What Are the Different Types of Soil Amendments?

person holding soil with a small plant growing from it, in both hands

Healthy soil is the foundation of a successful lawn and garden. But not all of us are blessed with rich earth right under our feet. That’s where soil amendments can help. A soil amendment is any material used to alter the soil’s physical properties, including texture, pH, and nutrients.

Different types of soil amendments have different effects. The right one for your garden depends on your soil’s current condition and what kinds of plants you want to grow. 

Using the wrong amendment can completely soil your soil, so choose wisely. Before you try to figure out which amendment is best for you, you need to know what kind of soil you have. To learn more about your soil:

  • Get a soil test to learn your soil’s pH level and nutrient content
  • Check our Guide to Different Soil Types to figure out your soil’s texture 

Do you already know all about your soil? Great! This guide will tell you which types of soil amendments can improve your soil to boost plant growth and make your lawn and garden better than ever. 

Organic vs. inorganic soil amendments

All soil-amending materials fall into one of two basic categories: organic or inorganic.

Organic soil amendments come from living things. Any kind of plant matter (such as grass clippings) or animal byproduct (such as manure) is organic. Organic materials have the benefits of adding plant nutrients to the soil and feeding helpful microorganisms such as earthworms. 

Inorganic soil amendments are mined or man-made materials. For example, minerals, sand, and rubber are all inorganic soil amendments. Inorganic materials can improve your soil’s texture or pH, but they don’t break down and add nutrients the way organic materials do.

Soil Texture Infographic - Soil texture is based on clay, sand, and silt percentages and coarseness
Infographic by Juan Rodgriguez

Soil amendments to improve soil texture

All soil is made of some combination of rock particles and organic matter (called humus). “Soil texture” refers to the size of the rock particles in your soil. The texture of soil affects water retention, nutrient holding capacity, drainage, and erosion. 

The most common soil textures are:

  • Clay: Very fine rock particles that stick together to form dense soil
  • Sand: Large rock particles that form loose soil 
  • Silt: Smaller rock particles than sand but larger than clay that form a slippery soil
  • Loam: A perfect balance of clay, sand, and silt that combines the strengths of all three and negates their weaknesses 

Your soil might contain mostly clay, sand, or silt, or it might be a combination of two or three textures. Loam is the ideal soil texture, and if you have it naturally, you should consider yourself very lucky. If not, adding the right amendments can turn unbalanced soil into perfect loam. 

Best soil amendments for clay soil

Problems with clay soil: Because it’s so dense, clay soil drains poorly and becomes waterlogged easily, which can drown plants. Dense clay soil also becomes compacted and blocks water, oxygen, and nutrients from reaching plants’ roots. Compaction limits root growth, as well.

How soil amendments can help: When adding soil amendments to mostly clay soil, the goal is to lighten and aerate the soil. You want to create space so that plant roots, water, and nutrients can move through the soil more easily. The following amendments will help to loosen clay soil.

Organic soil amendments to improve clay soil:

Inorganic soil amendments to improve clay soil:

  • Pea gravel
  • Perlite (a type of mineral commonly used in soil)
  • Finely shredded rubber 
  • Gypsum

Warning: A common misconception says adding sand to clay soil will loosen the texture, but don’t do it. Mixing clay and sand creates a concrete-like soil structure that’s even worse than the clay itself.

Best soil amendments for sandy soil

Problems with sandy soil: Sandy soil is so loose that it can’t hold water or plant nutrients for very long. The water drains too quickly, leaving nothing but dry soil for your plants, and the nutrients leach out without reaching plant roots. 

How soil amendments can help: With sandy soils, you want to improve the soil’s ability to hold water and nutrients. You need a soil amendment that adds mass to the loose soil and retains water well, such as the following. 

Organic soil amendments to improve sandy soil:

  • Peat moss (or a more eco-friendly alternative)
  • Coco coir 
  • Composted or aged manure
  • Compost

Inorganic soil amendments to improve sandy soil:

  • Vermiculite (a type of mineral commonly used in soil)

Best soil amendments for silty soil

Problems with silty soil: Soil with a lot of silt has a slippery texture that erodes easily. Basically, that means wind and rain can easily carry away the topsoil, which your plants need. 

