A Beginner’s Guide to Soil Amendments for a Healthy Garden

If there’s one gardening mantra that rings true, it’s that healthy soil leads to healthy plants. But not every soil starts out healthy! If you’re dealing with a garden that’s low in nutrients, poorly drained, or void of life, adding the right amendments will make it a more inviting place for plants.

There are hundreds of different amendments, so it can be difficult to choose the right ones for your garden. That’s why I’m here! I’ll introduce you to some of the most common amendments and help you determine which ones you should apply to your garden.

What Are Soil Amendments?

Amendments are products you add to the soil to improve it. These items can improve the chemical properties (fertilizers, pH adjusters), physical properties (peat moss, sand), or biological properties (beneficial microorganisms, compost) of soil. That’s right—amendments include more than just fertilizer!

Amendments often come in solid forms that you sprinkle on top of the soil. However, they can also include liquid products that you spray on top of the surface.

You can find both organic and synthetic amendments. Organic products come from naturally derived materials, while synthetic materials are man-made. You may also hear people classify amendments by whether or not they’re approved for use in Certified Organic production.

While organic products are generally less harmful than synthetic materials, it’s important to note that adding too much of any amendment can hurt your garden and plants. That’s why it’s important to understand the different types of amendments as well as when to add each one to your garden.

How to Determine Which Amendments to Use

Determining which types of amendments to apply can be overwhelming. After all, you have hundreds of products to choose from! Rather than blindly adding a little of this and a handful of that, I recommend conducting a soil test first.

These tests vary in complexity and scope, but most will tell you the pH, nutrient concentrations, cation exchange capacity (CEC), and the percentage of soil organic matter (SOM). While you can use an in-home test kit to test, I recommend collecting a sample and sending it to a certified testing lab. Both private and public labs exist, and one isn’t necessarily better than the other. However, you should always send your samples to the same lab since test procedures vary from lab to lab.

Learning how to read and interpret test results can help you improve your soil and, therefore, your plants. However, most labs allow you to receive nutrient recommendations based on your test results. All you have to do is tell them which crop you grew in that area in the past and what you plan to grow in the future. They use this information to provide you with recommendations you can use to guide your fertilizer applications. 

For example, if your results show your garden is low in calcium, you can add calcium-rich products like gypsum or crab shells. And if you need to add other nutrients, you can opt for a fertilizer blend that contains multiple nutrients.

You should also closely examine your overall garden. Look out for standing water and rapidly draining soil since both indicate you may need amendments that change the soil structure. You can also look for signs of life. While bacteria are impossible to spot with the naked eye, search for earthworms, white mycelium, and insects. If your soil looks like a clay pot or a sandy beach devoid of life, you probably want to add compost, worm castings, or another source of beneficial microorganisms.

Amendments that Impact Soil Chemistry

The first category of amendments I’m going to cover are those that alter the soil chemistry. Many of these amendments add nutrients, and a few also adjust the pH. You can find a range of fertilizers, including both synthetic and organic options. I’ll cover some common organic fertilizers that are helpful to many gardeners. Note that many of these take time to break down for availability to your plants.

Single-Nutrient Amendments

Check out these amendments if you only want to apply a single nutrient. They’re especially helpful if your garden is high in one nutrient and low in another. Although I’ve listed the NPK ratio for each product, note that the exact ratio can vary slightly between products.

Blood Meal

• NPK ratio: 12-0-0
• Trace minerals: none

While you may hesitate to apply dried blood to your garden, blood meal is one of the best organic nitrogen sources. Blood meal releases nitrogen much more quickly than other organic nitrogen fertilizers, so it’s a great option if your plants need more nitrogen ASAP. Water and make sure to mix blood meal into the top few inches of the soil after you apply it. Then add more water to help it settle in.

Feather Meal

• NPK ratio: 12-0-0
• Trace minerals: none

Feather meal is another slow-release form of nitrogen made from ground poultry feathers. Beneficial bacteria must convert the nitrogen present in feather meal into a plant-available form before your plants can take it up. Therefore, this fertilizer works best when the ground is warm and moist. Many organic fertilizer blends use feather meal as their nitrogen source.

