Yes, you can fertilize on top of mulch, but the amount of success you have depends on the type of mulch you have in place and how fast it decomposes, as well as the fertilizer you’re using.
- With fast-decomposing mulches such as seed hulls, grass clippings, and leaf mulch, you can apply fertilizer over the top of the remaining mulch prior to your yearly reapplication of mulch. Most of last year’s mulch will have decomposed by this time.
- With slow-decomposing and organic mulch, such as traditional wood and bark mulch, you should rake aside mulch before applying fertilizer to your garden soil or flower bed, to ensure the best contact with the soil surface.
- When using granular fertilizer, it’s always best to provide as much ground contact as possible. Granular fertilizers work best when they come in contact with moist soil, where they break down and release their nutrients.
- Liquid fertilizers are the best choice when fertilizing on top of mulch. They will work their way through the mulch to the soil below.
How to Fertilize on Top of Mulch
Timing is essential when applying fertilizer on top of mulch. If you’re using a fast-decomposing mulch, such as straw, grass clippings, or cocoa hulls, then you will typically need to reapply mulch each spring. Fertilizing before adding new mulch is a great idea because at this point most of the mulch in your garden will have broken down. You can fertilize, then add a layer of mulch with little fuss.
If you have a slow-decomposing mulch, such as bark mulch or nutshells, it may be more difficult to add fertilizer over mulch. This requires selecting a liquid fertilizer that works best at penetrating mulch and helping plants grow.
- In gardens where most of the mulch has decomposed (½ inch or less of mulch remains), apply granular or liquid fertilizer on top of old mulch, then add a layer of new mulch. Add at least 2-3 inches of mulch is discourage future weed growth.
- In gardens and areas with a significant quantity of mulch remains (greater than ½ inch), avoid using granular fertilizer. Use liquid fertilizer to make sure you reach all plant and tree roots.
Fertilizing Through Wood Chips
The best practice for fertilizing in a garden with wood chips is to remove the mulch first, or at least rake mulch away from the base of plants before fertilizing. This will yield the best results and deliver the highest amount of fertilizer to your plants.
In large gardens, it may not be feasible to remove or rake aside mulch to apply fertilizer. If you must fertilize through wood chips, remember the following:
- Avoid using granular fertilizer, which may get trapped in mulch and will not break down properly.
- Apply a liquid fertilizer on top of wood chips. Water after fertilizing to soak the fertilizer into the soil.
- Check the manufacturer’s instructions on your liquid fertilizer. They may recommend increasing the amount of fertilizer used in mulched areas.
Does Mulch Fertilize the Ground and Soil?
Mulches gradually fertilize the ground as they break down. Think of mulch as a thin layer of material that slowly turns into compost.
Over time, mulches will break down and add nutrients to the soil, which is generally very good. However, mulches with chemicals in them (as in some commercial wood mulches) or acidic mulches (such as pine needles) also could potentially add these qualities to the soil. So, be careful when choosing your mulch.
Does Mulch Tie up Nitrogen in the Soil?
Don’t believe the myth that mulch reduces nitrogen in the soil. This is completely untrue. As mulches break down, including wood and bark mulch, they actually add more nitrogen to the soil.
The myth that mulches steal nitrogen is common and still used by some to convince gardeners to add extra fertilizer after mulching. Mulch will not suck nutrients out of your soil. The worst that can happen is that some of your liquid fertilizer may be soaked up by mulch, which is why it’s best to rake mulch aside when fertilizing.
Can You Add Compost on Top of Mulch?
As with any fertilizer, it’s best to add compost and/or compost tea directly to the soil. This allows for the quickest and most powerful impact. However, there are some cases where you can add compost on top of mulch.
- In gardens with a ½ or less of mulch, add a 1–2-inch layer of compost, then apply new mulch on top.
- In gardens with greater than ½ inch of mulch, rake aside mulch at the base of plants to apply compost to the soil.
- Compost applied on top of a thick layer of mulch may not penetrate the soil and feed your plants. If left exposed, it can become a breeding ground for weeds.
Fertilizing on Top of Mulch
It’s best to apply fertilizer on top of mulch only when there is ½ inch or less of mulch. In areas with thick mulch, the best practice is to rake mulch away and apply your fertilizer or organic fertilizer of choice directly to the soil.
If you must apply fertilizer on top of mulch, use a liquid fertilizer and water thoroughly after fertilizing, to encourage nutrients to reach the soil and touch the roots of every plant and shrub. Granular fertilizers are generally less effective on top of mulch than liquid fertilizers.