Soil Mix

Asked April 28, 2016, 1:30 PM EDT


I'm considering using the potting mix: Organic potting mix
This has a lot less lime than the other mixes. Is that correct? Most of them use a lot more. What's the reason to use more or less lime?

New York

20 Responses


Without a brand name and list of contents, one would not be able to tell if limestone had been added to the mix or not. It should be listed. Limestone is added to serve a as a source of calcium and also to modify the pH. The pH affects the availability of all plant nutrients.

Thanks so much for the response.

Here's the list I was planning to follow:
Standard Blend (Cornell Mix): 1 bushel vermiculite, 1 bushel ground sphagnum moss, 8 tablespoons super phosphate, 8 tablespoons ground limestone, 2 cups bone meal.

Also, I was also only able to find Triple Super Phosphate.

Here are my questions.
1. Does this mix sound like a great mix?
2. Do I use Triple Super Phosphate in the same volume as I would have used Super Phosphate?
3. Based on this mix that I'm using for outdoor container vegetable gardening, would you suggest I intermittently add fertilizer?


The Cornell mixes have been used by growers for years until commercial mixes, like Jiffy Mix, Promix, Fafard were available. In fact, in one of my first horticultural jobs, we did make our own mixes. Then switched to Promix because the commercial mixes were a lot less work, had great physical properties and were consistent from year to year. So if you want to make your own mix, that is great but with today's commercial mixes, it really is not necessary.

Since triple superphosphate (0-40-0) is twice as strong as superphosphate (0-20-0) you would use half as much.

As far as would you have to add more fertilizer, you would need a source of nitrogen for sure and if you water a lot or it rains a lot, then any soluble nutrients would be leached out and you would need to replace them. You could use a water soluble fertilizer on a more frequent basis or a time release formulation less often or a granular formulation, whichever you prefer. Follow the directions on the package.

I grow 32 containers each year of flowers and bulbs and use Promix and Osmocote (with trace elements) to save time and am not disappointed with the results. The only container I put limestone in is the one with the Gloriosa lilies.

Thanks for the information, i's very helpful. Regarding my question about the mix, I had found another website before I found this site and that site referred to the recipe I posted above as the standard Cornell formula. I've foraged the internet trying to find a reference to the same "Cornell" mix and given up. Assuming it's not really a Cornell formula does the formula make sense? I'm asking because I purchased a ton (not literally) of these materials and I had planned to make this formula soon (thanks for saving me from accidentally doubling the super phosphate). I haven't opened any packages so I can probably return stuff and get other stuff but if the formula I have looks good I'll just make that.

- What is the best way to add Nitrogen? Is there something specific you would recommend?
- Regarding Promix, I just looked it up.The soil formula I've used contains many of the same ingredients except I don't have any Mycorise, maybe I can just get some of that alone?
- Thanks for the Osmocite recommendation

Thanks again for all the help.


I got really busy on Friday and did not have time to answer many emails. The Cornell mixes were developed by Cornell for commercial growers. I remember seeing the recipes in some of the older horticulture books. To add nitrogen, you could use about 8 tablespoons of bloodmeal to each batch. I do wonder if just with the peat moss and vermiculite if this mix will hold too much water and degrade physically over the growing season. Did it say anywhere that this was a seedling starting mix and not a container mix? You might want to add some perlite or composted bark to your mix for some larger particles.

Don't worry about adding microorganisms to your mix. They are everywhere and will colonize your containers whether you want them too or not. You could add a handful of compost to each of your containers to add microbes if you want.

Thanks, after 2 hours of making my mix (before I saw your answer) I accidentally halved the amount of vermiculite that I was supposed to add. Maybe I'll add perlite instead of the vermiculite I had planned to add. (I did't add water yet so it's really super light). It says it's a standard mix for self watering planters.

I'll mix in some compost too as long as I need to remix it anyway. Any thoughts about how much compost to add per cubic foot of soil or if I should also add some slow release fertilizer?


I don't know if I would add compost as you have a lot of organic matter in your mix already. Maybe if it was a low nutrient, very mature leaf based compost you could add about a cup per cubic foot. I would plant first and watch the plants before adding more fertilizer especially if you added a source of N, P & K. Maybe after a few weeks if you see the plants getting lighter in color then add slow release fertilizer to the top of the planting mix as directed on the package.

