Understanding Soil Additives: What To Add and When

By Garrett Cropsey
Published: November 12, 2019 | Last updated: April 23, 2021 01:43:10
Key Takeaways

There are a number of soil additives available to help gardeners create optimal growing conditions for their plants, but needs change throughout the growth cycle. Understanding when and how to apply these additives allows gardeners to cater subsurface conditions to the specific needs of their plants and soil throughout the growth cycle.

Caption: Manure fertilizer in hand pouring on the soil to grow seedlings of corn Source: kram-9/Shutterstock

As plants and micro-organisms grow, they constantly change the soil they inhabit. Gardeners can create optimal conditions for microbial and plant health throughout this process by adding soil amendments, which alter physical conditions, and fertilizers, which alter nutrient availability, to the substrate.


But, there can also be too much of a good thing, which makes the timing and application of additives as important as choosing the right ones. Understanding when and how to apply soil additives allows gardeners to cater to subsurface conditions to the specific needs of their plants and soil throughout the growth cycle. Here’s a list of mined and organic additives and how to use them.

Mined Soil Additives

Mined and man-made additives are considered inorganic and, for brevity’s sake, this article will focus on mined inorganic additives, as man-made additives are far too numerous to describe properly.


Inorganic additives can provide both immediate and slower changes to the soil, depending on the availability of the additive’s nutrients and structural properties. The immediate availability of nutrients does increase the possibility of “burning” plants, but inorganic additives also tend to be water-soluble and can be flushed from the soil if applied in excess.

Muriate of Potash or MOP. fertilizer in farmer handMuriate of Potash or MOP. fertilizer in farmer's hand. Source: Criniger kolio/Shutterstock

Potash: Potash is one of the most common sources of inorganic potassium and is used to describe any salts containing water-soluble potassium. Potassium is one of the three most important macronutrients for plants and, as a result, potash can be added to soils quite regularly. Sul-Po-Mag, or langbeinite, contains a high amount of potassium, sulfur and magnesium, is fast-acting and can be used to recondition used soil and boost potassium levels during growth.


Glauconite: Also known as greensand, glauconite is another inorganic source of potassium and a host of other trace minerals useful for plant growth like iron, magnesium, calcium, phosphorus and silica. Locked inside sand particles, these minerals are released over a much longer period of time than other inorganic additives. As such, greensand is more of a structural soil additive that also slowly adds nutrients and is most useful in growing systems that use the same soil repeatedly.

Gypsum: One of the oldest fertilizers used in agriculture, gypsum provides many benefits to soil. The calcium in gypsum additives helps reduce pH in alkaline conditions and its solubility makes the calcium and sulfur it contains readily available to plants. Gypsum also improves soil structure by breaking up heavy clays, which increases water infiltration and decreases bulk density. It is best to add gypsum when mixing soil before planting to ensure an even distribution.


Gardener mixing dolomitic limestone powder in garden soil to change the pH ant to provide more nutrients for plantsGardener mixing dolomitic limestone powder in garden soil to change the pH and to provide more nutrients for plants. Source: FotoHelin/Shutterstock

Dolomite Lime: Dolomite lime raises soil pH and is a source of calcium and magnesium. The amount of lime necessary to increase pH depends on the cation exchange capacity of the soil, with a larger increase in soils with low CEC. As plants take nutrients from the soil, the pH of that soil drops. Adding lime to depleted soils during the growth cycle or after harvest can help increase pH to suitable levels.

Rock Dust: Great for mineralizing or re-mineralizing soil, rock dust is created when glaciers, volcanoes and other forms of erosion allow access to the minerals held within rocks. The small particles are transported by the wind, carried by water or collected for sale by mining companies. While it does not contain enough nitrogen, potassium or phosphorus to be considered a fertilizer, rock dust contains a host of beneficial minerals and trace elements. To ensure even distribution of minerals, rock dust should be added when first mixing soil for planting.

Read More: How to Manage Soil Fertility

Phosphorite: Phosphorite, or rock phosphate, is a sedimentary rock containing phosphate-bearing minerals that provides a continued release of phosphorous for plants. Rock phosphate is not soluble and will not be leached from the soil with watering. Phosphorous is critical to early plant development and rock phosphate can be applied directly underneath seeds or transplants to allow easy access and in smaller amounts when mixing new soil.

Sulfur: While sulfur is not considered a macronutrient, it is important to plant development and a necessary additive to the soils. Adding elemental sulfur to the soil serves to lower pH when bacteria convert the sulfur into sulfuric acid. In sulfur-deficient soils, elemental sulfur should be added along with lime to balance changes in pH.

