What is the Correct pH for a Vegetable Garden?

Many gardeners don’t realize just how important an adequate soil pH is in growing vegetables. Photo: Krakenimages.com, depositphotos

By Larry Hodgson

Most gardeners have heard the term pH before and know that soil pH is important, but I’m not sure most know why … nor how to modify the pH of a garden soil when it isn’t suitable.

Here is some basic information about soil pH, especially as concerns growing vegetables:

Let’s start by looking at the pH scale. It’s numbered from 0 to 14, with 7.0 being neutral. Measurements between 0 and 6.9 are acidic and those between 7.1 and 14.0 are alkaline. For the vegetable garden, a slightly acidic pH of 6 to 6.9 is considered ideal, as it is suitable for most vegetables.

The importance of pH

The pH is important for plants because it determines the availability of almost all the nutrients essential for their growth. At a soil pH of 6 to 6.9, the most nutrients—such as nitrogen, phosphorus and potassium—are available for plant use. Most plant nutrients do not dissolve when the soil is too acidic or too alkaline and are therefore not available to plants under those circumstances.

If the pH of your garden is too acidic, some nutrients become less available, especially phosphorus, while other nutrients, such as manganese, may be too available and actually become toxic. Soils with a very acidic pH are also not very friendly to beneficial bacteria and microorganisms that live in the soil.

Alkaline soils, on the other hand, restrict the availability of nutrients like iron, manganese, copper, zinc and phosphorus. Plants dependent on high levels of iron, especially tomatoes, turnips, squashes, radishes, and sweet potatoes, often perform poorly in overly alkaline soils.

How to Correct the pH of Your Soil

The only way to know if the pH of your garden soil needs to be modified is to carry out a soil test.

You’ll discover that soil analysis services are widely available. In the USA, they are often offered through a local Extension Service. Here’s a link to find how to reach yours. In Canada, you’ll find most important garden centers offer the service, as do many of the big-box stores that have a plant nursery. However, don’t expect to see a small soil analysis lab at the heart of the store. You have to ask a clerk for a soil sample kit. Request the sample box or bag, take it home, and take the soil sample as requested, then return the sample to the store. Usually, you get the results in less than 2 weeks.

Having a soil analysis done is generally fairly inexpensive. Ideally, you’d bring in your soil sample early in the spring or in the fall, when the plants are not yet actively growing.

1. Soils that are too acidic

Soils that are too acidic need to be amended with lime, an alkalizing product. It increases the pH of the soil and makes it less acidic. The exact amount of lime needed to properly adjust the pH can only be determined by a soil test.

Be aware, however, that not all liming products are created equal. The results of your soil test will tell you whether you should use calcitic lime or dolomitic lime.

Calcitic lime or agricultural lime is extracted from natural limestone deposits and crushed into a fine powder. It supplies calcium, which is naturally alkaline, to your soil and therefore adjusts its pH. Calcitic lime is generally cheaper than dolomitic lime.

Dolomitic lime is derived in the same way, but from limestone sources that contain both calcium and magnesium.

If your soil test shows high levels of magnesium, use calcitic lime. If the test shows a magnesium deficiency, use dolomitic limestone.

Pelletized forms are easier to use and provide more even coverage. Note that the application rate of pelletized lime is lower than that of crushed lime. Usually, the ratio applied is 1:10. This means that you need ten times less pelletized lime than ground agricultural lime to achieve the same change in pH. So, if your soil test recommends adding 100 lb (50 kg) of crushed agricultural lime, you could add 10 lb (5 kg) of pelletized limestone as an alternative.

2. Soils that are too alkaline

If your soil is alkaline, or if you grow acid-loving plants, such as conifers, blueberries, rhododendrons and azaleas, you may need to lower the soil’s pH to make it more acidic. Usually either elemental sulfur or aluminum sulfate is applied.

The most economical way to lower the pH is to apply elemental sulfur to the garden, although it does take longer to act than aluminum sulfate. Indeed, it’s only when soil microbes have oxidized it that it begins to be effective. So, it takes from a few months to a more than a year to adjust the pH.

Working sulfur it into the soil will give better results than simply applying it to the soil surface, because it then acidifies the soil more quickly. Spring applications are generally the most effective.

Elemental sulfur is often found in pelletized form.

Although elemental sulfur may take some time to work, it is much less likely to burn plants than aluminum sulfate.

Aluminum sulfate reacts quickly with the soil and soon changes its pH, but there is an increased risk of burning plant roots.

Maintaining Soil pH

Never add more of any acidifying or alkalinizing product to the soil than the soil test recommends for adjusting the pH. Adding too much can shift the pH too far and cause another set of problems.

Do note that both lime and sulfur will eventually be leached out of the soil. Therefore, after a few years, the soil’s pH will return to a level similar to its original state. To maintain your vegetable garden’s soil pH at an optimal level of 6.0 to 6.9, it’s always best to have a new soil test done every 4 to 5 years and to make any applications recommended.