Understanding Soil Amendments

Soil Amendments are useful substances that augment your soil to be better suited for the plants you intend to grow.

The first step in choosing a soil amendment is determining what you want to grow. The second step is to determine the qualities of the soil you are working with. If your soil is mostly clay, then adding an amendment that can improve drainage is key. If your soil is sandy, then adding an amendment that will increase water retention is most beneficial. You should also keep in mind these four factors when selecting an amendment. How long will it last in the soil? What is your soil’s texture? What is your soil’s salinity and are you growing plants that are sensitive to salts? Furthermore, what is the salt content and pH of the amendment?

To do their job properly, soil amendments should be thoroughly mixed into the soil. This is rather easy to do with container gardening. When it comes to larger scale gardening such as in soil style, then a pitchfork, hoe or tiller might be useful in mixing the soil properly.

Amendments come in both organic and inorganic categories. Organic amendments come from something that was once alive while inorganic amendments are either mined or man-made. Just because something is inorganic doesn’t necessarily mean that it is harmful to a sustainable method of gardening. As always, research and careful inspection of labels is key into selecting the right product for you and your garden.

Improving Soil Texture

Soil TexturePermeabilityWater Retention

The chart above shows how the three primary soil types vary in permeability and water retention. The chart below shows how various soil amendments can aid in increasing or decreasing soil permeability and water retention.

AmendmentPermeabilityWater Retention

Hardwood BarkHighLow-Medium
PeatLow-MediumVery High
Wood ChipsHighLow-Medium

Aged ManureLow-MediumMedium


Soil pH

Soil pH is important because without it being in the correct range for the plants you are growing, they are unable to properly take up the nutrients they need. Add lime to raise the pH or sulfur to lower it. This should be done gradually if there are preexisting plants so as not to shock them. You will want to check your soils pH every year to see if further adjustments need to be made.


If your plants show signs of distress your soil might be lacking in nutrients. The three primary nutrients for plants are nitrogen, phosphorous and potassium. These nutrients are often labeled as NPK on commercial bags of fertilizer. I will be listing some organic fertilizer options with these nutrients’ as a numerical ratio after the name for your reference. Please keep in mind that these ratios can vary, so a particular brand may have slightly different numbers than what I have listed.

Alfalfa Meal 3-2-2

This is the plant alfalfa that has been ground up to increase it’s surface area thus making it easier to break down. I works at a moderate speed to augment your soil and is best when added in early spring before planting begins.

Blood Meal 12-1.5-0.5

This is an acidic fertilizer made from powdered blood of butchered livestock. It is high in nitrogen as the NPK numbers suggest, but one should be cautious when applying it. It is fast acting so could make for a tonic for plants in need.

Bone Meal 1-25-0

Bone Meal is rich in phosphorus and contains calcium and a trace amount of nitrogen. If your soil is not lacking in phosphorus then adding Bone Meal is not only unnecessary but could cause harmful yellowing of the leaves of your plants. You also want to make sure that your soil’s pH is below 7.0 as a higher pH won’t allow the soil to transfer the phosphorus to your plants. Bone Meal can be harmful to pets so make sure to work it well into the soil and keep this in mind for storage as well. Side note: as the name suggests, bone meal is made from animal bone waste from slaughterhouses thus people who enjoy a Vegan lifestyle should avoid this product.

Cottonseed Meal 6-1-1

This nitrogen rich fertilizer is slow working. It is a great addition to the Fall garden as that gives it plenty of time to break down in time for spring.

Compost 2-1.5-1.5

I love compost. There, it is in writing now. Not only can you make it yourself from things that you would otherwise simply throw away, but you can use it in a multitude of ways. Besides being a great fertilizer it can be used as a mulch and made into a nutrient rich tea. As with all things, keep a check on your soil’s pH as compost tends to be alkaline in nature. It is best added to the garden before planting.

Cow Manure 2.5-1-1.5

Manure works a moderate speed and works well when added with compost. Some undigested weed seeds could be present. Many manures are high in salt as well. Selecting an aged manure, or aging it yourself before adding it to your soil is a good option.

Poultry Manure 3.5-1.5-1.5

With slightly higher nutrient values and a slight faster working time, poultry manure might be a better choice to cow. Aging is also recommended so as not to burn your plants.

Earthworm Castings 2-1-1

Yes, we are still talking about… poop. No need to age and if you have earthworms then your soil already has some of this.

Bat Guano 10-10-2

Fast acting and great when used as a foliar spray (applied to the plant itself) or as a compost tea due to it being water soluble.

Fish Emulsion ranges from 2-4-0 to 5-1-1

Let’s cut to the chase. It’s dead fish stuff… so it stinks. It is also acidic so it could burn plants if not properly diluted. That said, it deploys it’s nitrogen load immediately so can save some sad plants who might be in need. Often used in a “cure them or kill them” situation.

Fish Meal 5-2-2

Fast acting a perfect for corn.

Greensand 1-1-5

Greensand comes from ancient sea beds. It is high in calcium and contains many other trace elements. These all vary depending on the source.

Liquid Kelp Fertilizer 1-0.2-2

Loaded with trace elements this kelp based fertilizer aids plants in growth. Mix with water and use as a foliar spray on applied to the soil.

Soybean Meal 7-2-0

This high nitrogen fertilizer tends to be absorbed at a moderate rate.

Shellfish Fertilizer or Shell Meal 5-2-5

Made from crushed up seafood byproducts, this fertilizer contains plenty of calcium as well as other trace elements. Noteworthy is the element chitin which aids in nematode protection.

There are so many more out there for you to explore! My suggestion is to first find out what your soil needs and then second, find out what your options are to provide it.