Organic Soil Amendments Explained

Use Soil Amendments to Improve Your Garden Soil!

April 12, 2019
Gardening Soil Amendments

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Healthy soil makes for healthy plants! Using soil amendments, you can turn your poor garden soil into a nutrient-rich paradise in which plants will thrive. Here’s an overview of the most commonly used organic soil amendments.

Which Nutrients Do Plants Need?

Just like humans, plants need a wide range of nutrients to keep them growing healthy and strong. Soil amendments contain these nutrients in varying amounts and can be used to supplement your garden soil if a nutrient is found to be lacking. Essential plant nutrients include:

  • Primary Nutrients: Nitrogen (N), phosphorus (P), and potassium (K). Aside from carbon, hydrogen, and oxygen, these are the nutrients used by plants in the greatest amounts. These nutrients help with major functions of the plant, including foliage, fruit, root, and flower growth, as well as disease protection. Learn more about the importance of N-P-K.
  • Secondary Nutrients: Magnesium (Mg), calcium (Ca), and sulfur (S). These nutrients are needed in lesser amounts, but are just as important to the overall health of the plant. Soil amendments may be used purely to boost these elements.
  • Micronutrients: Nutrients in this category are needed in much smaller amounts than primary and secondary nutrients. Most soil amendments will contain some amount of micronutrients in addition to the main nutrient. Micronutrients include boron (B), zinc (Zn) iron (Fe), manganese (Mn), chlorine (Cl), copper (Cu), molybdenum (Mo), nickel (Ni), and cobalt (Co).

Organic Soil Amendments

Organic soil amendments are a great natural alternative to chemical fertilizers, but before adding anything to your soil, you should perform a soil test to see what’s already there. After testing, you’ll know exactly which elements you need to add—and that’s where soil amendments come in. Read more about preparing your soil for planting here.

Mineral Supplements

  • Aragonite is a source of calcium that comes from mollusk shells. Since it’s low in magnesium, it’s 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 (acidic), 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’s ancient volcanic dust that merged with sea water 30 million years ago. It contains over 60 minerals that are good for plant growth.
  • Bone Char is burned bone meal that provides a readily accessible source of phosphorus.
  • Calphos Colloidal Phosphate is a good choice if your soil is low in calcium and phosphorus.
  • Dolomitic Limestone will not only raise your soil pH 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’s high in potassium and iron and has small amounts of magnesium and other trace elements. Greensand is good for loosening clay soils and improving sandy soil.
  • Gypsum is 23% calcium and 17% sulfur, which means that it can provide a source of calcium without raising pH levels. It helps improve drainage by aerating the soil, neutralizes plant toxins, and removes sodium from the soil. The sulfur reacts with water and forms a weak sulfuric acid that frees up calcium.
  • 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.

Organic Nutrient Meals

  • 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.
  • Bone Meal is used as a source of phosphorus and calcium.
  • 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.

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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. Remember: healthy soil makes for healthy plants!

Which soil amendments do you use in your garden? Let us know in the comments!

About This Blog

Get inspired by Robin Sweetser’s backyard gardening tips and tricks. 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.

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