by Mark Levisay
What is pH?
To answer this question we need to take the way back machine to high school chemistry class. Acidity is the relative activity of the H+ ion in a given environment, and there is a scale to measure that activity. The scale is from 0-14, where 0 is highly acidic, 7 is neutral, and 14 is highly basic or alkaline. The scale is logarithmic, so a change from 7 to 6 is actually a 10 fold increase in the acidity. In general, plants can grow in a pH range of 3.5 to 10, but most of the plants we’re interested in prefer a range from 6 to 7. Exceptions to this rule would be plants from forests or bogs which have evolved in a more acidic environment. Some acidic loving plant examples are azaleas, rhododendrons, laurels and blueberries.
Azaleas and rhododendrons are a few of the plants in our area that prefer acidic conditions.
The impact of pH on plant growth
The reason we should care about the pH of our soil is that it has a huge effect on fertility. If the soil is too acidic, manganese (Mn) can concentrate in toxic (to plants) levels. Calcium (Ca), magnesium (Mg) and phosphorus (P) can be tied up chemically and not available to the plant. On the other hand, in alkaline conditions phosphorus (P), iron (Fe), copper (Cu), zinc (Zn), boron (Bo) and manganese (Mn) can become unavailable. Some of these are major macronutrients, and plants can really suffer under the wrong conditions.
In central Virginia we rarely encounter alkaline conditions. Most of our native soils are going to be at least slightly acidic due to the underlying geology and the fact that most of this area was forested at one time. Adding mulch to your garden is an excellent idea but will lower the pH (making it more acidic) over time due to the organic processes of decomposition. Even rain is slightly acidic which doesn’t help the situation.
If you have trouble with your lawn grasses and/or have moss then your soil is probably more acidic than you’d like it to be. To get an exact answer, you need to do a soil test, which will give you an exact pH number and a recommendation of how much lime you might need to bring the pH into balance.
How to fix acidic soil
Raising a pH that’s too low (acidic) is pretty straightforward. Limestone is basically calcium carbonate (CaCo3). Adding this to the acidic soil breaks down the molecule, releasing the Ca+ ions along with carbon dioxide and water. Calcium is a nutrient needed by plants so limestone provides an added benefit. You may have used pelletized dolomitic limestone on your lawn, and this inexpensive product is also excellent for your vegetable and flower beds. Dolomite is a limestone that also contains magnesium (Mg), so you get a double benefit by adding both Ca and Mg to the soil. This can be especially important with fruiting plants such as tomatoes which can develop blossom end rot. This condition is caused by a calcium deficiency typically due to insufficient or infrequent watering. Having extra calcium in the soil can help prevent this.
Wood ashes from your fireplace can also be used to correct low pH, but it’s a little trickier. It takes twice as much ash to raise the pH than lime, but ashes also contain phosphorus, potassium and boron, so you shouldn’t use too much at any one time. The best practice is to spread a thin layer of ashes on your garden beds in the fall and let it incorporate over the winter. Never put ashes too close to seeds or seedlings, as it can damage the roots. Vegetables that seem to like things less acidic than others, like beets and spinach, may benefit from an application of wood ash. If you use ashes, it’s a good idea to do a soil test regularly to keep a close eye on the pH.
Keeping your pH at an “optimum” level of 6-7 will help ensure that your soil is able to deliver the nutrients your plants need. Your vegetables, flowers and lawn will reward you for it!
Have a garden question? Send me an email at marklevisay@gmail.com.