Perlite and Vermiculite: Perlite and vermiculite both provide increased aeration and moisture retention to the soil while decreasing the potential of compaction. The application of these additives depends on the preferred moisture level of the plants being grown. Vermiculite retains a great deal of water and should be used with plants that prefer lots of water. Perlite is more porous and loses water faster, making it ideal for plants that prefer less water.

Organic Soil Additives

Animal or plant by-products are considered organic additives and provide nutrients for plant growth across a variety of release times. Organic additives also provide useful changes to subsurface conditions by altering soil structure. The quality of organic additives is more loosely defined than that of inorganic additives, making reliable sourcing more important.

Manure: Animal dung provides a variety of benefits for soil and plants. The timing and application of manure varies depending on the animal from which it was produced. Manure increases soil aggregation by increasing organic matter and humus that, along with added nutrients, promotes the activity of soil microbes.

Guano and chicken manure both make excellent natural fertilizers as they contain large amounts of plant macronutrients. Manure can be mixed into the soil before planting or applied as a top dressing when needed. Some types of manure increase microbial activity dramatically, producing heat that can be damaging to plants, and should be composted before being applied.

Vermicomposting or Homemade Worm Composting into rich organic soil fertilizerVermicomposting or Homemade Worm Composting is method of turning kitchen food leftovers into rich organic soil fertilizer. Source: Andriana Swanych/Shutterstock

Vermicompost: Worm castings, the organic matter produced by earthworm digestion, provide a variety of nutrients and structural improvements to the soil. Castings contain almost all macro- and micronutrients necessary for plant growth, which are slowly released as the castings decompose.

Worm castings also provide a host of soil microbes to aid in decomposition. Soil aeration and water/nutrient retention are increased with the addition of worm castings. Castings can be mixed directly into the soil at a ratio of 1:4, or added to water with a small bubbler to create worm tea or compost tea for increased microbial activity.

Read More: Brew Up a Batch of Compost Tea

Blood Meal: A powder made from dried blood, usually from cattle, blood meal is an excellent source of nitrogen. The released nitrogen also lowers soil pH as microbes convert it to ammonia. Blood meal is fast-acting and should be used sparingly, as too much nitrogen can “burn” plants.

Feather Meal: Made from ground chicken feathers, feather meal is an excellent, slow-release nitrogen source that decreases the chance of “burning” plants.

Bone Meal: Made from ground-up animal bones, bone meal is a rich source of phosphorous and calcium. Bone meal generates strong root growth in early plant development and also promotes fruiting and flowering. However, bone meal’s accessibility to plants is severely limited in soils with a pH higher than 7.

Plant Meals: There are a variety of plant meals that provide both nutrients and beneficial chemicals for gardens. Alfalfa meal increases micro-organism activity and is a good source of nitrogen and triacontanol, a natural growth stimulant. Alfalfa meal generates heat as it decomposes, similar to certain manures, and should be added sparingly as a top dressing or with adequate time for composting before planting as part of a soil mixture.

Kelp meal is an excellent source of potassium and contains many valuable trace minerals in addition to plant growth promoters like cytokinins, indoles, auxins and gibberellins.

Mustard seed meal is a good source of nitrogen and contains glucosinolates that suppress soil diseases and increase pest resistance.

Overall, plant meals are all quite different and can be used in a variety of circumstances throughout the growth cycle.

Hand holding biocharBiochar Source: paroonkorn srichan/Shutterstock

Biochar: Biochar is pyrolyzed organic material, such as manure or wood chips, baked under pressure without oxygen, and has a long history of agricultural use. Biochar’s porous structure and high surface area retain nutrients and water, which prevents leaching and improves water quality while decreasing the need for additional fertilizers and irrigation.

Read More: Breaking Down Biochar

The negative charge across the surface of biochar helps increase pH and CEC, which improves plant uptake of positively charged nutrients like calcium, potassium and magnesium. The proportion of biochar intended for use in soil depends on the source material (animal or plant) and should be mixed into the soil before planting.

Soil additives provide nutritional and structural improvements to subsurface conditions throughout the growth cycle. Understanding the properties and combined effects of these additives allows growers to create the conditions that satisfy the full range of plant requirements. Whether gardeners are looking for a quick fix or a long-term solution, soil additives are an essential component in a grower’s toolbox.

Read Next: Growing Your Own Cannabis? Don't Forget These 3 Additives


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Written by Garrett Cropsey

Profile Picture of Garrett Cropsey
Garrett Cropsey is a biologist currently living in Boulder, Colorado. His love of nature began in Longmeadow, Massachusetts, where his first job was with the local parks department. He has a degree in ecology and evolutionary biology from the University of Colorado-Boulder, where he graduated Magna Cum Laude in 2010.

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