Mixing Old Mulch into Soil? How to and the Benefits

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Mulch is used to keep the ground moist and cool, reduce weeds, and provide nutrients for plants. The benefits of mulching are many, but what about when it becomes too aged or rotted? It can now be mixed in with the soil.

Old mulch can be mixed in with soil to help increase the soil’s organic matter. The aerobic decomposition of the mulch by the bacteria and microorganisms will add beneficial nutrients to the soil for the plants while building a healthy soil structure and increasing drainage and aeration for the plant’s roots.

In this article, we will explain the benefits of adding mulch into the soil, how it should be done and the types of mulch you can use. 

There are also some other things to take into consideration to ensure that your plants gain the full benefits from the mulch which we will also explain.

Can Old Mulch be Mixed into the Soil?

Mixing old mulch into the soil will help increase the organic matter within the soil.

However, there is one thing to remember when doing this,

when the microorganisms and bacteria begin to break down the wood chip there would be an initial deficiency of nitrogen in the soil as the microorganisms use it to decompose the wood chips. 

This would temporarily rob the plants that are currently in the soil of nitrogen. They will start showing signs of nitrogen deficiencies which will include yellowing of leaves because they are unable to make sufficient chlorophyll.

The decomposers consume the organic matter, and the nitrogen contained in the dead organism is converted to ammonium ions. The ammonium is then converted to nitrates by the nitrifying bacteria. 

This process usually takes somewhere between 2 to 6 months depending on the soil’s condition and population of microorganisms. 

The microorganisms use the nitrogen for the decomposition process, from which it is then released back into the soil as ammonium nitrate.

Plants love this and hence will flourish when the wood chips are broken down.

If the soil lacks oxygen, then this can lead to the anaerobic decomposition of the mulch, which will produce some very foul odors.

See our article on what causes soil to smell bad.

What are the Benefits of Mixing Old Mulch into the Soil?

There are many benefits of mixing old mulch into garden soil. First of all, you are adding organic matter to the garden, which is a plus.

Old mulch also supplies beneficial bacteria and fungi into the soil as it breaks down.

Increases Organic Matter

The microorganisms and bacteria will produce compost which is very rich in organic matter. This helps to create healthier plants and more resistant plants to diseases as well as creating a much better environment for the soil’s natural population of beneficial flora and fauna. 

Increase Drainage and Aeration in the Soil

Compost can help to increase soil drainage. Compost is a material that is created by microorganisms and bacteria when they break down the mulch. 

This will also provide a much better environment for the soil’s natural population of beneficial flora, such as earthworms and insects. When these organisms move through the soil they help create air spaces and increase drainage within the soil.

Increase Nutrients In the Soil

As previously explained, mixing mulch into the soil is like adding food for bacteria and microorganisms to have a feast. The result of this is the extra nutrients the mulch will add to the soil for the plants to use.

See our detailed article on how mixing mulch into soil can increase the soil’s organics as well as is microbial health.

How to Till Mulch into the Soil

You will need a shovel for this. 

Try to dig the mulch into the soil as deeply as possible without disturbing the roots of your plants. What is important is that you ensure no lumps or clumps of mulch end up in the soil.

1. Remove excess mulch from the soil (This is to prevent nitrogen deficiencies)
2. Using a shovel, till the leftover mulch into the first 2 to 4 inches of the topsoil.
3. Ensure that the soil totally covers the old much.
4. Add a top layer of organic compost 
5. Spread compost evenly over soil
6. Add a thin layer of new mulch and spread evenly over the newly added compost.

Mixing mulch with soil: A perfect combination – Old mulch is often made up of twigs and branches that have fallen off trees and shrubs.

Is Old Mulch Bad for Plants?

One of the most common misconceptions about mulch is that it’s bad for plants. Actually, mulch can be a great thing for plants as long as you’re using the right kind and applying it correctly. 

Old mulch can be mixed in with the soil which would then be broken down into nutrients for the plants. It can also be left alone while new mulch is added on top of it. Eventually, the old mulch will turn into the soil as time passes.

