What is the best soil mix for indoors plants?
Thank you
Michael

Your inquiry will produce lots of different and passionately conflicting results. It seems that everyone has their own favorite recipe that works for them. My take is a follows:

The livelihood of nursery growers depends on growing healthy plants quickly so they have determined the appropriate potting mix for the particular plants that they grow. That means that the soil that your plant is growing in in its nursery pot is probably the best for that particular plant. If you are repotting, you should try to duplicate that mix as closely as possible.

I will also add, that contrary to popular wisdom, most plants, especially newly acquired plants, do not need to be repotted. A lot can go wrong if you repot unnecessarily and incorrectly.

Finally, in general it is best to use soil-less, peat-based potting mixes for indoor plants. A good basic mix is 4 parts of peat moss and 1 part perlite. Popular packaged potting mixes often incorporate organic bark and other materials that harbor fungus gnats.

I have mixed my own potting soil this year. I have mixed feelings about it. I will go back to premixed, when I need more potting soil for many reasons. And, yes; as Will said, this is going to receive many replies with many opinions. I think we all need to do what works best for us. I will always have perlite and chicken grit in my potting shed, to assist drainage, but it will be mixed with a purchased potting mix.

@WillC, I was not aware bark harbors Fungus gnats. I do repot orchids from time to time. I will be asking for your advice on orchid mix when the time comes. Thanks for the tip.

Laurie - The pine bark used in may packaged potting mixes is finely ground so it decays easily and is a source of food for gnat larvae. Orchid mixes typically use larger bark chunks where decay is not so much of an issue unless you don't replace it for many years.

What is chicken grit? I wonder if that might be a source of gnats. Compost, which is excellent for outdoor use, is not recommended for indoor use where sterile material is preferred. Insects that are acceptable outside, are not embraced indoors!

Now I know where my gnats come from!

Will, chicken grit is usually crushed granite. There is a form that is crushed oyster shells, I believe. But, that's not the one used by gardeners. It is more readily available in the south, as opposed to pumice. Both chicken grit and pumice are used in place of perlite, which is very light and has the tendency to gravitate to the top of the potting mix, leaving plant roots with little air space to breath. These two products help to alleviate that problem.

Chicken grit is crushed rock, whatever that rock may be. Depending on where you are located, it is most often made from the regional local granite or limestone. It can be oyster shells, although I couldn't really say if it is from oysters or some other shell species. It is usually sold in 50 lb. bags of uniformly sized aggregate: common sizes are #1,#2,#3,#4. Number 4 is often referred to as turkey grit, and number 1 is often referred to as chick grit, although "chick" grit is also the general shorthand for all the sizes.

Rick! I learn here every day!

for the clarification, Rick.

I might also add that while both chicken grit and perlite do improve drainage in soils, their shared qualities pretty much ends there. Chicken grits are quite heavy (in weight) and add valuable minerals to the soil, while perlite weighs very little and does not contribute usable nutrients (unless you are growing a particular plant in the small subset of those that use silicon). What we call perlite is really expanded perlite, which is perlite heated to the point of expansion due to the water held inside the rock. Unlike vermiculite (another expanded rock), perlite does not absorb water, but will hold water on its surface due the the rough micro texture. The expanded perlite is usually contaminated with fluorine, but not so much as to cause any problems, except in particular circumstances.

Dang! Y'all really do know your stuff! I was about to ask about bark harboring fungus gnats but @lauriebasler beat me to it. And then I see there's something called 'chicken grit' and I'm a thinking 'What the what?' And whoa! It's sold in 50 lb bags (that's the weight of a small child!)

So thank you all for the education.

@synef - The only thing I will add is we know not where you live or where inside your house you will be placing your houseplants, and also what kindsa plants you have. These things might make a small difference, especially in the future.

@WillC - Do you ever use coir instead of peat moss? Or maybe I should just ask if you have an opinion about coir.

Yes, Sorry to have missed the question Will, but it looks like it was handled. I am not seeing gnats. I did find a mix I made was too wet, but my plants were only in that soil for under a day. I do like the addition of chicken grit, because I prefer the soil to dry rather quickly.

Rick, Deebie, and Barbalee, thanks. Great job all.

Living in the wilds of NYC, this country boy was completely clueless about chicken grit, thinking maybe it was related to chicken manure! Clueless! Thank you all for the clarification. Obviously, not a source of fungus gnats!

Joan - My opinion of coir (coconut husk) is very favorable. It can be used as a substitute for peat moss or mixed in with it. There is some evidence that it helps deter fungus gnats.

WillC said:Living in the wilds of NYC, this country boy was completely clueless about chicken grit, thinking maybe it was related to chicken manure! Clueless! Thank you all for the clarification. Obviously, not a source of fungus gnats!

Joan - My opinion of coir (coconut husk) is very favorable. It can be used as a substitute for peat moss or mixed in with it. There is some evidence that it helps deter fungus gnats.

Thanks, Will! I'm a not-alone country girl on chicken grit and now know more about coir!

Chicken grit, you could visit a petstore and get parrot and small bird grit in smaller sizes. We do add ground oyster shell to self mixed chicken feed to help the hens with calcium, but most lay pellets already include it. You might include a desert potting type mix if the garden 'soil' is too heavy, just saying.

FWIW, in the south, we have an agricultural store, Farm and Tractor Supply Store, which carries chicken grit in a smaller size bag. Just beware, you will pay more per pound (almost twice as much) for the dressed up package (marketing strategy). Also, make sure that it's crushed granite (rock) and not crushed oyster shell. The latter can damage plant roots due to its high salt content.

