Is pure compost suitable for planting without soil?
Written by
Liu Xiaohui
Reviewed by
Prof. Martin Thorne, Ph.D.Using pure compost for plantings causes serious difficulties. Its fine texture becomes waterlogged, smothering plant roots. Uneven ratios cause mineral imbalances, which can burn delicate roots. Compromise and use mixtures. I learned that lesson after losing my tomato seedlings a few seasons back from nitrogen toxicosis caused by using pure vermicompost.
Container Planting
- Ideal ratio: 30% compost + 60% potting soil + 10% drainage material
- Drainage materials: Perlite, vermiculite, or coarse sand
- Purpose: Prevents compaction while providing nutrients
Garden Bed Preparation
- New beds: Mix 3-4 inches compost into top 12 inches soil
- Established beds: Top-dress 1-2 inches compost annually
- Avoid: Pure compost pockets around plant roots
Seed Starting Mix
- Safe blend: 20% compost + 40% coco coir + 40% perlite
- Critical: Screen compost to remove chunks
- Benefit: Gentle nutrition without burning seedlings
Plants dislike the failure of pure compost to drain well. Water falls in pools that are not draining, causing root rot. The nutrients concentrate to great danger. Mixing causes air spaces and good roots. I use the squeeze test. The good mix lump gets soft, like a ball or lump, crumbling to the touch. Pure compost causes wet, lumpy mud.
Nutrient imbalances make pure compost toxic. Nitrogen levels surge to the point of burning the plant's roots. Phosphorus is rendered unavailable under anaerobic conditions. Potash rapidly leaches from the soil. The mixing of compost with soil alleviates these extremes. Test the compost mixture monthly. Look for pale foliage and stunted growth. These symptoms indicate a problem.
Make blends for specific situations. Ericaceous plants require acid soil, so prepare a compost and pine bark mixture. Succulents need a substantial amount of perlite in the mix to facilitate drainage. Careful consideration is needed to determine the soil best suited for native plants. Where clay soil exists, only a small amount of compost is needed to prevent water from forming around the roots in a bathtub-like fashion.
Transition plants slowly to compost blends. For example, container plants should have 10% compost. Increase the compost gradually during the seasons and monitor the root health when repotting. White firm roots indicate plants are doing well, while brown mushy roots mean you have something to change in your blend. This method builds soil safely.
Read the full article: Choosing the Best Compost for Gardens
