Gardening in clay soil is hard. It is like solid rock when dry and a sticky mess when wet, and it is only ever either dry or wet. For a long time, the go-to solution was to add sand to the clay. But unless you want to truck in a load of sand for your tiny garden plot, you’ll most likely mix in way too little sand to your clay and end up with a soil structure like cement.
What if there were a way to deal with heavy clay soil without a dump truck of additives? What if we could take the organic matter and organic compostable materials we already have lying around our home and yard and fix our soil with that? Stick around to learn how to mix compost with clay soil.
Can I Mix Clay with Compost?
You can mix clay with compost to improve the quality of your soil. Depending on the quality of your existing soil, you may need to add between ⅓ to ⅔ compost to properly improve it. Mixing clay and compost together will produce a nutrient-rich substrate that retains moisture and drains efficiently. Only mix aged compost with clay to ensure optimal nitrogen levels and prevent nutrient lock-out in new plants.
How to Mix Compost with Clay Soil?
How you mix compost into clay soil depends on the type of garden or yard you are amending and your growing goals. The most common ways to amend clay soil are to dig and mix, till/fork, or top dress. Each of these methods has its own advantages and disadvantages and may be practical or impractical in different situations.
- Dig and Mix – This technique is best used when amending a small flower bed or garden plot. Digging in clay is tough, so doing a large area could be tiring. It is most common to dig up clay and mix it with compost, then add it to a raised bed. You can also double dig, which allows you to amend the soil down to 12” to 16” deep directly where you want to plant.
- Till/Fork – A rototiller or broad fork can be used to work compost deep down into compacted clay soil. If you intend to amend an entire yard or large areas or want to do very deep soil rejuvenation before planting, this is an excellent method. Tilling should only be done once to establish a clay compost mix, as continuous tearing up of the soil ecosystem is counterproductive to all restorative practices.
- Top Dress- This method is best employed when garden plants, perennials, and trees, exist in the soil you want to amend. Adding 2 to 4 inches of compost for every 6 inches of clay topsoil is recommended. Apply the compost when the clay is not wet, then water the organic matter deeply. After that, you can add leaf mold and mulch to lock the moisture in.
Issues With Highly Clay Soils
There are many issues with heavy clay soils that can result in dead lawns and underperforming plants. When amending the ground, it is important to know what your current soil problems are, or else mixing might make it worse. Below are some of the common issues with clay soil that you may experience.
Compaction
Walking the same path through a clay yard every day is already enough to cause lasting damage. When clay particles are pushed together, they fuse, and the tiny pores close. The end result is a soil type that cannot drain and repels water simultaneously. This type of soil will not support plant life and becomes bare and ultimately erodes.
Drainage
When clay soil absorbs water, it holds onto it for a long time. While this is beneficial with sparse rainfall, clay can quickly become oversaturated in any extended deluge. With drainage problems comes flooding, drowning plants, and pest and odor problems.
Water loss
On the other extreme, clay that has dried out and becomes cracked has a hard time holding water. Clay soil that is exposed to elements will quickly lose water and lead to drought and heat stress for all surrounding vegetation. With continued water loss, the clay will completely dry out and become a desolate wasteland.
Holds on to Salts
The surface of clay particles has negatively charged ions. These ions attract positively charged nutrients like potassium and calcium and hold them where plants can access them instead of allowing rainfall to wash them away. If too much fertilizer is added, clay soil will have a hard time releasing it, resulting in burnt, stunted, and damaged plants or the need for excessive amounts of water to dilute build-ups.
What Is the Best Soil to Mix with Clay Soil?
There are 4 main soil types, and the combinations they make when mixed can be tweaked to fit your yard perfectly. While each soil has its own unique properties, what is most relevant to this article is what is the best soil to mix with clay soil. Below are common ingredients in improved clay soil mixtures.
- Sand – The largest of soil particles, sand is a mixture of fine rock particles and silicon dioxide. This soil type has stable particles that allow drainage, airflow, and ease of planting. Sand can be added to increase the workability of soil and aid in aeration practices.
- Silt – Typically somewhere in size between sand and clay, silt is the deposited treasures of streams and rivers. This fertile growing medium washes up on banks and represents the favored soil of the earliest civilizations. If you have access to this resource, then its use can result in greatly improved soil quality.
- Loam – This represents a blending of the 3 soils. Typically a ⅓,⅓,⅓ mixture of sand, silt, and clay will result in a fertile loam. This soil type represents the best in water retention, aeration, and drainage, as well as nutrient-dense zones. Loam is easy to plant in, easy to harvest from, and allows for quick, successive plantings of abundant yields.
- Compost – Decayed organic matter is one of the best resources to add to clay soil. Full of microbes that can enter clay pores and push air and water out, this soil amendment changes the very structure of clay soil, making it more workable. Compost also introduces bacteria and critters like worms and beetles that will endlessly work to aerate, decompact, and reinvigorate tired clay terrain.
- Soil Additives – It is possible that the soil type isn’t the only problem when growing plants. Sometimes there may be chemical imbalances or other invisible culprits. Adding lime or gypsum to change the pH may be needed. In other cases, large amounts of bone or blood meal, calcium, magnesium, or diatomaceous earth may be required.
Types of Soil and Soil Mixture Ratios
The different types of soil that make up the earth can be mixed together to achieve different results. When mixing soi,l it is good to have a solid plan on how to move and combine the soil types and then return them to the garden site. If choosing to top dress, you will want to anticipate several growing seasons before desired results can be expected.
Soil Types and Mixing Ratios
| Soil 1 | Soil 2 | Ratio | Results |
| Clay | Sand | Less than 50% | Cement-like soil |
| Clay | Silt | Less than 50% | Improved soil |
| Clay | Loam | Less than 50% | Nutrient-dense soil |
| Clay | Compost | Less than 50% | Living, organic-rich soil |
Break Down of Soil Types and Mixing Ratios results
- Clay + Sand – This combination of soil is great for earthworks and construction. By adding sand to clay and mixing, you can create the basic structure of bricks that construct natural buildings. Adding sand to clay can help build dams and add irrigation channels to your property.
- Clay + Silt – The smaller particles of silt allow it to blend well with clay without locking together, as is the case with sand. This union of clay and silt creates pockets of air that offset the drainage issues of pure clay soil. The added nutrients of silt can be captured by the clay and will not wash away with rainfall.
- Clay + Loam – One of the best ways to amend your clay is to add ⅓ loam and mix it together thoroughly. This combination provides the basis for a very healthy soil that can sustain large amounts of plants. Soil mixed this way will ensure that you can plant for many seasons and not worry about depleting your resources.
- Clay + Compost – The best way to incorporate compost with clay is to top dress. The benefit of compost is that it is alive and rich with microbes, mycorrhizae, bacteria, and worms. When left on top of clay, all of these lifeforms go to work, changing the structure of the clay soil. No amount of tilling or digging could ever come close to the amount of soil these organisms move each day.