How soil amendments can help: If you have silty soil, your main concern is to prevent erosion. This type of soil needs amendments that make it more stable so the elements can’t easily shift it. 

Organic soil amendments to improve silty soil:

  • Compost
  • Composted or aged manure
  • Composted wood chips 

Inorganic soil amendments to improve silty soil:

  • Pea gravel 
  • Finely shredded rubber 
  • Gypsum

Soil amendments to change soil pH

illustration showing the pH levels of soil

Soil pH has to do with the soil’s chemical makeup. Your soil’s pH level tells you if the soil is acidic, neutral, or alkaline. Here’s the breakdown of what each soil pH means:

  • Acidic: pH of 0 to 6.5
  • Neutral: pH of 6.6 to 7.3
  • Alkaline: pH of 7.4 to 14 

Soil pH affects how nutrients and minerals dissolve in the soil, which in turn affects your plants’ ability to soak up those nutrients. Some plants prefer acidic soil because of the nutrients they need, while others prefer alkaline. Some plants tolerate a range. In general, a soil pH of about 6 to 7 is best for growing most plants. 

If your soil is too acidic or too alkaline, you can use different types of soil amendments to alter the pH so it fits the preferred range of the plants you want to grow. 

Soil amendments to lower soil pH

Why would you need to lower soil pH? A soil pH of 7.4 or higher may be too alkaline for some of your plants. In alkaline soil, phosphorus is less readily available, which is a problem because phosphorus is one of the three most essential plant nutrients (along with nitrogen and potassium). Other nutrients, such as iron and manganese, are less available, as well.

If you have acid-loving plants or your soil is highly alkaline in general, you’ll need to use soil amendments that lower the pH and make the soil more acidic. 

Organic soil amendments to lower soil pH: 

  • Peat moss 
  • Ericaceous compost (compost made up of acidic materials such as pine needles, oak leaves, and citrus fruits)

Inorganic soil amendments to lower soil pH:

  • Sulfur
  • Ammonium sulfate 
  • Aluminum sulfate (use sparingly to avoid aluminum toxicity)

Soil amendments to raise soil pH

Why would you need to raise soil pH? A soil pH of 6.5 or lower may be too acidic for plants that prefer neutral or alkaline soil conditions. In highly acidic soil, nitrogen, phosphorus, and potassium (the three main nutrients a plant needs) don’t dissolve well and aren’t as available for your plants to soak up. Acidic soils also contain toxic levels of metals, such as aluminum, iron, and manganese.

You’ll need soil amendments that raise soil pH and increase alkalinity if you have highly acidic soil or if your plants need alkaline soil.

Organic soil amendments to raise soil pH:

  • Wood ashes

Inorganic soil amendments to raise soil pH:

  • Garden lime (aka agricultural lime or limestone)
  • Baking soda mixture: 1 tablespoon of baking soda to 1 gallon of water

Soil amendments to add nutrients

Your soil contains many nutrients that are necessary for plants to grow healthy. Plants need more of some nutrients than others. There are three categories of plant nutrients:

  • Primary nutrients: Nitrogen, phosphorus, and potassium — Plants need these nutrients most.
  • Secondary nutrients: Magnesium, calcium, and sulfur — Plants need less of these than the primary nutrients, but they still need a significant amount. 
  • Micronutrients: Boron, zinc, iron, manganese, chlorine, copper, molybdenum, nickel, cobalt — Plants only need a very small amount of these nutrients. 

Nitrogen, phosphorus, and potassium are the most important for your lawn and garden. If your soil lacks any of these three, you’ll probably have a hard time growing most plants. 

How do you know if your soil lacks nutrients? A soil test (which you can get from your local Cooperative Extension Service) will tell you. When a soil test shows your soil has a nutrient deficiency, you can use soil amendments to correct it. 

