Do this to change the soil pH in your raised garden beds
Q: I have two raised beds and need to lower the pH of the soil in them. Can I use vinegar to change the pH? If so, how much do I need and should it be applied diluted with water?
A: For folks new to gardening, the alkalinity and acidity of soil is measured using a pH scale. The pH scale ranges from 0 to 14. A pH of 7 is neutral, which is neither acid nor alkaline. A reading below 7 is acid and one above 7 is alkaline. Because the pH scale is a negative logarithmic scale, a pH of 5.5 is 10 times more acidic than a pH of 6.5.
The soil pH is important because it affects the availability of nutrients in the soil. Most nutrients needed by plants are most available in the soil at a pH between 6 .0 to 7.5. Because of this, most garden plants grow best in this pH range.
Watering with vinegar is not a recommended method for lowering soil pH for a couple of reasons. First, you would need a large amount of vinegar to move the pH of a large garden bed from 7.5 to 7.0 and because you would not want to apply vinegar to the soil without diluting with water, this could take a while.
The second reason is that applying vinegar only changes the pH of the water solution in the soil and it does not create a reaction to change the pH of the mineral portion. As soon as you started watering with regular water, the soil pH would soon return to the higher pH.
The University of California recommends using sulfur to lower the pH of soil, if the pH is high due to calcium deposits. The sulfur works to change the pH by converting to sulfuric acid in the soil with the help of soil bacteria. To apply granular sulfur to soil follow the recommendation on the bag by soil type, clay and organic soils need more sulfur to change the pH then sandy soils, but don’t exceed more than two pounds per 100 square feet.
Changing the pH with granular sulfur can take some time; however, the conversion rate of the sulfur is dependent on the fineness of it, the amount of soil moisture, soil temperature and the presence of the bacteria. Because of these factors, changes in soil pH after the application can be slow and take several months if the conditions are not ideal. Wait at least three months before testing to see if the pH changed.
Because sulfur takes so long, most people use aluminum sulfate. This is the other material commonly used for lowering soil pH of garden soils. Aluminum sulfate will change the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. But it can also damage plants if too much is applied.
Aluminum sulfate must be applied at a rate five to six times greater than sulfur to lower soil pH the same amount. Because excessive amounts of aluminum sulfate can injure plants, never apply more than 5 pounds per 100 square feet at any one time.
Both aluminum sulfate and sulfur can be found at local garden centers.
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Another way you can lower the soil pH in small garden beds is by adding sphagnum peat. The pH of sphagnum peat generally ranges from 3.0 to 4.5. Because it also adds organic matter to the soil, it effectively changes the pH for at least a garden season.
To use sphagnum peat, add a one- to two-inch layer to the top of the soil. Then thoroughly work it into the top 8 to 12 inches of soil before planting. Because of the expense and labor involved, addition of sphagnum peat to large areas would not be practical.
You should always test your soil before adjusting your pH. Contact the Master Gardner office for information on how to get your soil tested.
The Shasta Master Gardeners Program can be reached by phone at 242-2219 or email firstname.lastname@example.org. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.