Organic Soil Amendments Explained

Jul 20, 2017
Gardening Soil Amendments


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So, you just got your soil test results and they are recommending that you use things like greensand and sul-po-mag. What is this stuff? Here are some commonly used organic soil amendments explained.


  • Aragonite sounds like one of the three musketeers but it is a source of calcium found in mollusk shells. Since it is low in magnesium it is good to use if your soil needs calcium but does not need any extra magnesium. Too much magnesium can “tie up” other nutrients, making them unavailable for plants to use. If your pH is low, aragonite has almost as much sweetening power as limestone.
  • Azomite is a trademarked acronym for A to Z Minerals Including Trace Elements. Mined in Utah, it is ancient volcanic dust that merged with sea water 30 million years ago. It has over 60 minerals that are good for plant growth.
  • Bone Char is burned bone meal that provides a readily used source of phosphorus.
  • Calphos Colloidal Phosphate is a good choice if your soil is low in calcium and phosphorus.
  • Dolomitic Limestone will not only sweeten your soil better than pure limestone it also provides calcium and magnesium.
  • Granite Meal is a rock powder that provides slow release potassium and trace minerals without changing the pH of your soil.
  • Greensand is also called glauconite. It is high in potassium and iron and has small amounts of magnesium and other trace elements. It is good for loosening clay soils and improving sandy soil.
  • Gypsum is 23% calcium and 17% sulfur so it provides calcium without raising pH levels. It helps improve drainage by aerating the soil. It neutralizes plant toxins and removes sodium from the soil. The sulfur reacts with water and forms a weak sulfuric acid that frees up calcium in your soil.
  • Hi-Cal Lime is used to raise the pH and add calcium at the same time.
  • Sulfate of Potash contains 51% potassium and 18% sulfur along with trace amounts of calcium and magnesium. It is mined in the Great Salt Lake Desert in Utah.
  • Sul-Po-Mag, also called langbeinite, is used if you need magnesium and potassium but not more calcium. It does not raise pH.
  • Zeolites are found in volcanic ash and can improve water and mineral retention in sandy soils.


  • Alfalfa Meal is a source of readily available nitrogen for plant growth and also feeds soil organisms. It contains vitamins, folic acid, and trace minerals.
  • Blood Meal sounds like every vegetarian’s nightmare, but it is very high in fast-release nitrogen. It also repels deer.
  • Fish Meal is an excellent source of nitrogen and potassium. It is a byproduct of fish farming.
  • Kelp Meal is dried, ground up seaweed. It provides trace minerals, amino acids, and enzymes that stimulate plant and root growth and are beneficial to soil life. By improving soil structure it can help your soil hold moisture and reduce the effects of drought and frost.
  • Soybean Meal contains high amounts of nitrogen and potassium that are released slowly as it breaks down. Look for organic sources since most commercially grown soybeans are genetically modified.


Let your soil test be your guide when adding fertilizers and amendments to your soil. Too much of a good thing is worse than not enough so don’t overdo it. The idea is to feed the soil not the plants. Healthy soil results in healthy growth.

Read more about fertilizers. What is the NPK ratio and what do those numbers mean?

About This Blog

Get inspired by Robin Sweetser's backyard gardening tips. Robin has been a contributor to The Old Farmer's Almanac and the All-Seasons Garden Guide for many years. She and her partner Tom have a small greenhouse business and also sell plants, cut flowers, and vegetables at their local Farmer's Market.


Reader Comments

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Good grief...who could afford it all?

I have a compost pile of simply good leaves and good green grass clippings. I choose not to use household scraps to keep out vermin and so the worms will come to stay, and they are plentiful! My plants and garden thrive!

Azalea pH

This was very informative but I got here by following a link to soil amendments that increase acidity. I have an Azalea that was planted about 5 years ago and has been very stubborn. I added slow acting sulfur last year but the pH didn't budge. I have 3 Rhododendrons that flourish each year so I'm a bit perplexed.

That is perplexing since

That is perplexing since rhodies and azaleas are in the same family and like the same acidity of around 5.5 pH. Adding elemental sulfur is the preferred way to lower pH for rhodies but it does take time. The best way is to apply it to your soil in the spring. A soil test will tell you how much to use but don’t exceed 2lbs. per 100 square feet and wait at least 3 months between applications. Overdoing it can be harmful to your plants. If you have high amounts of calcium in your soil it will be even more difficult to lower the pH. Try adding more organic matter along with the sulfur. Don’t use aluminum sulfate on rhodies because it is toxic to them. For more information on growing rhodies and azaleas check out the American Rhododendron Society website at

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