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Soil Amendments: Types, Examples, and What They Do
Written by David S.
A soil amendment is any material you mix into the soil to improve it, better structure, drainage, aeration, and the ability to hold water and nutrients, as opposed to a fertilizer, whose job is to feed the plant. Common examples include compost, aged manure, worm castings, peat moss and coconut coir, perlite, vermiculite, gypsum, and lime. The right amendment depends on what your soil needs.
Quick answer: Soil amendments are materials mixed into the soil to improve its physical and chemical makeup, while fertilizers supply nutrients to the plant. Organic amendments (compost, manure, coir) are the workhorses that build structure and feed soil life; mineral amendments (perlite, vermiculite, sand, gypsum, lime, sulfur) tackle specific issues like drainage or pH. For most Southern California gardens, adding organic matter is the single best thing you can do, and no, gypsum does not break up ordinary clay.
What Is a Soil Amendment?
A soil amendment is any material mixed into a soil to improve its properties: better structure, water infiltration, aeration, and the capacity to hold water and nutrients. Because it changes the soil’s physical condition, it affects plant growth indirectly, which is a different job from a fertilizer.[1] In short, you amend the soil, and you fertilize the plant.
Soil Amendments vs Fertilizers
This is the distinction behind the “beyond NPK” idea. A fertilizer is a product that supplies at least one essential plant nutrient (the N, P, and K on the bag), feeding the plant directly. A soil amendment, by contrast, is material mixed into the soil to improve the soil itself.[2] Both have their place: fertilizer gives a quick nutrient boost, while amendments build the long-term health and structure of the soil (and the living things in it) that let plants thrive.
Common Soil Amendments and What They Do
Amendments fall into two broad groups: organic (from something that is or was alive) and mineral or inorganic (mined or manufactured).[2] Here are the common ones and what each does.
| Amendment | Type | What it does |
|---|---|---|
| Compost | Organic | Improves structure, water and nutrient holding; feeds soil life |
| Aged manure | Organic | Adds organic matter and slow-release nutrients |
| Peat moss or coconut coir | Organic | Hold water and lighten heavy soil |
| Worm castings | Organic | Add organic matter and gentle nutrients |
| Perlite | Mineral | Increases pore space and drainage |
| Vermiculite | Mineral | Adds pore space and holds water and nutrients |
| Coarse sand | Mineral | Improves drainage in clay (needs a large proportion to help) |
| Gypsum | Mineral | Helps high-sodium soils; does not change pH |
| Lime | Mineral | Raises pH (reduces acidity) |
| Elemental sulfur | Mineral | Lowers pH (adds acidity) |
Organic matter is the workhorse. In sandy soil it boosts water- and nutrient-holding capacity; in clay it improves drainage and aeration; and in both it supplies a slow, steady trickle of nutrients and feeds soil life.[3] Among the mineral amendments, perlite adds pore space and drainage while vermiculite also holds water and nutrients, and coarse builder’s sand only helps clay drainage if you add a large proportion of it.[4] If your issue is pH rather than texture, lime raises pH (makes soil less acidic) and elemental sulfur lowers it; most garden plants prefer a slightly acidic range of about 5.8 to 6.5.[5] For a deeper look at building good soil, see our guides to creating healthy soil and potting soil options.
Amending Clay and Sandy Soils in Southern California
A lot of Southern California soil is heavy clay or fast-draining sand, and the good news is that the same fix works for both: organic matter. Worked in, compost opens up clay for better drainage and aeration and helps sand hold onto water and nutrients.[3] Here is the honest part about a popular shortcut: gypsum does not “break up” ordinary clay. Gypsum is calcium sulfate, and its real use is leaching sodium out of high-sodium (sodic) soils; it does not change pH and will not loosen typical non-sodic clay.[4] Research backs this up: on non-sodic soil, gypsum showed no improvement in compaction, because its benefit is specific to sodium-affected soils.[6] Unless a soil test shows a sodium problem, skip the gypsum and add compost. For water-wise planting, see our guide to drought-tolerant plants.
How to Use Soil Amendments
An amendment only works if it is thoroughly mixed into the soil; left buried in a clump or laid on top, it does much less good.[7] Spread organic matter over the planting area and work it into the top several inches, amending the whole bed rather than just the planting hole, so roots can grow out into improved soil.[3] And do not overdo it: over-applying salty amendments like fresh manure can burn roots, so follow the product rate and build your soil gradually.[7] For containers specifically, see our guide to container gardening and soil amendments.
Not sure what your soil needs? Visit any Green Thumb Nursery location with a little of your soil or a soil-test result, and our team will help you pick the right amendments for your garden.
Soil Amendments FAQ
What is a soil amendment?
A soil amendment is any material mixed into the soil to improve its properties, such as structure, drainage, aeration, and its ability to hold water and nutrients. It improves the soil itself, which helps plants indirectly.[1]
What are examples of soil amendments?
Organic examples include compost, aged manure, worm castings, peat moss, and coconut coir. Mineral examples include perlite, vermiculite, coarse sand, gypsum, lime, and elemental sulfur. Compost is the most useful all-around amendment.[2]
What is the difference between a soil amendment and a fertilizer?
A fertilizer supplies nutrients to feed the plant; a soil amendment is mixed into the soil to improve the soil itself. You amend the soil and fertilize the plant, and the two work together.[1]
Does gypsum break up clay soil?
Generally no. Gypsum helps high-sodium (sodic) soils and does not change pH; it will not loosen ordinary non-sodic clay. For typical clay, adding organic matter such as compost is the reliable fix.[6]
How do you add soil amendments to soil?
Spread the amendment over the area and mix it thoroughly into the top several inches of the whole bed, not just the planting hole. Do not over-apply salty amendments like fresh manure, which can burn roots.[7]
Sources
- [1] UC Statewide IPM Program, Fertilizers vs. Soil Amendments: ipm.ucanr.edu
- [2] Colorado State University Extension, Soil Amendments: extension.colostate.edu
- [3] Oregon State University Extension, Add Organic Matter to Improve Most Garden Soils: extension.oregonstate.edu
- [4] UC Marin Master Gardeners, Other Soil Amendments: ucanr.edu
- [5] Clemson Cooperative Extension HGIC, Changing the pH of Your Soil: hgic.clemson.edu
- [6] Ohio State University Extension, Response of Soils and Crops to Gypsum Application: cfaes.osu.edu
- [7] Colorado State University Extension, Choosing a Soil Amendment: extension.colostate.edu
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