Turn your nutrient-poor garden soil into a nutrient-rich sanctuary where plants may thrive with the help of organic soil amendments.

Here’s a quick rundown of some of the most effective soil amendments for your garden.

Soil Amendments

Organic amendments

Organic soil amendments change the structure of the soil as they degrade, helping it to absorb and retain more water and nutrients. Insects, worms, fungi, and other organisms that live in the soil help breakdown organic matter, but they need energy from the nitrogen in the soil to do so.

As a result, when applying undecomposed organic additions to the soil, nitrogen is frequently required. Many organic soil additions are also categorized as natural organic fertilizers. Hay, straw, peat moss, leaf mold, and sawdust are some examples.

Inorganic amendments

In high-sodium soils, gypsum is an example of a nonorganic amendment used to promote water infiltration.

Synthetic versus natural organic fertilizers

Synthetic fertilizers

Chemically created products containing one or more of the basic elements required for plant growth: nitrogen, phosphorus, and potassium, are known as synthetic fertilizers.

Natural organic fertilizers

Natural organic fertilizers are made from plant or animal products that include a considerable amount of one or more of the three essential nutrients for plant growth: nitrogen, phosphorus, and potassium. The nutrient content of the product must be stated on the package.

Because most natural organic fertilizers contain considerable amounts of organic matter, they are also categorized as soil amendments. Manures, sewage sludge, and bone meal are among the examples.

Needed nutrients by plants

Plants, like humans, require a wide spectrum of nutrients to grow healthy and robust. These nutrients are found in variable concentrations in soil amendments, which can be used to supplement your garden soil if a nutrient is determined to be deficient. Plant nutrients that are essential include:

  • Primary nutrients: Nitrogen (N), phosphorus (P), and potassium (K) are the three primary nutrients (K). These are the nutrients that plants use the most, aside from carbon, hydrogen, and oxygen. These nutrients aid in the growth of the plant’s key functions, including as leaf, fruit, root, and flower growth, as well as disease resistance.
  • Secondary nutrients: Magnesium (Mg), calcium (Ca), and sulfur (S) are secondary nutrients (S). These nutrients are required in smaller amounts but are as crucial to the plant’s overall health. Soil amendments can be applied just to increase the levels of these elements.
  • Micronutrients: These nutrients are required in far smaller quantities than main and secondary nutrients. In addition to the main nutrient, most soil additions will include some micronutrients. Boron (B), zinc (Zn), iron (Fe), manganese (Mn), chlorine (Cl), copper (Cu), molybdenum (Mo), nickel (Ni), and cobalt (Co) are all micronutrients (Co).

Organic soil amendments

Compost

The most frequent organic soil supplement is compost. Fresh organic materials (also known as feedstocks) are composted into stable forms that release nutrients slowly. Yard debris, food waste, animal manure, wastewater biosolids, and woody materials are all examples of compost feedstocks.

Biosolids-blends

Another type of organic soil amendment is biosolids-blends. These aren’t compost, but they have a comparable effect. To manufacture biosolids-blends, biosolids are heated first to kill pathogens, then blended with aged woody materials and sometimes sand.

Compost is made up of more than just organic matter; it also comprises inorganic components found in plant tissue and other feedstocks, as well as any soil present in the feedstocks. As a result, when you add compost to soil, you’re mixing organic and mineral components together.

Compost typically contains 30 to 70 percent organic matter by weight. Backyard (homemade) compost and other compost with a lot of soil (like feedlot manure) are on the low end of the organic matter spectrum.

Advantages of organic soil amendments

Organic soil additives can loosen soil, improve water infiltration, and increase long-term nutrition delivery in the garden or landscape. Organic soil additives can increase water-holding capacity when added to sandy soils. Organic soil additions are especially beneficial to soils that are deficient in organic matter.

Urban soils that have been disrupted during building, landscaping, or utility work are common examples. Three inches of modification was the maximum application rate tested. Compost and biosolids amendments enhance the amount of soil organic matter in the soil, which helps to sequester carbon or carbon storage.

According to research conducted in Washington State, 20% or more of the amendment carbon was maintained in the soil 5 to 15 years after the last application.

However, in tilled soils, the amount of organic matter retained may be lower, and in soils depleted in organic matter, it may be higher. Compost and biosolids products help soils, the local economy, and the environment by recycling material from local waste streams.

How much amendment should you use?

