Guide to Soil Amendments: What They Are and How to Use Them

Unlike potted plants, your garden is subject to a lot of variables in nature and most will need soil amendments. Ideal soil—rich in nutrients and well-draining yet holding enough water so that the plants can absorb it—is rare. Luckily, soil is not static; there is a lot you can do as a gardener to improve it.

Before adding any amendment to soil, it is crucial to do a soil test. For accurate data on your soil and amendment recommendations, visit your local extension office for a specialized lab test.

Here's our guide to common soil amendments and their uses.

The 7 Best Potting Soils of 2024

01 of 23

Coconut Coir

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This fibrous material is extracted from the outer husk of coconuts. Coconut coir soaks up water, thereby improving the soil’s ability to retain water and nutrients.

Best for: Sandy soil
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Compost

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By adding compost—which must be fully decomposed—to your soil, you accomplish two things at once. You add nutrients and improve the soil density. Not only does compost loosen up dense soil, it also does just the opposite and helps to clump together soil that is too loose, which helps root growth.

Best for: Any type of soil that is short of organic matter; clay, sandy, and silty soil to improve soil structure
03 of 23

Vermiculite

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Vermiculite, a finely crushed mineral that looks like shiny flakes, helps the soil retain water and nutrients. It also promotes root growth and facilitates the anchoring of young roots. Vermiculite is a key ingredient in soilless potting mixes and it is also available separately in bags.

Best for: Sandy soil
04 of 23

Sawdust

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If you want to use sawdust, make sure it is well-decayed and not fresh when you work it into the soil. Because just like wood chips and shredded bark, sawdust ties up nitrogen at the expense of plants as nitrogen is consumed by soil microbes when they break the sawdust down. Mix it into the soil very lightly, otherwise it might clump and become a barrier to water flow.

Best for: Clay soil and other hard, compacted soil

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05 of 23

Perlite

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This crushed volcanic material is called perlite. It is mainly found in soilless potting mixes to help soil aeration and drainage, you can also use it for the same purpose in your yard.

Best for: Clay soil
06 of 23

Sand

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Contrary to popular belief, sand is not a good soil amendment for clay soil. It makes the soil dense like concrete. The only application where sand—and only coarse builder's sand, not play sand—is the top choice is for potted plants in order to increase the drainage.

Best for: Potted succulents
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Straw

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Inexpensive straw improves the soil structure of dense soils—it makes it more porous and the improved aeration helps roots to grow. You can directly work straw into the soil without detrimental effects on the nitrogen balance in the soil (unlike fresh wood chips or bark). You can chop up the straw before incorporating it into the soil by running over once or twice with rotary lawn mower. The straw provides micro-organisms like fungi and bacteria with carbohydrates as energy source.

Best for: Clay soil
08 of 23

Wood Bark and Chips

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Wood, either finely shredded bark or chips, do a good job in aerating dense, compacted soil and create spaces so that plant roots, water, and nutrients can move through the soil more easily. They also stabilize silty, slippery soil that is too loose, so it is less prone to erosion. However, if the wood is fresh, it ties up nitrogen in the soil, which leads to nitrogen deficiency in plants because the microorganisms in the soil use the nitrogen to break down the wood. That’s why it is important to only use composted wood chips and bark.

Best for: Clay soil, silty soil

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09 of 23

Sulfur

Not all sulfur is sold under that name, sometimes it is called soil acidifier. Sulfur lowers the soil pH and like other soil acidifiers, it is added to improve the availability of the macronutrient phosphorus as well as the micronutrients iron and manganese to plants.

Best for: Alkaline soil, especially when growing acid-loving plants
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Blood Meal

Dried animal blood is a byproduct from slaughtering animals. It is high in nitrogen.

Best for: Plants that require extra nitrogen such as green leafy vegetables, ferns, and any perennials with lush foliage
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Bone Meal

The finely ground animal bones and waste products from slaughterhouses add phosphate and calcium to the soil, which stimulates larger, improved fruits and blooms. In tomatoes, extra calcium can prevent blossom end rot.

Best for: Vegetables, tubers, flowers, bulbs; add bonemeal to the soil when planting tomatoes
12 of 23

Manure

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Similar to compost, manure is an excellent soil amendment that helps with water and nutrient retention. Whether you use chicken, horse, cow, or any other type of manure, it must be composted or aged, or else its high nitrogen content burns plants.

Best for: Any type of soil to add organic matter; sandy and silty soil to improve soil structure

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Garden Lime

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Also called agricultural lime or limestone, garden lime is a rock powder that is added to soil to make it more alkaline. When the soil pH is below 6.5, nitrogen, phosphorus, and potassium are not as available to plants as in soil with a neutral pH.

