Best Organic Soil Amendments For Gardens (& How to Use Them)

Soil is an interesting thing! Soil composition is never the same, but varies from garden to garden and even from year to year in the same garden. 

The universal growing medium for all plants, soil is a mixture of sand, silt, and clay, but it's not complete without organic matter, water, and air. Plus, the nutrients in organic matter help build a thriving ecosystem within the soil.

Why Soil Amendments Are Important

A soil amendment is anything added to soil that improves its quality and/or texture. Soil amendments go hand in hand with, but are not to be confused with, fertilizer that supplies essential nutrients to feed plants, but does not improve soil. (Check out these blog posts on fertilizing an organic garden, fertilizing fruit trees and perennials, and the best fertilizer for vegetable gardens if you are looking for resources about fertilizer.) 

If your soil has poor quality, too much clay or sand, poor moisture retention, or some other issue or deficiency, amenders are going to be a key ingredient in your garden's success! Here are some reasons that amending your soil is beneficial:

• Enhance Water Retention - Soil that is heavy in clay or lacks organic matter can have poor water retention. Amending your soil can help to improve its water-holding capacity and reduce the need for frequent watering.

• Improve Soil Structure - Soil that is compacted or heavy in clay can have poor structure, which can lead to poor drainage and root growth. Amending your soil with organic matter and gypsum can help to improve soil drainage and soil structure.

• Control pH - Soil that is too acidic or too alkaline can inhibit plant growth. Amending your soil with things like lime or sulfur can help to adjust the soil pH level, making it more conducive to healthy plant growth.

• Reduce Erosion - Soil that is bare or poorly protected can be subject to erosion. Amending your soil with cover crops and mulching can help to reduce erosion and protect your soil from the elements.

• Improve Fertility - Soil that is low in essential nutrients, such as nitrogen, phosphorus, or potassium, can lead to poor plant growth. But, adding fertilizer to a garden isn’t the only way to improve fertility. Amending your soil, especially with organic matter, can help to improve its fertility and promote plant growth.

When to Use Soil Amendments

Most soil amendments can be added in the spring when you are preparing for planting, but some can be used more often. Follow the instructions on the packaging for proper application intervals. 

When it comes to compost, though, I sometimes amend multiple times a year. For example, I reamend with compost in the spring before planting and then again after harvesting the spring crops before planting summer crops. Then, I reamend again when I pull out the summer crops before planting a fall garden.

Knowing Which Soil Amendments to Use

Photo courtesy of https://extension.umd.edu/

The first thing you want to do before amending your soil is assess the soil composition of your garden bed. Different types of soil will need different amendments than others. Now is also a good time to test your soil for pH and nutrient content with a Rapitest Soil Test Kit.

What you grew the prior season will also dictate how much amending you’re going to need to do as well. If you grew heavy feeders like tomatoes, pumpkins, zucchini, or even spinach, they used up a lot of macronutrients, as opposed to a bush bean that actually gives nutrients back to the soil. So I wouldn’t remineralize much in the bed where I grew bush beans last year, but I would focus on amending and fertilizing the bed where I grew my tomatoes.

Types of Organic Soil Amendments

As a baseline, I recommend amending most any garden with some amount of compost every year. The better your soil quality, the less compost you will need to reamend with each year. Then decide if you need other amendments from the following organic soil amendments.

Compost

Compost is broken down organic matter. It adds humus, humic acid, friability (looseness), and water & nutrient holding capabilities to soil. 

Compost also contains microbes that help increase soil fertility, increase plant defenses, and unlock nutrients otherwise inaccessible to plant roots. There really is no substitute for compost!

You can add as much fertilizer as you want, as much lime, sulfur, gypsum, or other amendments, but it won't replace using compost! When a garden has lots of rich compost, the garden can thrive! Are you ready to learn how to make compost to benefit your garden?

Application: You can apply compost one of two ways. If you already have really loose, fluffy soil, then you simply need to top off your beds by adding compost right on top. Don’t even work it into the soil because that will disturb the soil structure unnecessarily. 

If your soil is very poor or compacted and heavy, then I would consider working the compost into the soil. Double-digging or broadforking can help to loosen the soil to break up the soil in a gentle way.

