Is Leaf Mulch Acidic?

You may have been discouraged from using plentiful piles of leaves as mulch because of rumors that leaf mulch is acidic and could harm your plants. The truth is that the acid in leaves is no more harmful to your gardens than the mulch sold at the garden center. Remember that leaves are the natural mulch of the forest without human intervention, providing potassium and micronutrients that are helpful to plant growth.

It is likely the source of the old wives' tale about acidic mulch originated with oak trees (Quercus spp.), which grow in the wide range of U.S. Department of Agriculture plant hardiness zones 4 through 10, depending on species. It is true oak leaves contain more tannic acid than other trees, but the amount of acid is reduced as the leaves decay. Dry leaves have already begun the process of breaking down and are already on their way to being neutralized.

Leaf mulch isn't completely neutral when you place it in a garden, but even the low levels of acid in the leaves can't damage your plants. Because mulch is intended to be used on top of the soil, it can't alter the pH below the first 2 inches of soil and will only do so with repeated usage. If you mix the mulch into the soil before it's fully decomposed, it may affect the soil slightly, particularly in sandy soils. Place the leaves on top of the soil and your overall pH will remain steady.

Tannic acid has long been the villain behind leaf mulch fears. It is important to note that this acid isn't found only in leaves, but is nearly equally found in bark mulch. The acid is buffered by microbes that digest organic matter. The little creatures leave behind humus, the remainder of organic materials. This process, which neutralizes tannic acid, also neutralizes problems with pH. Balance issues in a garden usually happen with the addition of harsh chemicals and antifungals, which kill off the microbes that work to neutralize the acid. The chemicals you may have used in your yard aren't likely to cause this kind of microbe damage.

Mulches that decay quickly, like leaf mulch, can remove nitrogen from the soil. The microorganisms that make that all-important humus require a great amount of nitrogen to do their job. Materials that decay more slowly, like bark, don't have this problem. To balance the negative effects, you can add a nitrogen fertilizer. The usual recommended amount is 1 pound of actual nitrogen for every 1,000 square feet of soil area. Because no type of fertilizer contains 100 percent nitrogen, you must calculate the amount to add based on the percentage indicated on the bag. The amount of nitrogen in a fertilizer is represented by the first number of the three-number set on a bag of fertilizer. For example, 40-0-0 fertilizer would have 40 percent nitrogen. A 10 pound bag of 40-0-0 fertilizer would then have 4 pounds of nitrogen in it. This bag of fertilizer would be appropriate for 4,000 square feet of garden.