As plants and micro-organisms grow, they are constantly changing 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.
However, 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 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.
soil building conditioners/amendments
earth mineral amendments
animal based amendments
Plant based amendments
soil building conditioners and amendments
Compost: Compost will improve the quality of almost any soil, and for this reason it is most often considered a soil conditioner. Compost improves the structure and texture of the soil enabling it to better retain nutrients, moisture, and air for the betterment of plants. Incorporating compost into soil dramatically improves soil structure. A well-structured soil with lots of small aggregates stays loose and is easy to cultivate. Compost also adds nutrients to your soil. Compost contains a variety of the basic nutrients that plants require for healthy growth. In addition to the main three; nitrogen, phosphorous, and potassium, of special importance are the micronutrients found in compost such as manganese, copper, iron, and zinc. Micronutrients are only needed in small doses, like vitamins in our diet, but they play an important role in the plant's ability to extract nutrients from other foods. Compost is basically a free nutrient boost for your plants. Compost also supplies abundant microorganisms that provide a healthy soil food web to interact with the plants root systems. Research is showing us that soil treated with compost tends to produce plants with fewer pest problems. When mixed into the soil, compost helps keep the pH at optimum levels for nutrient availability.
Humus: All organic matter, as it decomposes, forms smaller and smaller particles. When it breaks down as far as it can and yet still can be identified as organic matter, it is called humus. Humus is the “life-force” of living organic potting soil. There are many benefits to plants which humus provides. Humus can hold up to 90% of its mass in water, and so enhances the soil’s ability to store water. The chemical composition of humus allows it to buffer abnormally high or low levels of pH in the soil. The biggest benefit of humus however is its colloidal characteristics. Humus is THE number one ingredient that will maintain nutrients (cations) in substrates so they will not wash away when watered.
Vermicompost: Worm castings, the organic matter produced by earthworm digestion, provide a variety of nutrients and structural improvements to 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.
Peat Moss: Peat moss is dead fibrous material that forms when mosses and other living material decompose in peat bogs. Gardeners use peat moss mainly as a soil amendment or ingredient in potting soil. It has an acid pH, so it provides a good buffer for alkaline amendments like lime. It holds several times its weight in moisture, and releases the moisture to the plants roots as needed. Peat moss isn’t considered a renewable resource. There is considerable controversy surrounding the mining of peat moss.
Coconut Fiber: Coconut fiber is also known as coconut coir. Unlike peat moss, which is highly acidic, coconut coir has a neutral pH level. Coir improves soil drainage in the bed while also helping to retain moisture in quick-draining soils. Since coir breaks down slowly, much like peat, it creates air pockets in the soil that allow excess moisture to drain away from plant roots. The coir itself holds onto some moisture so the drainage doesn't occur too quickly and the soil doesn't dry out completely. Coconut coir contains few nutrients so it doesn't add to the nutrient quality of the soil. Further fertilization is necessary in coir dominated amendments.
Biochar: Biochar is pyrolysed 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.
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 soil prior to planting.
earth mineral amendments
Azomite: Azomite is the registered trademark for a complex silica ore (hydrated sodium calcium aluminosilicate) with an elevated ratio of trace minerals unique to the Utah mineral deposit from which it is mined. Scientific analysis of this combination of volcanic ash and marine minerals reports over 70 trace minerals. Some of its claims include that it grows stronger plants, improves weight increase at harvest, re-mineralizes nutrient-depleted soils and is OMRI Listed for organic use.
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.
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 uptake nutrients from 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.
Soft Rock Phosphate: 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 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 soils. Adding elemental sulfur to 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.
animal based amendments
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 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.
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.
Fish Meal: Fish meal is a strong source of organic nitrogen and is often combined with kelp extract to enhance performance. It is slow release and feeds plants over a period of time. Fishmeal of high quality provides a balanced amount of all essential amino acids, minerals, phospholipids and fatty acids for vigorous growth.
