Amending Clay Soil With Peat Moss

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Sydney has over 15 years of experience in lawn maintenance, landscape design, and organic gardening. She has an A.A.S. in Landscape Design/Organic Produce Production from the Nebraska College of Technical Agriculture.

Peat Moss vs. Sphagnum Moss

Peat bogs produce two products:

Sphagnum peat moss (or just peat moss)
Sphagnum moss

Sphagnum moss grows in bogs. As the moss matures, old, dead plant material forms layers underneath the living moss. These deposits can grow to over 20’ in-depth, and form the material we call peat.

Bogs are wetlands, and as the dead plant material builds up, it is constantly saturated. This creates anaerobic conditions, which brings decomposition to a halt. It also creates extremely acidic conditions, with a resulting pH of 4-5.

Peat moss is a dark, crumbly, stable organic matter that is used as a soil amendment, potting soil ingredient, and seed-starter. It is sold in compressed bales and is labeled either sphagnum peat moss or peat moss.

Living sphagnum moss is also harvested from peat bogs. The moss is collected green and then dried, forming a loose, fibrous, moisture-retaining mat.

Sphagnum moss is used to line baskets, for arts & crafts, and as a soil amendment. Sphagnum moss can be milled and baled, or sold in loose in bags. Sphagnum moss has a neutral pH and helps with aeration.

Sphagnum moss and peat moss both resist decomposition, which makes them very stable soil amendments. However, they serve different purposes and are not interchangeable.

How Peat Moss Affects Soil

Peat moss is stable acidic organic matter. This makes it uniquely suited for clay soils since clay is usually low in organic matter, and alkaline.

Peat will have long-lasting effects, so it is important to understand how it can change your soil to avoid creating new problems.

Does Peat Moss Lower pH in soil?

Peat moss is one of the first amendments growers recommend for lowering pH in alkaline soils. However, peat moss only has mild, temporary effects on pH.

The pH of a soil is the product of its environment. The pH of irrigation water, parent material, organic material, and even rain can influence pH, and it is impossible for one application of peat moss to overcome all of these external factors.

Blueberries are a popular acid-loving plant, and most growers swear by peat moss. However, the addition of peat moss has less to do with pH and more to do with how blueberry roots access nutrients. Peat moss helps encourage fungi on the roots that enable the plants to grow.

Adding peat moss to your soil may help lower the pH of naturally-acidic soil, but it will have difficulty overcoming naturally-alkaline soil A better amendment for lowering pH is sulfur.

How Does Peat Moss Effect Soil Nutrients?

Peat moss helps improve the cation-exchange-capacity of soils, which helps retain nutrients.

Many nutrients are positively-charged ions. Cation-exchange-capacity (CEC) refers to the negative charge of the soil, which allows it to hold onto positively-charged nutrients. (source)

Organic matter, like compost, has a naturally-high CEC value, but it also breaks down easily. Peat moss is an extremely stable organic matter, which helps to increase CEC values long term.

Increased CEC means increased nutrient retention. Essentially, peat moss helps trap and hold nutrients so they aren’t lost due to irrigation.

This can be helpful in sandy soils, which have a low CEC. However, clay soils already have naturally-high CEC values, and therefore peat moss doesn’t have a significant impact.

Does Peat Moss Help Clay Soil Drainage?

This is where peat moss truly shines. Clay soil is made up of tiny particles that pack together and form impenetrable layers. This makes it incredibly difficult for water to penetrate, and if it succeeds, it is just as hard to drain.

This leads to clay soils that either shed water and are always dry or soils that are constantly waterlogged and slippery. Both cause issues for plants and both can be helped with the addition of organic matter.

Organic matter is porous, which helps improve absorption and drainage. Compost can be broken down quickly in clay soils and requires frequent applications to improve soil structure.

Peat moss is very difficult to break down, and the effects it has on porosity last much longer than compost. However, because it is difficult to break down, it doesn’t release nutrients into the soil. This is why you need to add peat moss and compost together to increase porosity and add nutrition.

What Soils Need Peat Moss?

Peat moss is generally recommended for alkaline soils as a way to lower pH. However, peat has a negligible effect on naturally-alkaline soils and only a mild effect on acidic soils.

This is good news for homeowners with sandy soils. Sand is generally acidic and suffers from poor water holding capacity. Peat moss would help soils retain water, but has not been recommended for sandy soils because they are already acidic.