Elemental Sulfur

• NPK ratio: 0-0-0
• Trace minerals: sulfur

If you’re a certified organic grower who wants to add sulfur to your garden, elemental sulfur is the best option. Although this product is slow-acting, it’s one of the only options approved by the Organic Materials Review Institute (OMRI), the body that determines organic growing standards.

Elemental not only adds sulfur, but it also lowers the pH over time, making it more acidic. Therefore, it’s a great amendment if you’re dealing with a pH above 7.0.

Multi-Nutrient Amendments

If your soil is deficient in a few different nutrients, look for a single product that contains these nutrients. All of the following amendments come from a single source yet they contain sizable amounts of two or more nutrients.

Alfalfa Meal or Pellets

• NPK ratio: 2.5-0.5-2.5
• Trace minerals: calcium, sulfur

This amendment is made from dried and ground alfalfa. People use it as an animal feed and soil conditioner since it’s a great source of nitrogen and trace minerals. It also contains the plant growth regulator triacontanol which impacts enzyme activation and helps limit unfavorable responses to stress. Therefore, adding alfalfa meal to your garden does more than provide your plants with nutrients.

Soybean Meal

• NPK ratio: 7-1-2
• Trace minerals: calcium, manganese, sulfur, iron

This slow-release fertilizer is made from ground soybeans. It’s an excellent nitrogen source if you’d like to stay away from using fertilizers derived from animal products. If you choose to apply soybean meal, be aware that many soybeans are sprayed with herbicides.

Cottonseed Meal

• NPK ratio: 6-2-1
• Trace minerals:

You probably think of fluffy, white cotton bolls when you think of cotton, but these plants also produce nutrient-rich seeds. Applying these ground seeds adds a moderate amount of nitrogen as well as phosphorus, potassium, and numerous other nutrients. Like most organic fertilizers, cottonseed meal slowly releases nutrients.

Crab Shell

• NPK ratio: 5-3-0
• Trace minerals: calcium and magnesium

A byproduct of the crab industry, crab shell is rich in calcium and contains a notable amount of nitrogen, phosphorus, and magnesium. If you need to add calcium as well as nitrogen and phosphorus, it’s a good product. Crab shell can raise the pH, but it won’t impact it as much as limestone.

Gypsum

• NPK ratio: 0-0-0
• Trace minerals: calcium and sulfur

If you want to add a large amount of calcium without raising the pH, gypsum is a great choice. Also known as calcium sulfite, gypsum is a soft rock that contains around 29% calcium and 23% sulfur.

Dolomite Lime

• NPK ratio: 0-0-0
• Trace minerals: calcium and magnesium

If you’re dealing with acidic soil and want to raise the pH, dolomite lime is one of your best options. This type of limestone primarily consists of calcium carbonate and magnesium carbonate. As these compounds dissolve, they release their carbonate base which helps raise the pH.

This type of lime is high in calcium and magnesium, so adding it to your soil not only raises the pH but also adds essential elements. If you don’t need to add magnesium, opt for calcitic lime instead.

Greensand

• NPK ratio: 0-0-3
• Trace minerals: iron, manganese, silicon, and cobalt,

Greensand is made from green sandstones. It contains numerous trace elements essential for plant growth as well as 50+ other minerals. Some researchers also believe that greensand helps improve the physical properties of soil and enhances plant growth.

Azomite

• NPK ratio: 0-0-0.2
• Trace minerals: calcium, magnesium, zinc, boron, copper, cobalt, manganese, and others

Azomite is made up of volcanic ash deposits found in Utah. It contains a variety of trace elements, including calcium, magnesium, zinc, and manganese. Therefore, azomite is one of the best products for adding a swatch of trace minerals. 

You can find this product in a fine powder as well as a pelletized form. The powder form is easier to evenly apply over a small area, so it’s a better choice for home gardens. 

