Garden Clippings

Soil pH level determines color of hydrangea blooms

| April 2006

But many gardeners don't understand the role that soil pH plays in the health and vigor of garden plants, or how to change the soil to give the plants a better chance.

To bring out the best blue color of a hydrangea, the soil pH should be between 5.0 and 5.5. You can continue adding amendments to the soil until the favored color is achieved, but this is similar to adding salt to a cake recipe. Without knowing the correct amount, you might guess right, but you're more than likely going to add too much or too little. Taking a soil test of a certain area of the garden for pH levels can give you the right amount.

Adjusting pH levels in the garden is not difficult to do, especially if you know what the current level is and where you want to go. In my little garden, pH levels tend to be around 7.0, so a hydrangea bloom would most likely be pink if nothing was done to the soil for adjustment. To adjust the pH to the 5.0 to 5.5 levels that produce the best blue blooms, I would need to add an amendment to make the soil more acidic.

Two commonly used materials for lowering pH are sulfur and aluminum sulfate. With aluminum sulfate, the results are immediate, whereas sulfur requires the aid of moisture and bacteria to break it down before the pH change can occur.

However, if large areas need to be amended, sulfur would be the better choice because of cost and the amount needed to make the change in pH. For example, in a 100-square-foot garden, to lower a pH of 7.5 down to 5.0, which would be optimal for the blue hydrangea, you'd need to incorporate 35 pounds of aluminum sulfate into the soil. The same change in pH would require only 5 pounds of sulfur.

To raise the pH of the soil, limestone particles are used to lessen the acidity of the soil. Lime can be applied in palletized granular or fine dust form. The finer the particle size, the faster it will work in the soil. It's often easier to run a granule or pellet through a spreader to disperse the product, and these particles are more evenly applied to an area than a dust or powder.

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