If there’s one thing you should know about your vegetable garden, it’s the soil pH. The pH scale runs 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. The target vegetable garden pH is 6.5.
Soil pH is important because…
1. pH is so important to plant growth because it determines the availability of almost all essential plant nutrients. At a soil pH of 6.5, the highest number of nutrients are available for plant use. See the USDA chart below for a visual explanation.
2. If the vegetable garden pH is too acidic, certain nutrients become less available, phosphorus in particular, while other nutrients, like aluminum and manganese, can become toxic. Acidic pH levels are also unwelcoming to beneficial soil bacteria.
3. Alkaline soils impede the availability of nutrients like iron, manganese, copper, zinc, and also phosphorous. Plants dependent on high levels of iron, evergreens in particular, perform poorly in alkaline soils.
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How to adjust pH of your soil:
The only way to tell if your garden soil pH needs to be adjusted is to get a soil test. These are available in the U.S. from the Extension Service of your state’s land-grant university. Here’s a link to determine where to go. There are also a number of independent soil testing laboratories. In Canada, check with your local agriculture office. A garden pH test is not expensive and should be performed every four or five years.
1. Acidic soils are amended with lime to raise the soil pH and make the soil less acidic. The exact amount of lime necessary to properly adjust pH can only be determined by a soil test. Do be aware, however, that not all liming materials are equal. Look to your soil test results to determine if you need calcitic lime or dolomitic lime.
– Calcitic lime is mined from natural limestone deposits and crushed to a fine powder. It is also called aglime or agricultural lime and supplies calcium to your soil as it adjusts the pH.
– Dolomitic lime is derived in a similar manner but from limestone sources that contain both calcium and magnesium.
If your soil test comes back showing high levels of magnesium, use calcitic lime. If the test shows a magnesium deficiency, then use dolomitic limestone. Pelletized forms are easier to use and allow for more uniform coverage, and the application rate for pelletized lime is lower than for crushed. A 1:10 ratio is the rule of thumb. Meaning you need ten times less pelletized lime than crushed agricultural lime to garner the same pH change. So, if your soil test recommends adding 100 lbs of crushed agricultural lime, you can add 10 lbs of pelletized as an alternative.
2. If you’re growing acid-loving plants, such as evergreens, blueberries, rhododendrons, and azaleas, you may need to lower the soil pH into the acidic range. If this is necessary, turn to elemental sulfur or aluminum sulfate.
– Elemental sulfur is applied to the garden and is eventually oxidized by soil microbes. It takes a few months to adjust pH. Working it into the soil will yield better results than adding it to the surface because it is more rapidly processed when it’s mixed into the soil. Spring applications are generally the most effective. Elemental sulfur is often found in pelletized form, and while it may take some time to work, it is far less likely to burn plants than aluminum sulfate products.
– Aluminum sulfate reacts quickly with the soil and makes a rapid soil pH change, but there is an increased potential to burn plant roots.
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Soil pH maintenance:
It’s important to remember to only add the recommended amount of any pH adjusting product as per the results of a soil test. Adding too much may shift the pH too far and cause a different set of problems.
Because both lime and sulfur will eventually be processed out of the soil, the pH will revert to a less-than-ideal level every few years. To keep the vegetable garden soil pH at the optimum 6.5, a new soil test should be performed in the vegetable garden every four to five years.