Mulch can benefit plants by :

• Retaining nutrients that might otherwise leach away from the soil with heavy rain, snow or drought.
• Preventing weeds and grass.
• Reducing harmful ultraviolet rays which heat the soil.
• Retaining moisture, increasing drainage and aeration in the soil.

When used properly, however, mulch can be a valuable tool for gardeners who want to create a beautiful vista, an attractive exterior, or even a safer environment for their home from fire ants or other pests.

If you are using some type of inorganic mulch, then you should reconsider mixing it into the soil as it will not add any beneficial nutrients or help increase the soil’s drainage and aeration.

Our recommendations for mixing old mulch in the Soil.

• Don’t mix too much ( prevent nitrogen deficiencies)
• Add excess mulch which was removed to a compost
• Plan the mixing – Mix the old mulch in before winter so it will have enough time to decompose
• Use only organic mulch when mixing into the soil.

Types of Garden Mulch

Garden Mulches falls under two general categories; Organic and Inorganic mulch

Organic Mulches

Organic mulches are mulches that will break down over the course of time which will increase soil fertility and boost the growth of your plants.

Inorganic Mulches

Inorganic mulches are the ones that is synthetically made such as rubber mulch and landscaping fabric. See our article on how to remove old landscaping fabric from garden soil.

These mulches do not break down over time and even if they do, the chemicals contained in the mulch can have a negative effect on plants when released in the soil. 

Not to mention, the chemicals can get their way into the fruits or vegetables and into your plate. This is why you should only consider mixing organic mulches into the soil and know what type of mulch you have is a good start in preventing any unwanted events.

Types of Organic Mulches

1. Wood Chips, Nuggets, or Bark
2. Straw
3. Newspaper or Cardboard
4. Grass Clippings
5. Cocoa Chips
6. Coffee Grounds

Types of Inorganic Mulches

1. Landscaping Fabric
2. Rubber Mulch
3. Stone or rock Mulch

Too much Mulch can be Bad for Plants

As they say, too much of anything can be bad. The same goes for mulch. Too much mulch on the soil can be a problem for the plants. It will reduce air circulation which can result in poor soil aeration and even the plants.

If you decide to use mulch, then make sure to choose a mulch that is light, easy to spread, and will break down over time. 

Remember not all mulched soil is the same as each type of soil is different. Also, remember to mix hardwood mulches with softwood or food-grade mulches with compostable mulches and other garden supplies.

Additionally, mounding mulch around plants can lead to insect and rodent infestation as they will now have a place to hide and nest. 

Too much mulch can also cause the soil to retain excess moisture which would have been given up to evaporation. This moisture retention can lead to root rot and other plant problems.

Can I Mix Mulch with Potting Soil?

Mulch can be mixed into potting soil but do take into consideration that the potting soil you use should not be contaminated with insecticides or fungicides. 

These chemicals could be detrimental to plants. 

You should also mix mulch into potting soil at smaller amounts and then allow time for it to break down. This will help the plants avoid any pest problems and ensure proper drainage.

The time that is given for the small amount of mulch to be broken down in the potted plant will prevent the intermittent nitrogen deprivation associated with the increased bacterial activity from the breakdown of the mulch.

be sure not to mix any inorganic mulch into your potting soil as they will not be beneficial in any way to your plant. 

Inorganic mulch should only stay on top of potted plants for decoration purposes. 

You can check out our detailed article on the benefits of covering potted plants with rocks.

Do I Need to Remove Old Mulch before Adding New Mulch?

You do not need to remove old mulch before the addition of new mulch. However, you will have to ensure that with the addition of new mulch, the overall mulch mass doesn’t create a barrier to the soil beneath.

If the layer of mulch becomes too thick, it can lead to excess moisture retention in the soil, not to mention the heat retention within the soil on hot days.

To prevent the effects of creating a soil barrier you should mix some of the mulch into the soil and remove the rest which can be placed in compost for later use.

Will Weeds Grow through Mulch?

Weed can sometimes grow through mulch. Weeds can grow through mulch layers by tunneling underneath the mulch and so it is important that you ensure that the layers are evenly spread on all sides of the garden bed or potted plant.

You should also keep in mind that weeds can also be killed with mulches than those mentioned above such as Stonewool, lava rocks, pebbles, or cordwood.

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