Good organic growing is all about getting the soil or potting mix well prepared in the first place, before any plants have gone anywhere near it! Here we discuss the various ingredients that organic growers use to improve their soils and potting mixes.
What is a Soil Amendment?
People get unduly freaked out when they hear terms like "soil amendments" as if they need to don a white lab coat before doing something as scientific as tinkering with their soils. Sure, a generally scientific approach to improving your soil is certainly not a bad thing, but it doesn't have to be complex either.
Think of soil amendments as simply the just stuff you add to soil to make it better. Soil quality can be improved in terms of:
The quantity and diversity of microbial life present in it.
The amount of composted material in it.
The amount of sand, clay, and general size of particles affects water and air-holding abilities.
Acidity / Alkalinity
The pH value of soil has a direct effect on biological life present in the soil and the availability of nutrients.
You can add soil amendments in the form of single ingredients or a blend to help combat micronutrient deficiencies, adjust pH, combat salts, introduce microbial activities, and so much more.
What About Adding Fertility / Nutrients?
Growers often confuse soil amendments with fertilizers. "What's the difference between a soil amendment and a fertilizer?" you may ask. Fertilizers and organic inputs (such as bat guano or chicken manure) help to increase the fertility of the soil. Soil amendments, on the other hand, are all about improving the surrounding qualities - they can be thought of as 'supporting' the work of the fertilizers. Soil amendments are used to treat the soil itself. It's worth noting that many soil amendments are also used in blended fertilizer products to provide feeding nutrition so it's very common to see them together - hence the confusion!
What are the Different Types of Soil Amendments?
Soil amendments come in many different forms; most are organic, which is preferable for your soils, some are considered “natural” which are derived naturally though not necessarily organic, and some are synthetic. Organic amendments as well as naturaly derived amendments are the best way to change your soils from a dead clay heap to a living, symbiotic organism. The categories of amendments include: bark products, plant byproducts, animal byproducts, manure-based, compost and mulch based, and rock and mineral powder based.
Soil Amendments made from Plants
Feeds with 3% nitrogen and is known to contain growth factors and mineral content. Very common ingredient in many blended organic fertilizers.
Great aerator. Excellent soil amendment or a standalone grow medium. Coconut not only provides great aeration but also efficient transfer of nutrients. Check your source though, as coconut can be very salty and this can harm soils and plants.
Nitrogen super booster! (Between 6-7%.) Only “organic” if it was grown that way - most cotton farms use chemical foods and insecticides. So, check your source. Cottonseed meal is also a very common ingredient in blended fertilizers.
Adds micronutrients and plant hormones! Derived from varieties of kelp harvested from the ocean then dried and ground into a powder form. Contains small amounts of nitrogen, phosphorus, and potassium. Many forms include kelp extracts in which compounds are extracted from the seaweed to concentrate the micronutrients and other helpful plant hormones. They are not high enough to correct deficiencies, but kelp provides plants and soils with a vitamin like effect.
Adds vital nitrogen to the soil. Usually one of the more expensive amendments, but very useful.
Sphagnum Peat Moss
Great to add to rough soils. Improves moisture retention.
Another soil softener. Adds essential phosphate and potash. Can be a little high in pH (very alkaline) so go easy on it. Great to add to very acidic soils that are high in nitrogen.
Soil Amendments Made from Animals
Use with caution or risk burning your plants. It’s dried and ground waste from slaughterhouses, and is a strong source of nitrogen.
Slow release. Another common ingredient in fertilizer mixes, and due to its high amounts of phosphorus, should be used with care like blood meal.
Clean, usually odorless, and can be used on all soil and plants. Castings, when concentrated, are rich in nitrogen but they will not burn plants. Castings vary but can contain magnesium, calcium, potassium, potash, micronutrients, and some trace elements. Grain fed castings tend to be the best and stay in the soil longer, while manure fed castings are a lower grade, but release their nutrients faster. More importantly, worm castings contain tons of beneficial organisms and microbes that help to restore soil life and begin recreating the soil web.
Products of the poultry industry, the feathers are ground into a meal which contains levels of nitrogen which releases a little slower than other nitrogen sources.
Ground and dried fish waste. A good source of nitrogen and some phosphorous too. Not to be confused with fish emulsion, which is a liquid form of the fish waste products and less stable.
Oyster Shell Lime
Sometimes grouped with dolomite lime, however it is derived from the shells of finely ground oyster shells rather than being rock-based like dolomite lime. Oyster Shell lime will raise pH, add calcium, and many micronutrients. The microbial life inside of your soil loves oyster shell lime!
Phosphorous Bat Guano
King of the phosphorus amendments! Fantastic crop sweetener! Not to be confused with nitrogen bat guanos, which are more of a manure product, phosphorous bat guanos are ground up fossilized remains of bat poop. The phosphorus releases slowly into the soil over time and contains beneficial microbes too.
Shrimp, Crab, and Sea Meals
Ground waste products from sea going animals that are not fish. Their waste is primarily their shells and exoskeletons which provide an excellent source of major nutrients as well as many micro-nutrients. They break down slowly, thus providing some staying power.
Soil Amendments made from Poop!
Nitrogen Bat Guanos
Bat droppings that are fresher provide a great source of nitrogen. They contain some phosphorus, though not as high of levels as the fossilized bat guanos. Nitrogen bat guano is considered to be the best of the manures as they are nutrient rich, but very stable.
Quick and dirty fix of nitrogen - releases very fast. One of the poorer quality manures, but effective if used quickly and properly treated. It can burn your plants, so it must be amended into the soil and watered in before planting. Fish meal is preferable as it is more stable and does not decompose as quickly.
Sewage Sludge / Bio-Solids
Very cheap, very dirty: derived from human waste and whatever else got flushed, such as traces of household chemicals, bleaches, and paints. It can contain many harmful pathogens and heavy metals. Not nice.
The farmer’s stinky favorite. Derived from steers and some equine. Very potent source of nitrogen and should be used truly to amend soils in preparation of planting. Best for large gardens and not small landscapes or containers. In bagged form, it is still very volatile and breaks down quickly.
Soil Amendments made from Composted Organic Materials
Derived from decayed plant matter such as your left over vegetable scraps. Compost does include decaying animal matter as well, but for most gardeners it comes from vegetable and fruit scraps from your home.
Plant and bark materials that are not fully decomposed. Aids moisture retention, decreases temperatures (protects from hot temperatures), weed reduction, and more.
Soil Amendments made from Rocks and Minerals
Provides calcium, magnesium, and lowers pH quickly. It also helps with the breaking up of clay soils.
A clay-type mineral supplement, greensand will provide potassium on a slow release schedule. It is very effective at improving soil structures though not so much as a fertilizer.
Hard Rock Phosphate
Derived from volcanic deposits and highly mineral in composition. The phosphate is not as available as in more traditional, soft rock phosphates, so it is not always the best amendment. However, with its mineral qualities, it can provide slow and steady mineral release into soils.
Derived from leonardite, humic and fulvic acids, humates help with the active part of soil’s humus. These goodies help with nutrient uptake by plants and assist under the soil too.
Volcanic rock “rice crispies.” Perlite is inert and provides drainage and aeration in to compacted soils. Perlite rises to the top when watered, so it is not the best to add to in-ground plantings, though very important for containers and raised beds.
Excellent, slow releasing form of phosphorus. Many sources have been mined deeply and contain levels of arsenic, so get it tested or check your manufacturers MSDS (Material Safety Data Sheet).
Vermiculite is very light and can float in water. It is a great medium for starting seeds and amending soils as it contains some minerals and will help with aeration. Perlite provides better results for drainage and aeration, though.