Calculator Inputs
Example Data Table
| Container | Initial Volume (L) | Final Volume (L) | Volume Lost (L) | Shrinkage (%) |
|---|---|---|---|---|
| Herb pot (small) | 6.000 | 5.400 | 0.600 | 10.00 |
| Tomato pot (medium) | 18.000 | 16.380 | 1.620 | 9.00 |
| Raised bed section | 120.000 | 111.600 | 8.400 | 7.00 |
Use these as a starting point. Your shrinkage depends on organic matter, particle size, and how much the soil was pre-wetted and compacted.
Formula Used
- Volumetric shrinkage (%) = ((Vi - Vf) / Vi) * 100
- Volume lost = Vi - Vf
- If estimating from linear shrinkage: Vf approx Vi * (1 - LS/100)^3
Here, Vi is the initial (wet) soil volume and Vf is the settled/dry soil volume. Linear shrinkage is applied equally in three directions for a quick estimate.
How to Use This Calculator
- Fill the container with your soil mix and water thoroughly.
- Measure the soil volume (or inside dimensions and soil depth).
- Let the soil settle and dry to your typical maintenance level.
- Measure the final volume (or final dimensions/depth).
- Enter values, calculate, then top up using the suggested amount.
Professional Article
Why soil shrinkage matters in containers and beds
Soil mixes settle after first watering and early dry cycles. Organic particles compress, air pockets collapse, and fine material shifts downward. The surface drops, reducing rooting depth and exposing crowns in some plants. Settlement can also change drainage by creating uneven wetting through the profile.
Key drivers: texture, organics, and compaction
Compost- or fiber-rich blends often shrink more because they compress over time. Loose filling increases settlement, while gentle layering and pre-wetting reduce it. Large wet-to-dry swings raise shrinkage, especially in shallow pots. Consistent irrigation helps stabilize the mix.
Turning results into top-up and mix decisions
The calculator reports volumetric shrinkage and volume lost. Treat “volume lost” as your top-up amount. Using the same mix keeps structure consistent; topping up with compost can increase water-holding and nutrients. After top-up, water lightly and re-check the soil line the next day.
Interpreting shrinkage levels for plant performance
Many mixes fall near 5% to 12% shrinkage. Higher values can signal excessive fines, under-compaction, or very fibrous material. Large settlement increases stem wobble risk and may leave fertilizers concentrated near the surface. If shrinkage is high, improve pre-wetting and reduce extreme drying.
Example data and actionable thresholds
Test one container, then apply the same percentage to similar pots. Use these sample outputs to guide actions.
| Mix Type | Initial (L) | Final (L) | Shrinkage (%) | Suggested Action |
|---|---|---|---|---|
| Compost-heavy blend | 20.0 | 17.8 | 11.0 | Top up 2.2 L; settle with light watering. |
| Balanced potting mix | 12.0 | 11.0 | 8.3 | Top up 1.0 L; add mulch to slow drying. |
| Sandy raised bed fill | 100.0 | 96.0 | 4.0 | Minor top-up; increase organic matter gradually. |
If results stay above 12%, pre-wet thoroughly and compact gently in layers.
FAQs
1) What is soil shrinkage in gardening?
It is the reduction in soil volume after watering, settling, and drying. Organic matter compresses and air gaps collapse, lowering the surface and reducing rooting depth.
2) What shrinkage range is considered normal?
Many potting mixes show about 5% to 12% volumetric shrinkage. Values vary with compost, fibers, fines, and how loosely the container was filled.
3) Should I top up immediately after planting?
Wait until the first settling cycle completes, then top up based on the volume lost. Topping up too early can lead to repeated settlement and uneven watering.
4) Can I use the dimensions method instead of volumes?
Yes. Measure inside length, width, and soil depth for wet and dry states. The calculator converts those measurements into volumes and reports shrinkage.
5) How accurate is the linear shrinkage estimate?
It is a practical approximation when you only know a linear shrinkage percent. It assumes equal shrinkage in three directions, which may differ slightly in real containers.
6) What should I add when topping up?
Use the same mix for consistent structure. Compost can boost nutrients and water-holding, while extra perlite can improve aeration. Match the plant’s needs and your drainage goals.
7) How can I reduce shrinkage in future batches?
Pre-wet the mix thoroughly, fill in layers, and compact gently without crushing structure. Reduce extreme dry-down cycles early, and avoid overly fine materials that settle heavily.