Azomite Rock Dust

Over time, minerals in soil are depleted due to leaching, water runoff, plant intake, and other factors. Typical fertilizer only replaces the macronutrients causing plants to become deficient in trace minerals over time. 

Azomite rock dust contains over 50 trace minerals that are readily available for plants, it will not affect the pH of your soil, and it can be used in any garden without burning plants. Adding azomite each season replenishes minerals to promote healthy soil that can sustain exceptional life. 

Application: The recommended application rate is ¼ cup per every 10 sq feet during soil amending. 

Molasses

While molasses does not change the overall structure of soil, it can improve the quality indirectly because it is a carbohydrate source that beneficial bacteria can use to multiply in number. These bacteria and fungi associate with plant roots and mine minerals and nutrients for the plants found in the soil, help ward off bad bacteria, assist with drought and heat resistance, and reduce stress. 

That’s why molasses is the main ingredient in our Sassy Lass soil microbe booster! Application: I recommend diluting 2 Tbsp of Sassy Lass per gallon of water during watering as needed.

Worm Castings

It is so important to amend with worm castings because they bring beneficial bacteria, humic acid, and fulvic acid to the party! Worms break down organic material in a natural way, increasing nutrients, but also loosening soil. 

Many gardeners will add composting worms right to their garden, while others practice vermicomposting in worm bins and add just the worm castings to the garden. Application: The recommended application rate is ¼ - ½ cup of worm castings per 100 sq feet.

Lime

Lime is an organic soil amendment that can help to raise the pH level of your soil to make it more alkaline. Very few soil types are too acidic, but if your soil is, you may need to add lime to it. Or if you are growing Brassicas, you may want to amend with lime because they prefer a slightly alkaline soil. 

You will need to add lime yearly to maintain the desired pH. Pro-Tip: A soil pH test should be done before applying to ensure your soil truly needs it and you are not adding too much! 

Gypsum

Gypsum is a natural soil amendment that can help to improve the structure of your soil. As I explained in How to Improve Soil Drainage, it can help to break up heavy clay soils and make them more porous, allowing water and air to penetrate more easily. Gypsum is great too because it does not affect soil pH.

Rock Phosphate

Rock phosphate is an organic soil amendment that can help to promote healthy plant growth. It is a great source of phosphorus, which is an essential nutrient for plants. It also helps to regulate soil biology because many beneficial bacteria and fungi use phosphate to grow and multiply.

Epsom Salt

Epsom salt is a soil activator that allows plants to uptake vital macronutrients such as nitrogen and phosphorus. This nutrient uptake will increase the production of foliage and blooms, and create bushier, stronger plants. 

Epsom salt is one of the only soil amenders that plays a vital role in chlorophyll production. Without it, photosynthesis would not be able to happen at all. 

Application: To apply, mix 2 teaspoons of Epsom salt per gallon of water when watering plants. Use no more than every 2 weeks. 

Blood Meal

Blood meal is an organic soil amendment that is high in nitrogen. It can help to promote healthy plant growth and can be used to improve the fertility of your soil.

Application: Add 1 tbsp to soil per transplant when planting or 2 tsp per gallon of soil in a seed starting mix.

Greensand

Greensand is a naturally occurring sandstone that is rich in iron, potassium, and other trace minerals. This pulverized rock contains hundreds of beneficial micronutrients that will be like a multivitamin for your plants and soil. It can help to promote healthy plant growth and can be used to improve the structure of your soil.

Biochar

Biochar is the material that remains when the wood is burned without oxygen. Biochar is porous, meaning it holds onto water and there is a lot of surface area for bacteria to colonize. It can also help to hold nutrients in the soil rather than them being leached down beyond where plants can reach. 

Biochar can be made at home by burning sticks in a metal container, then when the entire bin is on fire, spray down the fire with water. The cooled remains can be sifted out, the ash discarded, and the biochar placed into a bucket for use in the garden. 

Cover Crops

It may surprise you that plants, such as clover or rye, can amend soil, but it's true! Cover cropping is another great way to improve your soil, add organic matter, and help to control erosion. Here’s more information on how and why you should use cover crops in your garden.

Remember that soil improvement is a gradual process and it may take a while to see the results of your efforts. Regardless of what you do, just remember you are making a difference!