Crab Meal: Crab meal is loaded with nitrogen, phosphorous, calcium and magnesium. It is slow release which is healthier for plants and it also helps with nematode and fungus problems in the soil. Because it is also high in chitin it encourages soil microorganisms to discharge enzymes called chitinases, which break down the chitin that are a part of the nematode egg shell. Crab meal can be considered a bio-pesticide (preventing, destroying or repelling) for this reason. It can be worked into the soil or add it in the planting hole when transplanting. Crab meal is also a great activator to add to your compost. It really gets the microorganisms on the move and helps with the breakdown process for faster and more robust compost!
Oyster Shells: Oyster shells are a natural source of high quality Calcium that is ground into varying sizes to ensure an immediate and sustained release of this valuable nutrient. Calcium promotes strong root development, improves nutrient uptake and boosts plant immunity. Oyster Shell builds optimum tilth by improving the texture, aeration and water penetration of soils.
Frass: Insect Frass is insect poop (like guano or castings) But unlike bats or birds, Insect Frass actually comes from plants. In all natural ecosystems (where there are no man-made chemical pesticides), insects feed on and digest vegetation, and give it right back to plants in the form of nature's perfect plant food. Plants benefit insects - Insects benefit plants! Insect Frass naturally contains the nutrition plants require, beneficial micro-organisms, and the only immediately plant-available source of chitin. Chitin fortifies a plant from the inside out, causing an "auto-immune" response that signals a plant to produce natural toxins which fend off its natural enemies like pests and fungal pathogens. The EPA says that chitin and chitosan defend against botrytis (grey mold), powdery mildew, early and late blight, fungal pathogens in the root zone (root rot) and root-feeding nematodes.
Plant based amendments
Alfalfa Meal: Alfalfa is a good source of nitrogen, along with several other minerals. Alfalfa builds organic matter in your soil providing nutrients to plant roots. The microorganisms in your soil love alfalfa because of the protein, amino acids, fiber and sugars in its stalk. Alfalfa contains triacontanol, a hormone which stimulates the growth of plant roots, enhances photosynthesis, and increases beneficial microbes which help to suppress many soil-borne diseases.
Kelp meal: 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: Mustard seed meal is a good source of nitrogen and contains glucosinolates that suppress soil diseases and increase pest resistance.
Cottonseed Meal: A by-product of the cotton industry, cottonseed meal is an excellent source of slow-release nitrogen, phosphorus, potash and numerous trace elements. Because it breaks down over time, this popular fertilizer promotes long-lasting growth. Cottonseed Meal will also acidify the soil and must be used with the right management plan.
Soybean Meal: One of the benefits of soybean meal is that it is very high in nitrogen for an organic fertilizer often reaching 7%. Soybean meal has a typical analysis of 7-1.5-1 (N,P,K) and is a naturally a slow release fertilizer. Soybean meal is not water soluble hence does not leach and pollute waterways like artificial fertilizers. Soybean meal is a good organic fertilizer if it is certified as "Organic". If not certified as organic, the CHA would advise not to use it as there are many other safe choices.
Neem Seed Meal: Neem Seed Meal is an excellent all natural fertilizer for enriching garden and agricultural soils and encouraging optimum plant development. Cold pressed from the seeds of the fast growing Neem tree (Azadirachta Indica), it is also referred to as neem cake. It provides fertility benefits for short and long season crops and improves organic matter content of soils. Neem Seed Meal can be mixed into soils or potting media, used as a top dress around established plants or steeped to make a potent liquid solution. Neem cake organic manure protects plant roots from nematodes, soil grubs and white ants probably due to its residual limonoid content. Neem cake is effective in the management of insects and pests. The bitter principles of the soil and cake have been reported to have seven types of activities (a) antifeedant (b) attractant (c) repellent (d) insecticide (e) nematicide (f) growth disruptor and (g) antimicrobial.