Nutrient Blends

If you don’t want to deal with the hassle of buying three or five different fertilizers to keep your plants happy, you can look for a nutrient blend designed for specific types of crops. These blends typically contain the nutrients that plants need as they grow, so they’re best used after you fix any major nutrient deficiencies.

When you buy a nutrient blend, pay attention to the crop it’s designed to be used on. For example, a general vegetable and herb fertilizer has an NPK ratio of 3-4-4 and is perfect for most leafy greens and herbs. However, if you want to grow healthy tomatoes, opt for a tomato fertilizer with a higher amount of potassium and calcium.

Amendments that Impact Soil Biology

You may know that plants need certain nutrients to grow, but you may not know that many organic fertilizers rely on microorganisms to convert their nutrients into a form plants can take up! Beneficial microbes also improve a plant’s ability to fight disease, handle the stress of drought, and produce hormones. Therefore, fostering and maintaining a living soil will help your plants remain healthy. 

Adding organic matter and keeping the soil moist can help encourage naturally occurring microbes to colonize the soil, but you can also add biological amendments to add healthy bacteria and fungi.

Compost

First off, it’s important to realize that different types of compost can drastically vary. A well-managed compost pile often contains much richer microbial life than a pile left to sit, overheat, or grow cold. Healthy composts also contain a large amount of bacteria and fungi, while unhealthy composts tend to only contain bacteria. If you really want to geek out on the biological richness of compost, you can grab a microscope and check for the different elements present in a healthy food web.

Many people think the main point of compost is adding nutrients, but I like to focus on the biological life present in the material. While adding compost adds nutrients, it also adds microbes and organic matter. The microbes help make nutrients available to plants, help plants respond to stress, and offer other benefits. And the organic matter helps feed the microbes already present through numerous processes including moisture retention.

It’s difficult to add too much compost to your garden, but you should pay attention to the pH of the material. Some compost has a low pH, so adding a couple of inches of it can lead to a pH too acidic to support crops. The opposite is less common, but also possible.

Worm Castings

Worm castings, also known as worm poop, are a rich addition to your garden. While they don’t provide a large amount of nutrients, they are loaded with beneficial bacteria that help improve nutrient availability and fight disease. They also contain plant growth regulators that help stimulate root growth and cell division. The castings also help improve water infiltration and water holding capacity.

If you aren’t afraid of adding a bunch of wiggly new friends to your garden setup, you can try vermicomposting at home. However, if keeping worms isn’t appealing, you can purchase bagged worm castings. You only need to sprinkle a small amount of the castings to add a boost of life.

EM-1 Microbial Inoculant

If you don’t want to guess which beneficial microbes are in your compost, turn to EM-1. Also known as Effective Microorganisms, this product contains 83 bacteria and fungi to help your soil and plants thrive. You can easily dilute the product in water and apply it to your soil.

Amendments that Impact Soil Physical Properties

If your soil is poorly draining, compacted, or otherwise lacking proper structure, you can add amendments to change its physical properties. Some of these amendments are inert products, while others are organic materials that will also increase the amount of organic matter present.

Perlite

Perlite is an expanded volcanic rock that resembles tiny pieces of white Styrofoam. You often see it in potting mixes, which makes sense since it improves drainage and aeration.

Vermiculite

Vermiculite is a type of expanded rock that’s often used in mixes for raised beds and potted plants. It helps with both water drainage and retention and, therefore, stabilizes soil moisture.

Coco Coir

Coco coir is an eco-friendly alternative to peat moss made from shredded coconut husks. It helps improve aeration and drainage while also improving the water-holding capacity. Therefore, it’s a good addition whether you’re dealing with heavy clay or sandy soil.

Sand

If you’re dealing with poorly draining soil, adding sand can help improve drainage and aeration. That’s because sand particles are larger than clay and silt. However, be aware that adding sand to heavy clay can lead to a concrete-like material rather than the loose loam you were imagining. If you’re dealing with heavy clay, adding organic matter like coco coir or compost is a better alternative.