Leftwood, for the additional info on chicken grit. There is so much to learned about gardening, and I'm thankful to those who share their knowledge with us.

Laurie

FWIW, in the south, we have an agricultural store, Farm and Tractor Supply Store, which carries chicken grit in a smaller size bag. Just beware, you will pay more per pound (almost twice as much) for the dressed up package (marketing strategy). Also, make sure that it's crushed granite (rock) and not crushed oyster shell. The latter can damage plant roots due to its high salt content.

The chicken grit I use is like that of size 1 chicken grit as shown on Rick's photo, that's the size we get from the chicken grit bags we get at our local Ace hardware. I use it to top dress my succulent containers. Before I use either perlite, pumice, crushed lava rock, kanuma or akadama. I also mix in some of this to my tropical indoor plant's soil, it helps to give air at the root zone. Though knowing what type of plant you are growing indoors is important, the more tropical ones will appreciate some more moisture retention in its media than the succulents. And some succulents do go partially dormant during cold season, so they need really fast draining soil and proper watering intervals.

I have used coco coir with my epiphyllums, and some orchids, they are okay with it. Some orchids are more water needy like the Oncidiums and coco coir is really good for them since it holds water longer. My Phals, I use chunky bark mixed with clay rocks and have experimented with baggies of clay rocks. I forgot the name of this material I used as baggies, but it is really nice, lots of holes, porous enough to allow quick air flow but at the same time holds some moisture which the Phal roots needs.

So pretty much the best soil mix for indoor plants will depend on what type of plant you are going to grow. Different type of plants, different types of media.

synef said:What is the best soil mix for indoors plants?
Thank you
Michael

Hi Michael. Welcome to NGA!

Short answer: something that drains quickly so that air can flow back into the soil mix and diffuse through it, once enough water flows out the bottom holes.

If you buy a potting mix, there are "professional" brands that are a little pricy and tend to come in huge bales of 3.8 cubic feet each. Pro-Mix, Fafard, Sunshine and Black Kow are some "pro" brands.

If you buy a cheaper mix that doesn't drain as fast and is not as well aerated, you should add something like Perlite, grit, screened crushed stone, or screened evergreen bark shreds to "open it up".

Long answer:

I think the pro mixes are mostly peat-based, but with a good grade of sphagnum moss plus amendments to open it up more, not just the thin brown dusty "peat moss" that packs down densely and can be hard to re-wet once dry. The "pro" mixes are careful to include ingredients to keep them "lofted" or "open" or "airy".

That is crucial in pots, since potted soil mixes tend to pack down so densely that they hold or retain too much water. "Too much water" means that all the small crevices and too many medium-size crevices and openings fill with water and are then held in place by capillary force.
Then they also have a "perched water layer" that can completely flood the bottom few inches in a pot.

Once the open spaces fill with water, air diffusion is slowed by a factor of 10,000!
( http://garden.org/thread/view_... )

When air diffusion is slowed, the roots can't get the oxygen they need to live. They drown. Then rot.

That's why you want potting mix so open that some of crevices, openings, air gaps or voids are large enough that they still have some open space remaining after capillary films and perched water try to fill them. Air diffusion can occur through the remaining open space.

It seems to be a good rule of thumb that, if excess water flows out the bottom quickly, there must have been enough open space that the roots can breath within a few minutes of watering.

If you don't buy a professional-quality potting mix, there are many brands that sell "stuff" they call potting mixes in big box stores like Home Depot and Lows. My experience with the HD near me is terrible. Lowes had some decent bagged bark products.

When I have some cheap potting mix, I "open it up" by mixing with grit or screened bark chunks and shreds. I like the shreds to be around 1-2 mm in small dimension, and up to 1/4 inch to 1/2 inch, depending on size of pot. If the bark is mostly chunks, not long shreds, I like them to be around 1/10 inch (2.5 mm). Much smaller than that, and they start holding too much water also.

(P.S. Since bark is REALLY cheap, and I can screen it myself, I mix 25-50% screened bark with an expensive potting mix, so that the mix in effect cost only 1/2 as much. And some things seem to grow pretty OK in mostly-bark.)

Other have told me that they can buy almost any old potting mix from HD and get good results, for example with vegetables in big pots, outdoors.

Maybe that's a regional difference, or my HD has a mean-spirited garden manager, or maybe I just over-water so much that I can't use ordinary cheap potting mixes. You'll have to see whether your plants drown and roots rot with any given potting mix ... but my thinking is, if I water heavily and DON'T see the water come right out the bottom, I should have lightened the mix more before holding the plants' roots under water.

@RickCory. You make such a good point for this poster to think about. You inform with such clarity.

In closing above, you said "maybe I just over-water so much that I can't use ordinary cheap potting mixes". That is the truest clearest fact for me and others here too.

@synef. Rick gave us a nice blue print to remember and also tweak a bit for our own differences, in climates, light and watering style. Picking soil for the plants needs is a must, but if you overwater, then you should pick the soil best for the plant first, and then add a component for fast drainage. I don't think an exact recipe is really the answer. I tend to have to add a fair bit more grit or perlite, for the fast drainage. Cheap peat soil is an expensive waste of money for me. I will kill things I love, and spent money on.

Rick, I am going to print out your post, and tack it up in my potting shed. Great info, that will be an important reminder, not to forget to take in my own watering shortcomings when choosing soil.