What Are Soil Amendments?
Soil amendments are materials you add to existing soil to improve its physical properties, nutrient content, or biological activity. Unlike fertilisers that feed the plant directly, amendments improve the soil itself so it can better support plant growth over the long term. Understanding which amendments to use is essential for gardening success in the Philippines, where soils range from heavy clay to sandy loam depending on the region.
This guide covers what each amendment does, when to use it, and how much to add. For testing your current soil conditions, see our soil pH testing guide. For ready-made mixes that already include the right amendments, browse our soil shop.
Compost
Compost is decomposed organic matter and the single most valuable amendment for any garden. It improves soil structure, adds slow-release nutrients, increases water retention in sandy soil, and improves drainage in clay soil. Good compost smells earthy and looks like dark, crumbly soil.
When to use: Add compost to every garden bed at planting time and again every 3 to 4 months. Use it as a top-dressing for established beds and around trees.
How much: Mix 5 centimetres of compost into the top 15 to 20 centimetres of soil. For containers, make compost 20 to 25 per cent of the potting mix by volume.
Cost in PH: Homemade compost is free. Bagged compost costs 50 to 150 pesos per 5 kilograms at garden centres.
Vermicast (Worm Castings)
Vermicast is the premium organic amendment for Philippine gardens. Produced by earthworms, it contains 5 times more nitrogen, 7 times more phosphorus, and 11 times more potassium than regular compost. It also introduces beneficial microbes that protect plants from soil-borne diseases.
When to use: Add vermicast when planting vegetables, fruiting plants, and flowering plants. Use it as a monthly top-dressing during the growing season. Mix into seed starting blends for vigorous seedlings.
How much: Mix 1 cup of vermicast per potted plant when repotting. For garden beds, spread 1 kilogram per square metre and work it into the top 10 centimetres.
Cost in PH: 80 to 150 pesos per kilogram at garden shops. Bulk pricing is available for orders over 10 kilograms.
Perlite
Perlite is expanded volcanic glass that looks like small white balls. It creates permanent air pockets in the soil that improve drainage and root aeration. Unlike organic amendments, perlite does not decompose, so its benefits last for years.
When to use: Add perlite to soil that drains slowly, compacts easily, or stays wet too long. Essential for container plants, succulents, and any plant prone to root rot.
How much: Mix perlite at 15 to 25 per cent of the total soil volume. For a typical potting mix, that means 1 part perlite to 3 parts soil and compost.
Cost in PH: 80 to 150 pesos per litre at garden shops and online marketplaces.
Vermiculite
Vermiculite is a heat-expanded mineral that looks like small golden flakes. It holds moisture and nutrients like a sponge, releasing them slowly to plant roots. It works opposite to perlite: where perlite improves drainage, vermiculite improves water retention.
When to use: Add vermiculite to sandy soil that dries out too quickly. Excellent for seed starting mixes because it keeps seeds consistently moist. Useful for moisture-loving plants like ferns and calathea.
How much: Mix vermiculite at 10 to 20 per cent of the total soil volume. For seed starting, use up to 30 per cent.
Cost in PH: 100 to 200 pesos per litre. Less commonly available than perlite, but stocked at specialty garden shops and online.
Coco Peat (Coconut Coir)
Coco peat is made from processed coconut husks, making it one of the most sustainable and affordable amendments in the Philippines. It holds 8 to 10 times its weight in water while maintaining a loose, airy structure. It is naturally pH neutral and free from weed seeds.
When to use: Add coco peat to improve water retention in sandy or fast-draining soil. Mix into potting blends for container plants. Use as a growing medium for seedlings and hydroponic systems.
How much: Mix coco peat at 20 to 30 per cent of the total soil volume for containers. For garden beds, work 5 centimetres of coco peat into the top 15 centimetres of soil.
Cost in PH: 30 to 60 pesos per compressed brick. One brick expands to fill a large bucket when soaked.
Rice Hull
Rice hull is a byproduct of rice milling that is abundantly available across the Philippines. Fresh rice hull improves drainage and slowly releases silica, which strengthens plant cell walls. Carbonised rice hull (CRH) is even better because the charring process makes it pH neutral and adds trace minerals.
When to use: Add rice hull to heavy clay soil to improve drainage. Mix into container soil as a lightweight alternative to perlite. Use carbonised rice hull as a soil conditioner and mulch.
How much: Mix rice hull at 15 to 25 per cent of the total soil volume. For improving heavy clay soil, use up to 30 per cent.
Cost in PH: Fresh rice hull costs 15 to 30 pesos per kilogram from rice mills. Carbonised rice hull costs 30 to 60 pesos per kilogram at garden shops.
Charcoal
Charcoal is a powerful soil amendment that absorbs toxins, reduces odours, improves drainage, and provides a habitat for beneficial soil microbes. The porous structure of charcoal creates millions of tiny spaces where bacteria and fungi thrive, building a healthy soil ecosystem.