However, we know now that peat moss has a small effect on pH, and is safe to use in acidic soils in reasonable amounts.

Peat is also an excellent amendment for clay soils, but it must be used as one part of a comprehensive soil improvement plan. Peat will increase drainage, but it will also increase the soil’s ability to retain water.

If peat is used in conjunction with compost applications, a 1-2-3-2-1 irrigation schedule (source), and annual aeration, it will help the overall health and structure of the soil.

Applying Peat Moss To Bare Soil

Peat moss is sold in bales at most hardware stores and nurseries. Before you purchase any, measure the area you want to amend. Divide the total square footage by 90 to determine how many large bales you will need.

For example, if your lawn and garden space is 35’ x 25’, you need to amend 875sq’. Each large bale of peat moss contains 3.8 cubic feet of material, which is enough to cover 90sq’ at a 1” depth.

So, if you divide 875 by 90, you get 9.72, which means you need 10 bales of moss to cover this area.

Follow these steps to incorporate peat moss into bare soil:

Soak the moss in a large bin or tub of water for 24 hours. Peat is difficult to saturate, especially in the soil. Soaking it first helps it incorporate into the soil instead of creating dry patches.
Till the area as deep as possible. Wait until the soil is moist before tilling.
Use this opportunity to incorporate both compost and peat deep into the soil:
- Spread 3” of compost over the area
- Spread 1” of peat over the area
Lightly till the organic matter into the soil, being careful not to over-till and ruin the structure
Wait 2-3 months before planting to allow material time to break down

Incorporating large amounts of organic matter into your soil can cause a temporary loss in nitrogen. As the material decomposes, it draws nitrogen from the soil to support the process. Once decomposition is complete, nitrogen is released back to the soil and is available for plants.

It is best to till and amend bare soil late in the fall before the ground freezes. This will prevent weeds from germinating on bare soil, and it allows organic matter to begin to break down. The soil will be loose from tilling, and it is important to keep it from getting compacted. Don’t allow people to walk or drive on the tilled area, or else you will need to till again before planting, which can destroy the structure.

Applying Peat Moss To Existing Lawns & Landscapes

If your lawn or garden is suffering, but not enough to rip it out and start over, you can add peat in small doses to gradually improve the soil.

Use peat as part of a larger soil improvement plan to get the most benefits.

To add peat moss to an existing lawn or landscape, follow these steps:

In the spring, use a core aerator and aerate as deeply as possible (make sure soil is moist, not wet, before aerating)
Soak peat moss in a large bin or tub for 24 hours before application
Spread peat moss ¼” thick layer over the area, and rake it into the holes
Spread ¼” thick layer of compost over the area, and rake it into the holes
Water thoroughly

For an existing landscape, you will need to pull back mulch and weed fabric to aerate. You can use a handheld aerator to work around large shrub roots and flower beds.

Once the area has been aerated and amended, employ a 1-2-3-2-1 irrigation schedule, and mow twice per week. This will encourage roots to grow deeper into the soil, which helps incorporate the compost and peat into the soil and improve drainage.

Looking for more ways to improve clay soil in an existing lawn? Read Amend Clay Soil Without Digging – 5 Steps To Lasting Results

Is It Working?

This can be difficult to gauge. The most common problem with clay soil is overwatering, and peat should help with drainage over time.

Your lawn should begin to absorb water readily during irrigation, instead of running off into low spots. When the lawn is wet, it should be less slippery and more springy. Puddles should drain quickly, and yellow spots should turn deep green.

It will take consistent applications over a few years before the structure is noticeably improved.

Peat moss is not going to fix clay soil on its own. However, it is a valuable ingredient in a soil improvement plan, and it provides long-lasting benefits.

It does not improve pH, but it will improve porosity, which is the major issue facing homeowners with clay soils.

For more information on improving clay soils, visit our articles on in-ground composting, soil preparation, and understanding alkalinity.

Product Name	Sphagnum peat moss (or just peat moss)
Characteristics	Improves drainage. Helps soil retain nutrients. Improves water retention Lowers pH (slightly)
Best For These Soil Types	Clay soil (to improve drainage) Sandy soil (to improve water retention)
Clay soil applications	Can be used to improve both bare soil and existing lawns.