All the Dirt on Soil Amendments
We have all been there: we buy a potting mix or soil and see that it retains too much moisture, or not enough. Or you buy a plant and when looking up care you see that people are saying the mix needs to retain moisture or be well-draining. So what do you do? Well, the answer is very simple, you add something to either improve drainage or moisture retention. Thankfully there are options, and none of them are very expensive.
Vermiculite: This is the shiny, brownish flake that you see in potting mixes that can sometimes feel spongy. Vermiculite is partially used to help aerate the soil. Since it has a higher ability to retain water by acting like a sponge, it is better to use it with plants that need soil that retains more moisture. Another of its good qualities is that it is pH neutral, which means that it is neither acidic nor basic. Also, due to it being sponge-like, it can hold nutrients from soluble fertilizers, and it can add magnesium and potassium. Vermiculite is actually a mixture of 19 different minerals that resemble, contain, or consist of mica, and iron oxide, silica, aluminum oxide, and magnesium oxide. This mix is heated to 1472-2012 degrees Fahrenheit (800-1100 Celsius), to make it expand eight to 20 times in volume.
Perlite: This is the white additive that looks a bit like styrofoam that you see a lot in potting mixes. Perlite is commonly used to aerate soil to help achieve better drainage. This is due to it having a rigid shape that creates spaces around it that can hold air, which is beneficial for plants such as succulents that need good drainage. Due to it only holding water in the pits on the outside, it only retains water in those small spaces, which means that it drains quickly. It is ph neutral, which means that it will not alter the pH of the potting mix. Perlite can hold some nutrients from soluble fertilizers, but not much and overall it does not add any nutrients to the soil. In its natural form, it is a volcanic rock that is gray and glassy with a pearl-like shine on its surface. It mainly is comprised of silica with water molecules trapped inside of it, which cause it to pop and puff up to 15 times its volume, like popcorn, when crushed and heated to 1472-1562 degrees Fahrenheit (800-850 Celsius).
Pumice: This is a volcanic rock that is filled with holes due to gas bubbles getting trapped in lava as it cools. These holes cause it to be fairly light and porous. It resembles perlite, but can be a light grey in color and is a little bit heavier. Pumice is great to use if you have a plant that needs a well draining soil. It also provides great aeration due to those tiny holes. Also, it has a neutral pH, is easily leached, and is odorless. Pumice contains trace elements such as magnesium, calcium, potassium, phosphorus, sodium, manganese, and copper. Due to it being so porous, it can also hold nutrients from soluble fertilizers. It can also hold small amounts of water, which can help reduce the amount of watering needed.
Orchid Bark: This is generally pieces of bark from fir trees that usually comes in three grades, fine, medium, and coarse. This is used to help improve air flow in soil mixes, and the finer, the slower it will dry out. The bark of the fir, which is a pine, is naturally acidic, with a pH of about 4-5.
Sphagnum Moss: This is dried out moss, not to be confused with sphagnum peat moss. It grows on top of peat bogs. Sphagnum moss is also pH neutral, soft, purely moss, and has long fibers. Since the plant fibers are like sponges, they suck up any water they can and retain it, so it is used to help keep soil moist.
Peat Moss: This is generally comprised of sphagnum moss that has fallen below the surface of a peat bog, and is therefore decayed. Due to that fact, it is used to retain moisture in soils because it acts like a sponge and holds water well once it has absorbed it. Peat moss is also used to lighten up potting mixes, which can help lessen the weight of a large pot, and add some air flow. It is also acidic and high in tannins. The downside is if you add too much, it can retain too much water and become a waterlogged mess. Oddly, if it is too compressed it won't hold water well. Also, it can be difficult to get it to absorb water once fully dried out. You see it commonly compressed into blocks, bales, discs, etc.
Coir: This is coconut fiber that is collected from the husks of coconuts. Coir retains water well, will improve air flow in soil, and helps make potting mixes lighter. It can be high in salts though, so it is best to rinse it out before using. It is also high in potassium, iron, manganese, zinc, and copper. It has a pH level that is nearly neutral, but only slightly basic at 5.2-6.7. It sometimes comes in compressed blocks and when water is added it can expand to up to seven times its dry weight. Coir is also easier to re-wet once fully dried out than peat moss is. Another positive for coir is that it is a renewable source, as it comes from coconut husks which are readily available.
Sand: This is found in a lot of succulent/cactus potting mixes because it helps improve drainage and soil aeration. Sand is also very inexpensive, but can add weight to potting mixes. The best type to use is a course grade construction sand, not a beach or silica sand. Those two types are too fine to really do too much other than add considerable weight.
Charcoal: This is horticultural charcoal, not to be confused with other types, and is basically just charcoal in its purest form without the additives found in other types. Charcoal is carbon that is created using plant matter, and is very porous. Due to this porous nature, it helps increase air flow and can hold water and nutrients. Adding charcoal helps fertilizer be more efficient, improves the pH of soil, draws out toxins, and water absorption. Also, it does not decompose like some other potting mix additives.
Rocks or Pebbles in the Bottom: Do not ever do this! It creates an environment where the roots become cramped faster. Also, since soil holds water, and the rocks take up space that would have been soil, the roots are exposed to the raised "water table" created by the rocks. In the long run it will cause your plants to become cramped faster, and the roots to be sitting in wet soil for longer. Both of those are a recipe for disaster! Also, it adds considerable weight to the pot.
Hopefully all of these options help you decide what will help you improve the quality of your potting mix. Remember, it is always best to find out what type of soil each specific plant likes, so you can pick out the correct thing to add to the mix to help the plant flourish. If you need any help trying to figure out what type of soil - like fast draining or moisture retaining - your plant needs, head to our Facebook group House Plant Hobbyist, where we are happy to help!