Add 1 to 3 inches of organic soil amendment to the soil for new garden or landscape plantings, and incorporate to a depth of at least 6 to 8 inches.

Add 2 or 3 inches of amendment if your soil lacks organic matter (it’s usually light colored and has poor physical qualities).

Add less if your soil already has enough organic matter or if salts are an issue (or none at all).

Consider lowering or eliminating amendment applications if a laboratory soil test reveals high levels of phosphorus (P) in your soil.

If you’re planting landscaping plants, don’t simply modify the planting holes; amend the entire bed. After the initial application, permanent landscape beds do not require organic soil amendments.

Decomposition of leaf litter and organic surface mulches will aid in the preservation of organic matter, resulting in a soil environment that is similar to that found in forests.

Established gardens and landscapes require fewer organic soil amendments—about 12 inch each year on average. Gardeners can also produce cover crops to keep the soil’s organic matter levels at a healthy level.

If you’re growing in raised beds, make sure you add enough organic soil amendment or soil mix to keep the beds’ original soil depth. The soil mix settles in the bed, and the amendments decompose, thus volume loss in raised beds is unavoidable.

Because of the deep, fibrous root systems that create organic matter in the soil as the roots grow, die, and disintegrate, established lawns rarely require organic matter supplements.

Organic soil additives, on the other hand, may help new lawns if they are integrated into the soil before planting. This is especially true if the topsoil has been removed or if the soil is devoid of organic matter.

New lawn amendment rates are typically 15 to 20% by volume, or 1 inch of compost integrated to a depth of 5 to 6 inches, which is lower than for new gardens. Spreading a thin layer of compost, no more than 12 inch deep, over an established lawn that is growing in low-organic-matter soil is the easiest technique to modify the soil.

Then, using a hollow-tine aerator, aerate the lawn multiple times to remove soil cores and integrate the compost into the holes. Applying a thick coating of compost to turf without mixing it into the soil can create a barrier that prevents water from moving through the soil and roots from growing.

Downsides of over-amending soil

Is it true that if adding organic matter is beneficial, adding more will make it even better? Plants can be grown in straight compost as long as the porosity is appropriate and salts are not an issue. Nonetheless, significant applications of organic soil additions on a regular basis have downsides, including:

Excessive amount of phosphorus

Although phosphorus (P) is an essential plant nutrient, most organic soil amendments contain more than plants require. Excessive P levels in the soil can result from repeated applications, increasing the risk of P runoff into lakes and other bodies of water.

Excess P in lakes causes eutrophication (algae blooms that deplete oxygen levels in the water), causing ecosystem harm and lowering the recreational value of the lake. When gardens are positioned near sidewalks and roads, phosphorus runoff is a particular concern in urban areas because it enables for easy passage of garden runoff into storm drains.

Diminishing returns on organic matter advantages

Organic amendments will assist the soils with the least amount of organic matter at the start. However, when soil organic matter accumulates, incremental advantages decrease and organic matter decomposition accelerates. As organic matter levels rise, soil’s ability to store more carbon decreases.

Deteriorated turf quality and usability 

Turfgrass is the only living groundcover that can withstand persistent wear from foot traffic throughout the growing season if it is properly maintained.

If there is too much compost in the soil, the root zone can retain too much water, making the area unsuitable for foot circulation during wet weather (particularly in areas west of the Cascades). Top-dressing, which is the application of thick layers of compost on the surface of turfgrass to hold excess water, can also make the region unfit for foot circulation.

Salt buildup in arid regions 

Plants are harmed by excess salts because they reduce their ability to absorb water. Manure-based soil additives include a significant amount of salts, which can build up in the soil. This can be a problem in desert areas where there isn’t enough rain for the salts to naturally leach out of the soil.

In arid climates, this may limit the amount of organic soil amendments (especially manure-based) that can be applied, unless the salts can be leached from the soil.

Enhancing soil via soil amendments

Beneficial soil organisms, organic matter, and moisture retention can all benefit from organic soil amendments. The following list includes soil amendments derived from animal, mineral, and plant sources. Some products are free and easy to come by, while others must be purchased.

In general, before planting the garden, add soil amendments in the fall or spring. Soil amendments are split into three categories: animal-based, mineral-based, and plant-based amendments.

Animal-derived soil amendments

In addition to enhancing soil structure, some animal-derived soil supplements can enhance beneficial soil organisms. Apply untreated animal products nine months before harvest, or at least two weeks before planting, in a safe manner.

