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Soil Conditioner vs. Compost What’s the Difference?


What's the difference between compost and soil conditioner?

One of the most common pieces of advice for improving soil is to add compost. It improves drainage in clay soil, and it helps sandy soil form aggregates that hold moisture. Over time, compost will release nutrients that feed plants and create an overall healthier soil.

A much less common suggestion for improving soil is to add a soil conditioner, which is great advice…

…but, what’s a soil conditioner? Is it better than compost?

Actually, compost is considered a soil conditioner and in fact, it is the best general-purpose soil conditioner available. A soil conditioner is any soil amendment that improves the physical properties of the soil.

While compost is almost always a beneficial soil amendment, there are other soil conditioners that can improve specific soil properties.

The first step is to get a soil test to identify potential problems with the structure. You can purchase an inexpensive soil test kit (link to Amazon) and get a clear picture of the conditions you are up against.

Once you know the specifics about your soil texture, you can find soil conditioners to target and improve certain properties.

Common Soil Structure Problems

  • Soil texture refers to the ratio of sand, silt, and clay to each other.
  • Soil structure refers to how well the soil holds itself together, how well it holds or drains the water, and how easily oxygen is able to penetrate.

Soil structure is something you can influence and improve with the addition of soil conditioners.

Soil texture, on the other hand, is not something you can change. Sandy soil will remain sandy, and clay will remain clay.

Sandy Soils

Sandy soils have a difficult time holding moisture and nutrients.

In technical terms, sandy soils have a low CEC, or cation exchange capacity.

Plant nutrients are either negatively-charged anions, or positively-charged cations. The more clay present, the stronger the negative charge in the soil.

This negative charge is what holds on to the cations, which results in more fertile soil. These cations can be held or exchanged with other cations, which is where we get the term cation exchange capacity. (source)

Sandy soils also have large pore spaces, which makes it difficult to hold on to moisture, and since sand already has a low CEC, this makes it difficult to hold on to nutrients with a weaker charge.

As water washes through the large pore spaces, essential nutrients are quickly lost to leaching.

Silt Soils

Silt is a smaller particle size than sand, and a larger particle size than clay. Silt deposits can be quite fertile, although the texture is susceptible to compaction.

Silt generally has an average CEC and average porosity. While it is technically the middle ground between sand and clay, silt behaves like clay under most circumstances.

When silt gets wet, it can form a slick, impermeable layer that prevents water from saturating the soil. As it dries, silt will form a crust that makes it difficult for plant roots to expand.

Silt will respond to soil conditioners quicker than clay soils because it has a slightly larger particle size.

Clay Soils

Clay is one of the most difficult soils to improve. If you work it when it’s dry, it turns to dust. If you work it when it’s wet, it turns into a giant concrete slab.

However, clay has the highest CEC out of all three soil textures, and these soils can be incredibly beneficial if you can increase porosity.

It takes longer to improve clay soils than any other soil texture. Changes in structure and pH happen slowly and only with consistent applications of soil conditioners.

Loam Soils

Loamy soils are any mixture of sand, silt, and clay. The ideal loam is 40% sand, 40% silt, and 20% clay.

Balanced loamy soils respond quickly to soil conditioners and can be extremely fertile if they are managed correctly.

How Soil Conditioners Affect Soil

Soil conditioners affect the structure of the soil, so these amendments will be used to improve aeration, water-holding capacity, pH, and CEC (source).

General-purpose conditioners, like compost, will improve all of these factors gradually. You must add compost on a consistent basis in order to see a reliable change in your soil’s texture.

Soil conditioners are divided into two basic categories: organic and inorganic.

Organic Soil Conditioners

Organic means something that is or was alive. Living organisms are made up of many different chemical elements that are bound together.

Once the organism dies, those bonds break down, and the elements are released back into the air and soil.

Organic soil conditioners improve structure as they decompose. Decomposition can take time.

The purpose of adding these amendments is to create a gradual, stable improvement.

Examples of organic soil conditioners include:

  • Animal manure (improves structure & adds nitrogen)
  • Worm castings (adds nitrogen)
  • Sawdust (lowers pH)
  • Compost (improves all aspects of soil structure & nutrition)
  • Wood chips (improves structure & acts as weed barrier)
  • Cover crops (prevents erosion & adds nutrition)
  • Peat moss (improves porosity)
  • Any item added to the soil that used to be alive

If you purchase bagged conditioners at a garden center or hardware store, they should be aged enough to add into your soil with no adverse effects. If you decide to source your own conditioners, many will need to be composted or aged before you add them into your soil.

Fresh manures and worm castings can cause nitrogen burn if they are added directly to your soil.

Wood chips and sawdust can significantly change the pH if they are not aged or applied properly.

Tilling under cover crops can actually cause a decrease in nitrogen while the organic material breaks down.

If you opt for bagged organic conditioners, you can read the label and apply as soon as you get home. If you source your own, you may have to wait up to a year before the organic material is able to be added into your soil, and you may have to process them.

The only exception is adding fresh organic material into fallow land where you don’t expect to grow for at least 6 months.

Inorganic Soil Conditioners

The term organic has come to mean “healthy” or “superior” in many foodie circles. While I definitely recommend buying organically-grown produce, the term organic simply means something that is or was alive. An apple that was not grown organically is still, technically, organic.

Inorganic soil conditioners are not sub-par or less useful than their organic counterparts. They are simply materials that were never alive. These amendments focus more on adjusting pH and CEC rather than porosity.

Generally speaking, an organic amendment is good for most soils all the time. An inorganic amendment is targeted at a specific problem at a specific time.

Inorganic Soil AmendmentBenefits
Gypsumadds calcium
Pulverized limestoneincreases pH & adds calcium
Vermiculiteincreases porosity & water holding capacity
Perliteincreases porosity & water holding capacity
Polysaccharidesreduces erosion
Polyacrylamidesreduces erosion (source)

It is nearly impossible to source your own inorganic soil conditioners. The best place to find these are garden centers, hardware stores, and feed stores.

Many will require additional equipment to spread the amendment, and some may require gloves or a respirator. Make sure you follow the label to keep yourself safe and to ensure you apply the correct amount.

How Compost Conditions Soil

Compost is the best all-around soil conditioner. No matter what difficulties you may be facing with your soil, compost can improve the situation.

This is due, in part, to its ability to improve structure. Organic matter helps soil become spongey and porous, which leads to a healthy balance of moisture retention and drainage, as well as creating a structure that holds nutrients.

However, compost also provides nutrients and beneficial bacteria. The reason compost is considered a soil conditioner, and not a fertilizer or inoculant, is that its chief contribution to the soil is improving structure.

To a lesser degree, compost is a slow-release organic fertilizer and is home to billions of beneficial microbes.

This is why compost is considered the best all-around soil conditioner. While the organic matter in compost is dead, compost is very much alive. It infuses your soil with living, breathing organisms that add life into poor soils, which create chain reactions that improve every element of your landscape or garden beds.

Soil conditioners are a major component of sustainable, healthy growing conditions for your lawn and garden.

Healthy soil will help cut down on irrigation costs, pesticide applications, and fertilizer needs. Conditioners are long-term solutions that work slowly, but their benefits are much more effective and permanent than synthetic sprays.

Learn how to make your own high-quality compost by reading our articles on Compost For Beginners and The Definitive Guide To Compost.  

Learn more about improving clay soil by reading our articles on Troubleshooting & Prevention and Amending Clay Soil.

Sydney Bosque

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.

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