Soil amendments to increase nitrogen in the soil:

  • Composted or aged manure
  • Alfalfa meal
  • Blood meal

Soil amendments to increase phosphorus in the soil:

  • Bone meal
  • Rock phosphate 

Soil amendments to increase potassium in the soil:

  • Greensand (aka glauconite, a sediment from the ocean floor)
  • Wood ashes
  • Gypsum
  • Kelp or seaweed

Salts in soil amendments

Beware that some soil amendments are high in salts. Too much salt can dry out your soil or burn your grass and plants. 

If your soil is already high in salts, it’s best to avoid salty soil amendments altogether. If your soil doesn’t have a salt problem, you can use these amendments, but only use them in moderation. 

Common soil amendments that increase soil salinity:

  • Manure
  • Biosolids (byproducts of sewage treatment)
  • Gypsum
  • Sulfur
  • Baking soda

FAQ about soil amendments

1. How do you add soil amendments to the soil? 

Here are some general tips for adding soil amendments to your soil:

Step 1: Pour the recommended amount of the soil amendment (varies by type) on top of the existing soil in an even layer.
Step 2: Use a garden fork or spade to mix the amendment into the top 6 to 8 inches of the existing soil. 
Step 3: Immediately after amending, water the soil thoroughly. 
Step 4: Wait at least two weeks before planting anything in the amended soil. 

2. How do you amend the soil in an existing garden?

Adding soil amendments to an existing garden is different from the above process because your plants and their roots will be in the way. In an existing garden, you’ll “topdress” the soil instead of digging several inches deep. 

To topdress the soil in an existing garden:

Step 1: Spread a thin layer of the soil amendment on top of the soil (about ½ inch to 2 inches). 
Step 2: Scratch the soil amendment into the top inch or top few inches of the existing soil using a garden fork or spade. 
Step 3: Be careful scratching around the base of your plants, so as not to damage the roots.
Step 4: Water well. 

3. How much soil amendment do I need?

Each type of soil amendment has its own recommended application rate, or how much of it you should apply per square foot, 10 square feet, 100 square feet, or whatever the measurement is. 

Check the package of your soil amendment for the recommended application rate. Note that the number may vary based on soil type. Once you know the recommended application rate, you just have to calculate how much you need using the square footage of the area you’re going to amend. 

For example, the recommended application rate of garden lime is about 50 pounds per 1,000 square feet. Let’s say your lawn is 10,000 square feet, and you want to amend the soil in the whole lawn. Because 10,000 square feet is 10 times more than 1,000 square feet, you would multiply 50 pounds by 10 to calculate that you need 500 pounds of garden lime.

4. When should you amend soil?

Most soil amendments take time to have significant effects on the soil. For best results, add amendments in the fall and let them break down into the soil for several months before you plant in spring. 

5. What’s the difference between soil amendments and mulch?

Soil amendments and mulch seem similar because they can both benefit the soil and they can both be either organic or inorganic. But mulch and soil amendments serve different purposes.

Soil amendments are mixed into the soil to change its physical properties.

Mulch sits on top of the soil to suppress weeds and help retain moisture. Organic mulches break down into the soil over time to add nutrients, but in smaller quantities than soil amendments. 

Healthy soil is only the beginning

Yes, healthy soil is the foundation of a healthy lawn and garden, but you don’t stop building at the foundation. Once you’ve used soil amendments to improve your soil, you still have a lot left to do to keep your grass and plants growing strong. Lawn Love can guide you every step of the way. 

Check out these other guides on lawn and garden care to find out how to keep your landscape lush and healthy:

With the help of soil amendments, you’ve got a good foundation. From here, you can build something beautiful. 

Maybe you’ve realized lawn care and garden maintenance are a lot of work. Let Lawn Love’s local pros take some of that work off your hands. 

Main Photo Credit: VisionPics | Pixabay

Jordan Ardoin

Jordan Ardoin is a writer and editor with a passion for sustainable, earth-friendly gardening and lawn care practices. When she isn't sharing her knowledge about lawn care and landscaping, you can find her curled up with a good book and a cat in her lap.