Quick Answer
Topsoil alone is generally not recommended for growing plants. While topsoil contains important nutrients for plant growth, it lacks the texture and structure that subsoil provides. Using a mix of topsoil and subsoil will create better conditions for root development. However, in some cases where only poor quality subsoil is available, amending topsoil with compost can create a suitable growing medium. Testing the soil and making appropriate amendments is key for healthy plants.
What is Topsoil?
Topsoil is the uppermost layer of soil and has the highest concentration of organic matter and microorganisms. It is usually darker in color than the layers below due to increased organic content. Here are some key facts about topsoil:
- Contains decomposed organic matter like plant residues, manure, and compost
- Has a high concentration of microorganisms that break down organic matter
- Provides many nutrients for plant growth like nitrogen, phosphorus, and potassium
- Has a crumbly, loose structure that allows for air circulation and root growth
- Moisture retention capabilities due to high organic matter content
- Prone to erosion since it is the surface layer exposed to wind, rain, and other elements
The depth of topsoil can range from a few inches to a couple feet depending on factors like parent material, climate, vegetation, and terrain. Areas with minimal disturbance and abundant native vegetation often have relatively deep topsoil layers.
What is Subsoil?
Subsoil is the layer of soil beneath the topsoil and above the parent material. In contrast to topsoil, subsoil has lower organic matter content and microbial activity. Here are some defining features of subsoil:
- Lower in organic matter and microorganisms compared to topsoil
- Higher clay content which gives it a dense, compact structure
- Less total pore space and larger pores than topsoil
- Less ability to retain moisture and nutrients for plant use
- Light brown to red color from iron oxide coatings on clay particles
- Can have harder pans or clay layers that restrict root growth
While subsoil has less available nutrients than topsoil, its texture plays an important role in soil health. The clay content in subsoil improves water holding capacity and cation exchange capacity – the ability to hold positively charged nutrients. The larger pore spaces also allow for drainage and gas exchange. Roots need to grow through subsoil to anchor plants and access moisture and nutrients deep in the soil profile.
Benefits of Mixing Topsoil and Subsoil
Using a blend of topsoil and subsoil can provide the ideal balance of nutrients, texture, structure, and drainage for plant growth. Here are some of the benefits of mixing topsoil and subsoil:
- Combines nutrient-rich topsoil and moisture retaining subsoil
- Improves drainage while still holding adequate water and nutrients
- Allows roots to grow easily through the soil to access nutrients
- Prevents compacted layers that restrict root growth
- Balances aeration and water retention
- Provides physical support and anchoring for plants
- Blending reduces need to amend single soil type to reach ideal conditions
The ideal ratio of topsoil to subsoil often falls in the range of 20-40% topsoil to 60-80% subsoil. However, the optimal blend depends on the soil properties and plant requirements. Lighter soils with more sand may warrant higher topsoil ratios, while heavy clay soils perform better with more subsoil. Testing the soils and amending when needed is important for getting the mixture right.
When to Use Just Topsoil
In some cases, using 100% topsoil may be warranted if the available subsoil is of very poor quality. Reasons to use just topsoil include:
- Subsoil is extremely dense, compacted, or restricted by hardpans
- Subsoil is mostly heavy, dense clay that retains too much water
- Subsoil is very shallow over bedrock or the water table
- Subsoil has high salinity or toxicity issues
- There is not enough subsoil available to blend
Using just topsoil is generally only a short-term solution and will require extra management to create a suitable growing environment. The lack of subsoil can lead to problems like:
- Poor drainage and aeration
- Insufficient anchoring for plants making them prone to uprooting
- Nutrient deficiencies if organic matter breaks down too rapidly
- Soil compaction over time as structure deteriorates
- Erosion issues as light topsoil particles wash or blow away
To make pure topsoil more suitable for plant growth, adding organic amendments and aerating regularly is recommended. Cover crops can also help maintain soil structure. Container gardens and raised beds with carefully formulated soil mixes are other options for using primarily topsoil.
Amending Topsoil
When using mostly topsoil, either by necessity or design, it should be amended to create a better growing medium. Here are some amendments to consider:
Compost
Compost serves as a nutrient reservoir and helps promote microbial activity and good soil structure. Compost percentages between 20-40% are typically recommended when blending with topsoil. High-quality, well-aged compost is ideal.
Sand
Adding sand can improve drainage and aeration in dense topsoil. Coarse sand particles create larger pore spaces for air and water movement. About 20-30% sand by volume is usually suitable when amending topsoil mixes.
Perlite or vermiculite
These lightweight minerals can also help lighten heavy topsoil textures when used at amendment rates of 10-20%. Both perlite and vermiculite improve aeration and moisture retention.
Peat moss
Peat moss provides moisture retention but is also acidic. It should be used sparingly at 10% or less when amending topsoil.
Biochar
This highly porous, carbon-rich material can enhance nutrient retention and water holding capacity. Use biochar at 5-10% amendment rates.
Fertilizers
Since topsoil loses nutrients faster without subsoil, organic fertilizers may be needed. Compost provides some fertility, but additional nitrogen, phosphorus, and potassium should be supplemented based on soil testing.
Soil conditioners
Materials like coir, humates, or natural polymers can help retain moisture and improve topsoil structure. Follow manufacturer’s rate guidelines.
Key Considerations
When deciding whether to use just topsoil, the following factors should be considered:
- Soil testing: Test topsoil and subsoil to determine realistic amendments and optimal plant growth conditions.
- Available materials: Assess how much quality topsoil and subsoil can realistically be obtained for the project.
- Plant factors: Choose plants suitable for amended topsoil if subsoil cannot be mixed in.
- Drainage: Address potential drainage issues with sand, perlite, or elevated beds if using pure topsoil.
- Irrigation: Supplemental irrigation may be needed to account for lower water holding capacity of topsoil alone.
- Erosion control: Use cover crops and mulch to protect topsoil from wind and water erosion.
- Soil building: Topsoil alone will require more intensive soil building with compost, amendments, and cover crops.
With the right amendments and management practices, it is possible to grow plants in pure topsoil. However, mixing with quality subsoil is still the best option for creating ideal garden soil.
Conclusion
Topsoil forms the fertile, life-giving skin of the earth. But other soil layers play crucial roles in providing structural support, drainage, and nutrients for plants. The ideal approach is blending topsoil and subsoil to harness the benefits of each.
In situations where only topsoil is available, with careful amendment and management, it is possible to create a suitable growing medium. But the lack of subsoil’s stabilizing texture must be addressed through additions of compost, minerals, and organic fertilizers. Extra steps will also be required to prevent erosion and maintain soil structure in the long-term.
While plants can grow in pure topsoil, the blend of topsoil and subsoil best mimics nature and provides ideal conditions for vigorous vegetables, flowers, and other plants. The complexity of soil ecosystems means it is difficult to truly isolate and improve just one component like topsoil. Working to enhance the interplay of biological, chemical and physical properties throughout the soil profile results in the healthiest, most productive soils.
