A Gardener’s Guide to Soil Amendments

How can we build a soil that will feed our plants? The goal in building soil is to create soil that is nutrient rich and is biologically active. We have learned, through the actions of Big Ag, that adding quick acting chemical fertilizers actually decreases the health of plants and the nutrient value of our foods (in addition to damaging aquatic life, birds and beneficial insect populations.) In gardening, less is actually more. If we add what plants need into the soil and make sure our soil is chock-full of a healthy microorganisms, then we actually will gain more.

Start by adding compost. Choose compost that is fully composted, rich in humus, and full of microbes. Plan on adding 1-2” on any surface area. Then add amendments that feed the microbiology, and add macro and micronutrients.

Most soil amendments tell you the NPK of that product. NPK is the reference to how much Nitrogen, Phosphorous and Potassium exist in an amendment. Nitrogen is the element that helps plants grow. Phosphorous helps plants fruit, flower and seed. Potassium encourages a healthy immune system. For example, feather meal, which has an NPK ratio of 12-0-0, has a high amount of nitrogen in it and is used primarily in the vegetative state of a plant’s life.

Other macronutrients, micronutrients, and trace minerals are needed to encourage the health of a plant. Calcium, Iron, Sulfur, Zinc, Silica are common ones you will see at any garden store. Each nutrient has its role in the function of a plant. Iron for example will green up your plant because it aids in chlorophyll production. Magnesium increases oil production and helps plants chelate other nutrients. Zinc stimulates flower formation. Sulfur aids in oil and aroma.

Below is a list of common soil amendments we can use to build our soil. Next to the product is its NPK rating and a brief description of what it is used for.

Alfalfa meal, 2.5-1-1, Alfalfa meal is a gentle source of Nitrogen, Phosphorus and trace elements. It is also a great fungal and bacterial food. Don’t let the low numbers fool you. Bacteria and Fungi are what break down the nutrients into usable form for plants. So Alfalfa meal is all used by the microbes, which then feed your plants. Alfalfa pellets breaks down slowly and add air pockets to the soil.

Azurite contains calcium, sodium, magnesium and many trace minerals. It comes in both a micronized and a coarse form. It can be mixed into the soil and added all year long to supplement trace minerals.

Blood Meal 13-0-0 is very high in nitrogen and is used for the growth phase. Blood meal is not totally water soluble, and is not considered fast acting, but it does release quicker than feather meal.

Bone Meal, usually steamed has a varying NPK, but is always high in phosphorous. It breaks down slowly and is excellent for fruit and flowering, and has a high amount of calcium.

Cottonseed Meal, 6-2-1 encourages vegetative growth and also helps to lower the PH of soils. It is most often used with acid loving plants such as blueberries.

Crab Meal, 5-4-3, made from the crushed shells of crab helps in all stages of plant life. It contains chitosan, calcium and bacteria and fungi. Chitosan helps plants build a strong immune system, increases germination rates, and repels parasitic insects and nematodes.

Feather Meal, 12-0-0, made from the crushed up feathers of poultry contains a high amount of slow release nitrogen. It is also a great fungal food.

Fish Bone Meal varies in NPK but is usually high in phosphorous and calcium. It is an excellent fungal food. This is my favorite phosphorous source. And sadly, it is also the local bear’s favorite!

Fishmeal, 9-6-2, is high in nitrogen and has a fair amount of phosphorous. The bags at our store are always very oily, so it maintains the natural fish oil. It stimulates microbial growth. It can be added in a dry form or and added later in a more soluble form.

Glacial Rock Dust contains trace minerals and food for microbes. It is a good source of Calcium, iron, and magnesium.

Greensand, 0-0-3, is an iron potassium silicate. Often collected from marine sediments, it contains trace elements from the sea. It helps break down clay soils, and holds moisture.

Gypsum contains calcium, a little sulfur and trace minerals. Soluble gypsum is known to break down salt deposits and clay soils.

Humic Acid generally comes from shale deposits or from mined peat bogs. Humic Acids also contain a high amount of fulvic acid. Both humic and fulvic acid act as natural chelators and help plants uptake nutrients, feed bacteria and fungi, and contain beneficial microbes.

Kelp Meal, 0-0-1, is an excellent source of potassium, contains naturally occurring growth hormones, and has a high amount of trace minerals. It also feeds microbes.

Magnesium sulfate or KMS contains high amounts of magnesium, sulfur and potassium. It tends to be soluble, so can help plants absorb the nutrients quicker. Magnesium is a chelator and can help plants use other nutrients as well. Magnesium Sulfate, however, can also damage some microbes, as it is a salt.

Oyster Shell contains a high amount of calcium and increases ph.

Rock Phosphate, 0-3-0, is another excellent source of calcium (27%) and iron. It is used for flower and fruit development. It is essential in these foothill soils.

Shrimp Meal, 6-6-2, also a source of chitosan, is a rounded source of nitrogen and phosphorous.

Sulfur, lacking in our local soils, helps lower ph. It can act as a fungicide and an insect repellant. Small quantities actually feed some microbes.

Sulfate of Potash, 0-0-50 super high source of organic potassium


Bat Guano and Seabird Guano come in a variety of NPK packages. Nitrogen bat guanos tend to contain naturally occurring calcium nitrate and are pretty water soluble, they are best added in small quantities or as you grow. Phosphorous bat and seabird guanos can be added to the soil or top dressed later.

Chicken and Steer manure NPKs vary, but with Nitrogen always being the highest amount. They are quick acting and are known to feed the bacteria that break down organic matter, say in a compost pile. . Manures also contain a high amount of bacteria. Adding a small amount as a top dress will feed the soil.

Many companies have made life easier for you and offer packages containing a variety of the above amendments. Some include beneficial bacterial and fungal spores. And importantly, they have directions for use! Adding too many nutrients can lead to nutrient lock out or leaching into our precious watershed.

Build your soil and the soil will feed your plants.

July Sales and News

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