Mineral-based amendments

To remedy mineral deficiencies, use mineral-derived soil additives. Mineral-based amendments take a long time to break down, thus they can be used excessively. That is why it is critical to do a soil test ahead of time to ensure that you do not over-apply them.

Plant-based amendments

To improve soil structure, use plant-based soil additives. To avoid contaminating the soil, it’s critical to find herbicide-free plant-based additives. Low germination rates and curled/yellowing leaves are the effect of herbicide contamination.

Organic soil amendments

Organic soil amendments are a terrific natural alternative to artificial fertilizers, but you should first examine your soil to see what’s already there before adding anything. You’ll know exactly which elements you need to add after testing, and that’s where soil amendments come in. 

Mineral supplements

  • Aragonite is a calcium mineral derived from the shells of mollusks. It’s useful to apply if your soil needs calcium but not extra magnesium because it’s low in magnesium. Magnesium in excess can “lock up” other nutrients, rendering them unavailable to plants. Aragonite has approximately as much sweetening power as limestone if your pH is low or is acidic.
  • The term azomite is a trademarked abbreviation for “A to Z Minerals Including Trace Elements.” It’s old volcanic dust mined in Utah that combined with sea water 30 million years ago. It has more than 60 minerals that are beneficial to plant growth.
  • Bone Char is a phosphorus-rich bone meal that has been burned.
  • Calphos If your soil is lacking in calcium and phosphorus, Colloidal Phosphate is a good option.
  • Dolomitic Limestone not only raises the pH of your soil more effectively than pure limestone, but it also adds calcium and magnesium.
  • Granite Meal is a rock powder that offers potassium and trace minerals in a slow-release form without affecting the pH of your soil.
  • Glauconite is another name for greensand. Potassium and iron levels are high, with tiny amounts of magnesium and other trace minerals. Greensand aids in the loosening of clay soils as well as the improvement of sandy soil.
  • Gypsum is 23 percent calcium and 17 percent sulfur, thus, it can act as a calcium source without altering pH levels. It aerates the soil, neutralizes plant poisons, and eliminates sodium from the soil, all of which help to enhance drainage. When sulfur combines with water, a weak sulfuric acid is formed, which releases calcium.
  • Hi-Cal Lime is used to improve the pH while also adding calcium.
  • Potash’s sulfate is made up of 51 percent potassium, 18 percent sulfur, and trace amounts of calcium and magnesium. It’s found in Utah’s Great Salt Lake Desert.
  • If you need extra magnesium and potassium but not more calcium, Sul-Po-Mag, also known as langbeinite, is a good choice. It has no effect on pH.
  • In sandy soils, zeolites, which are present in volcanic ash, can improve water and mineral retention.

Organic nutrient meals

  • Alfalfa Meal is a readily available nitrogen source for plant growth as well as a source of food for soil organisms. Vitamins, folic acid, and trace minerals are all present.
  • Blood Meal may sound like a vegetarian’s worst nightmare, but it contains a lot of fast-release nitrogen. It also keeps deers away.
  • Phosphorus and calcium are obtained from bone meal.
  • Fish Meal is a great source of potassium and nitrogen. It’s a byproduct of the fish aquaculture industry.
  • Kelp Meal is kelp that has been dried and powdered. It contains trace minerals, amino acids, and enzymes that help plants and roots thrive while also benefiting soil life. It can help your soil hold moisture and lessen the effects of drought and frost by enhancing soil structure.
  • Soybean Meal is abundant in nitrogen and potassium, which are slowly released as it breaks down. Because most commercially farmed soybeans are genetically modified, look for organic sources.

How should soil amendments be applied, and how much should be used? Get a soil test for free or at a reasonable cost.

When it comes to applying fertilizers and amendments to your soil, this will be your guide. Don’t overdo it since too much of a good thing is worse than not enough. The goal is to nourish the soil rather than the plants. Keep in mind that healthy soil equals healthy plants!

Things to consider

When choosing a soil amendment, there are at least four elements to consider: how long the amendment will persist in the soil, soil texture, soil salinity and plant salt sensitivity, and the salt concentration and pH of the amendment. Organic amendments’ salt content, pH, and organic matter can all be determined using laboratory testing. After that, the quality of bulk organic amendments for application in large-scale landscapes can be determined.

Last update on 2021-10-23 / Affiliate links / Images from Amazon Product Advertising API