation from the garden soil surface: weeds, plants, organic matter, and garden debris.
  • Create a simple drawing of the area from which you’ll take samples, just to help you keep track of what you’re doing.
  • Take your first sample. Dig a 4-6” deep plug with a trowelspade, or shovel soil probe. Place the sample in the bucket. Make a note of the approximate place from which you took the sample.
  • Move to another area of the garden and take another sample.
  • Repeat the sampling process 6-10 times to get a representative from the entire area you’re testing – more samples if the area is large, fewer if it’s a smaller garden. To avoid skewing the process, don’t sample areas that have been treated chemically (especially within the last 6-8 weeks).
  • With a trowel or shovel, mix all the samples together thoroughly in the bucket.
  • Conduct the soil test according to test kit directions. Or, if you’re sending your sample to the lab, then follow lab instructions for submission. 

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