How to Test and Improve Your Soil

Good soil gives vegetable roots a balance of all the things they need: moisture, nutrients, and air. And knowing your soil type enables you to counteract problems that you may face when gardening on that piece of land. I explain the basics in the following sections.

Distinguishing different types of soil

Clay soil tends to have a lot of natural fertility but is heavy to work with and doesn’t drain water well. Sandy soil, on the other hand, drains water well (maybe too well) but doesn’t have a lot of natural fertility. Loam, the ideal soil, is somewhere in between the two.

Here are general characteristics of the three basic types of soil:

• Sandy soil is composed of mostly large mineral particles. Water moves through this soil quickly, taking nutrients with it. Sandy soil is well aerated, quick to dry out and warm up, and often lacks the nutrients that vegetables need.
• Clay soil consists of mainly small particles that cling tightly together and hold water and nutrients. It’s slow to dry out and warm up, and has poor aeration, but it’s fertile when it can be worked.
• Loam soil is a happy mixture of large and small particles. It’s well aerated and drains properly, but it can still hold water and nutrients. This is the soil to have for a great vegetable garden.

The squeeze test

• Sandy soil falls apart and doesn’t hold together in a ball when you let go. It feels gritty when you rub it between your fingers.
• Clay soil oozes through your fingers as you squeeze it and stays in a slippery wad when you let go. Rubbing clay soil between your fingers feels slippery.
• Loam soil usually stays together after you squeeze it, but it falls apart easily when you poke it with your finger.

Testing your soil's chemistry

In addition to nutrient levels, soil pH also is an important factor in plant growth. The right pH enables vegetables to use nutrients from the soil. Soil is rated on a pH scale, with a pH of 1 being most acidic and a pH of 14 being most alkaline.

If your soil’s pH isn’t within a suitable range, plants can’t take up nutrients — like phosphorus and potassium — even if they’re present in the soil in high amounts. In addition, if the pH is too low, the solubility of certain minerals, such as manganese, may increase to toxic levels.

Most vegetables grow well in a slightly acidic soil with a pH between 6 and 7. Potatoes, including sweet potatoes, prefer a slightly more acidic soil, in the 5 to 6 range. But in general, if you aim for a soil pH between 6 and 7, your vegetables should grow well.

• Use a do-it-yourself kit. This basic pH test measures your soil’s acidity and alkalinity and sometimes major nutrient content. Buy a kit at a nursery, follow the instructions, and voilà — you know your soil’s pH. However, the test gives you only a rough picture of the pH and nutrient levels in your soil. You may want to know more about your soil.
• Have a soil lab do a test for you. A complete soil test is a good investment because a soil lab can thoroughly analyze your soil. Here’s what you can find out from a soil lab’s test in addition to the pH level:
  • Your soil’s nutrient content: If you know your soil’s nutrient content, you can determine how much and what kind of fertilizer to use. In fact, many soil tests tell you exactly how much fertilizer to add; see Chapter 15 for more on fertilizer.
  • Soil problems that are specific to your geographic region: A soil test may help you identify local problems. The soil lab should then give you a recommendation for a type and amount of fertilizer to add to your soil. For example, in dry-summer areas, you may have salty soil; the remedy is to add gypsum, a readily available mineral soil additive.

Fall is a good time to test soil because labs aren’t as busy. It’s also a good time to add many amendments (materials that improve your soil’s fertility and workability) to your soil because they break down slowly.

1. Fill a cup with soil from the top 4 to 6 inches of soil from your vegetable garden, and then place the soil in a plastic bag.
2. Dig six to eight similar samples from different parts of your plot.
3. Mix all the cups of soil together; place two cups of the combined soil in a plastic bag — that’s your soil sample.

If you’re testing a soil imbalance — a known problem that you’ve identified in either pH or nutrients — you may want to test your soil every year because changes in pH and most nutrients are gradual.

A home testing kit is a good way to test a pH imbalance. For nutrients, you may want to do a yearly test at a lab until the imbalance (high or low levels of a nutrient) is fixed. To maintain balanced soil, test it every three to five years.

Adjusting soil pH

• If the number is below 6, the soil is too acidic, and you need to add ground limestone.
• If the measurement is above 7.5, the soil is too alkaline for most vegetables, and you need to add soil sulfur.

Calculating how much lime or sulfur you need

Pounds of Limestone Needed to Raise pH (per 1,000 square feet)

Pounds of Sulfur Needed to Lower pH (per 1,000 square feet)

West of the Mississippi, where less rainfall occurs, soils are more alkaline. But regardless of where you live in North America, you should easily be able to find the lime or sulfur that you need at your local garden center.