Best for: Highly acidic soil; when growing plants that need alkaline soil
14 of 23

Pea Gravel

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This popular hardscaping material can also be used as a soil amendment, as long as it’s jagged-edged, not rounded pea gravel. Working a 2-inch layer of pea gravel into clay soil opens up spaces for plant roots, water, and nutrients. Pea gravel is also suitable to stabilize soft, slippery soils that are prone to erosion.

Best for: Clay soil, silty soil
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Wood Ashes

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If you have ashes from a wood-burning fireplace or a pellet stove, you can use it as a soil amendment to increase the soil pH and make it more alkaline. Wood ashes are often used as a no-cost alternative to garden lime. However, wood ashes should be used only in limited amounts, no more than 20 pounds per 1,000 square foot annually. While ashes contain important plant nutrients (potassium and calcium), they are also high in salt, which can affect germination.

Best for: Highly acidic soil; growing plants that need alkaline soil
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Worm Castings

The castings of earthworms, also called vermicompost, contain nutrients and microbes while also improving soil aeration and water retention and anchor nutrients so they don’t wash out with water. Worm castings have a texture similar to compost. They are also sold in bags but they are more expensive than compost.

Best for: Any soil outdoors, potted plants

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Aluminum Sulfate

Aluminum sulfate is used to lower the soil pH and make the soil less alkaline because phosphorus, one of the three macronutrients, is less available in alkaline soil. Aluminum sulfate has the same effect on iron and manganese, two important micronutrients. By adding aluminum sulfate, you improve the nutrient uptake of the soil.

Best for: Alkaline soil, especially when growing acid-loving plants such as blueberries and azaleas
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Ammonium Sulfate

It acidifies the soil similar to aluminum sulfate, but ammonium sulfate also adds nitrogen to the soil.

Best for: Alkaline soil that also benefits from additional nitrogen for foliage growth
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Cottonseed Meal

The by-product from food-grade cottonseed oil production delivers slow-release nitrogen and some phosphorus to the soil.

Best for: Trees, shrubs, and perennial beds
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Greensand

Mined from natural deposits of glauconite, greensand provides iron, an important plant micronutrient. Greensand also loosens clay soils, binds sandy soils, and increases the water holding capacity of soils of all types.

Best for: Flower beds, gardens, lawns

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Kelp Meal

Kelp is based on seaweed that has been dried and ground. It contains small amounts of nitrogen and potash and is used as a supplement to other soil amendments.

Best for: Flowers, trees, vegetable gardens
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Alfalfa Meal

Alfalfa meal is made of ground-up alfalfa plants that are either shredded into a meal or pressed into pellets. Alfalfa is high in nitrogen.

Best for: Soils in need of extra nitrogen, such as vegetable gardens whose nitrogen gets depleted from leafy crops. Go easy when using it for landscape ornamentals, as excess nitrogen leads to lots of foliage but few flowers.
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Rock Phosphate

Rock phosphate is derived from mined soft rock phosphate. It adds phosphorus to the soil, which promotes blooming and root growth. Rock phosphate is mostly sold in pelletized form.

Best for: Flowering plants

What Soil Amendments to Avoid

There are several common soil amendments that should be avoided:

Gypsum: Although gypsum is commonly listed under recommended soil amendments, its downsides outweigh its benefits. There are other alternatives for compacted soils, first and foremost adding organic matter.
Peat moss: Because of its superior moisture retention capability, peat moss used to be the first choice to break up clay soil and increase water retention of sandy soil. It was also an important ingredient in potting seed-starting mixes and used to lower soil pH. But due to the depletion of peat bogs, peat moss is no longer a sustainable material. Look for eco-friendly alternatives such as coco coir, pine bark, worm castings, and rice hulls. For container growing, there are peat-free potting mixes.
Shredded rubber: Made from scrap tires and sold in garden centers as mulch or soil amendment, the use of shredded rubber is not recommended, as it leaches toxic substances into the soil. Remember, tires are made from oil, not something you’d pour into your landscape.

FAQ

Are topsoil and a soil amendment different?

Topsoil is the upper layer of the soil where many nutrients and microorganisms are located. It includes everything that makes up the soil. An amendment is blended into the soil to enrich it and make it healthier.
Is there a difference between fertilizer and soil amendments?

Both fertilizers and amendments are added to soil to promote healthy plant growth. Fertilizers are concentrated plant food delivered to increase nutrient supply while amendments may change the pH, improve soil structure, or balance nutrients.
How often should soil be amended?

Soil should be amended regularly, as precipitation washes nutrients away, growing crops deplete nutrients, and the soil compacts over time. How often you need to add amendments depends on how good or bad the original soil was and other site conditions, as well as the type of plants. The soil of a vegetable garden that produces baskets full of produce every summer needs more frequent soil amendments than a perennial bed.

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