When to use: Add charcoal to pots without drainage holes to absorb excess moisture. Mix into garden soil to improve long-term soil health. Use as a drainage layer at the bottom of containers. Essential for orchid mixes.
How much: Crush charcoal to 1 to 2 centimetre pieces. Add a handful per pot or mix at 5 to 10 per cent of the soil volume for garden beds.
Cost in PH: Regular cooking charcoal costs 20 to 40 pesos per kilogram. Activated charcoal costs 100 to 200 pesos per kilogram.
Bone Meal
Bone meal is ground animal bones rich in phosphorus and calcium. Phosphorus promotes strong root development and flowering, making bone meal ideal for fruit and flower gardens. Calcium strengthens cell walls and prevents blossom end rot in tomatoes and peppers.
When to use: Add bone meal at planting time for fruiting vegetables, flowering plants, and fruit trees. Use it to boost phosphorus in soils that show deficiency symptoms like purple-tinted leaves and weak root systems.
How much: Mix 2 tablespoons of bone meal per potted plant when planting. For garden beds, spread 200 grams per square metre and work it into the top 10 centimetres of soil.
Cost in PH: 80 to 150 pesos per kilogram at garden centres and agricultural supply shops.
Premium Soil Ready to Use
Our loam soil and garden soil come pre-amended with the right balance of nutrients. Same-day delivery across Metro Manila via Lalamove.
Tips for Success
- Test your soil first. Knowing your soil pH and texture helps you choose the right amendments. Clay soil needs drainage amendments. Sandy soil needs moisture-retention amendments.
- Amend before planting. Work amendments into the soil 1 to 2 weeks before planting. This gives the soil time to stabilise and microbes time to colonise new organic matter.
- Mix thoroughly. Amendments dumped on the surface and not mixed in create layers that block water and root movement. Work everything into the top 15 to 20 centimetres of soil.
- Combine organic and structural amendments. Use compost or vermicast for nutrients and perlite or rice hull for drainage together. One without the other gives incomplete improvement.
- Keep records. Write down what you added, how much, and when. Track plant performance to learn which amendments work best in your specific conditions.
Common Mistakes to Avoid
- Over-amending with nitrogen-rich materials. Too much compost, vermicast, or manure pushes excessive nitrogen into the soil. Plants produce lots of leaves but few flowers or fruit. Keep organic amendments to 30 per cent of total volume maximum.
- Using fresh manure. Fresh chicken, cow, or carabao manure burns roots with high ammonia levels. Always use manure that has been composted for at least 3 months.
- Adding amendments without mixing. A layer of coco peat on top of clay soil creates a barrier that repels water. Always mix amendments thoroughly into the existing soil.
- Ignoring soil drainage needs. Adding nutrients to waterlogged soil does nothing because drowning roots cannot absorb them. Fix drainage first with perlite, rice hull, or sand before adding nutrient amendments.
- Using the wrong amendment for the situation. Vermiculite in an already wet soil makes things worse. Perlite in dry sandy soil dries it out further. Match the amendment to the specific problem you need to solve.
- Amending only once. Organic amendments decompose over time. A single application does not permanently fix poor soil. Plan for regular additions every 3 to 4 months.
Frequently Asked Questions
What is the best soil amendment for clay soil in the Philippines?
The best amendment for heavy clay soil in the Philippines is a combination of coarse sand, rice hull, and compost mixed into the top 20 to 30 centimetres. Add one part coarse sand and one part carbonised rice hull for every two parts clay soil. This breaks up the dense structure and creates drainage channels. Follow up with compost at 25 per cent of the total volume to add nutrients and feed soil microbes. Gypsum also helps clay soil without changing pH. Apply 1 kilogram per square metre and work it in. The full improvement takes 2 to 3 seasons of regular amendment.
How often should I add amendments to garden soil?
Add organic amendments like compost or vermicast every 3 to 4 months for actively planted beds. At the start of each growing season, work 5 centimetres of compost into the top 15 centimetres of soil. Top-dress with vermicast monthly during the growing period. Structural amendments like perlite, rice hull, and coarse sand only need adding once during initial bed preparation or when you notice drainage problems. Coco peat breaks down over 12 to 18 months and should be replenished when the soil feels heavy and compacted.
Can I use too many soil amendments at once?
Yes, over-amending is a common mistake. Adding too much compost makes soil too rich in nitrogen, causing leafy growth at the expense of flowers and fruit. Too much perlite creates a mix that dries out too quickly. Too much coco peat holds excessive moisture and promotes root rot. A balanced approach works best. Keep organic amendments to no more than 30 per cent of the total soil volume. Keep structural amendments like perlite and rice hull to 15 to 25 per cent. Test your soil with a simple squeeze test after amending. It should hold together briefly, then crumble when poked.
