Check out our quick guide to soil amendments and find out how to fine-tune your soil to vastly improve the quality (and quantity) of your crops.

THE BASICS

What is a Soil Amendment?

A soil amendment is just stuff you add to soil – either a single ingredient or a blend – to help combat micronutrient deficiencies, adjust pH, combat salts, introduce microbial activities, and so much more. But hey, don’t confuse soil amendments with fertilizers – these add formulated food for the purpose of the plant, while soil amendments are used to treat the soil itself. However, that said, many soil amendments are also used in blended fertilizers and provide feeding nutrition.

Types of 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 naturally 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.

Do I Need a Soil Amendment?

As a rule of thumb, all soils love to be amended; the question actually lies in: which amendment do you need? It is best to have your soil tested for pH, micro nutrient deficiencies, salt content and more before applying products that will alter the nutrient value of your soil. Just because lime is a great amendment, it does not necessarily mean that you need it. When selecting a soil amendment you look for how long the amendment will last in the soil depending on your growing season; the soil texture and salinity; the plants you are planting; and the salt and pH content.

Which Amendment is Right for Me?

PLANT BYPRODUCTS

First, let’s take a look at the soil amendments that are made from plants:

Alfalfa Meal

Feeds with 3% nitrogen and is known to contain growth factors and mineral content. Very common ingredient in many blended organic fertilizers.

Coconut Fiber

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.

Cottonseed Meal

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.

Seaweed

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.

Soybean Meal

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.

Wood Ash

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.

ANIMAL BYPRODUCTS

Blood Meal

Use with caution or risk burning your plants. It’s dried and ground waste from slaughterhouses, primarily steer, and is a strong source of nitrogen.

Bone Meal

Slow release. Another common ingredient in fertilizer mixes, and due to its high amounts of phosphorus, should be used with care like blood meal.

Earthworm Castings

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.

Feather Meal

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.

Fish Meal

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.

MANURE-BASED AMENDMENTS

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.

Poultry Manure

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.

Steer Manure

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.

COMPOST-BASED AMENDMENTS

Compost

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.

Mulch

Plant and bark materials that are not fully decomposed. Aids moisture retention, decreases temperatures (protects from hot temperatures), weed reduction, and more.

ROCK & MINERAL-BASED AMENDMENTS

Dolomite Lime

Provides calcium, magnesium, and lowers pH quickly. It also helps with the breaking up of clay soils.

Greensand

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.

Humates

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.

Perlite

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.

Rock Phosphate

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

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.