So it sounds like you wouldn't add anything and see what happens. Did I get that right (other than adding the perlite (in lace of the vermiculite that I forgot to add) Also, I added pelleted lime instead of crushed, Does that matter? Any thoughts about how to fix? Speaking of seeing what happens I planted a few tomato seeds about 7 weeks ago. I use growing lights. I put them outside on a few nice days. All my other plants look okay. Any thoughts about what happened to my tomatoes?


Correct. I wouldn't add anything else. See how the plants are doing and if they look pale add a water soluble or time release fertilizer. Either form of limestone is fine as neither is particularly quick release. Your tomatoes were left out too long and got some sun scald and probably some wind damage. They will grow more leaves. To harden plants off, start with full shade, then part shade, then 30 minutes of sun, then 60, etc. Don't put them out unless the temperatures are over 60.


Thanks, I thought it was yellow and curling so it was a nutrient issue. Oops, I thought they would enjoy a nice day outside. They also got a little floppy. You can't tell from the picture but they are leaning up against each other since they will flop over without that. If they look too weak in a week or so when I'm ready to transplant I'll cut my losses and buy some healthy tomatoes at the nursery.

Thanks so much for all your help.

I'm pretty sure your plants will recover. I have been guilty of leaving them out in the sun too long myself. If yours are leggy, move them into larger pots and stake them. Growers use growth regulators to make their plants stocky. We can help promote stronger stems by petting them a couple times of day - not as effective as growth regulators but it helps somewhat. When finally transplanted into the garden and staked or caged, they will be fine.

Unfortunately it doesn't look like most of my tomato plants will make it. I had planned to plant them on the outdoor planters last week but it's getting imp the 40's at night so I going to wait a few more days. I'll post some pictures but I think there might be some additional problems. They are looking weaker and weaker.

I would not plant any tomatoes or peppers or other tender crops until Memorial Day weekend. I brought all mine back inside Saturday night and they are back under plant lights. Tomorrow I will put them out in the shade during the day. It really is too cold for them when the temps go much below 50 F.

Okay, I'm going to look at the long range forcast on Thursday, If it looks like it won't dip below 50 I'll plant outside. I've got the weigh the balance of my plants that I started indoors way too early and getting them outside before it's too late and they won't transplant too well. Especially my squash and cucumber. Do you have any experience with aphids? I noticed little green bugs and I feel almost certain based on my online research that's wht they are. There are a few on each of my plants. (I've got some stuff outside already (lettce, oregano, cilantro). This morning I found about 40 of them and killed all of them but can't keep it up.

You can use insecticidal soap or just wash them off with a strong (not too strong) spray of water. You might need to use either 2 or 3 times.

Thanks, what about Neem oil?

It may injure small tender, leafed seedlings. I just use it on more mature plants.

Okay thanks. The plants I started from seed are over 2 months old so hopefully they aren't overly sensitive.


I got a soil analyzer and test my soil it showed a PF of 5. I used about 5 cups of lime for about 6-7 cubic feet to soil but then I ran out (I had the regular kind of pelletized garden lime). I went to a nursery to get more and they insisted I needed the fast acting kind (which was more than 4 times more expensive). I used that on the remaining soil. I'm curious if you have an opinion? I'm growing mostly vegetables (tomatoes, peppers, cucumbers, squash and herbs, Did I need that. Is that the wrong thing? The package referred to lawns or potted plants but not preparing soil for large planter boxes.


Without you telling me what the label says, I can't really tell you if the material you bought would be quicker acting or not. Some of the soluble forms of calcium, like Solucal or Maxical claim that they will change the pH more quickly than ground limestone but from what little research I've seen on them, they still do not work that fast. Also often the quicker acting liming materials do not have a long lasting effect and may need to be reapplied several times throughout the growing season especially if a lot of rain or irrigation occurs. Either material would suit your purpose.

The soil pH should be 6.4 - 6.6 for growing vegetables in a mineral garden soil. When using a soilless potting mix, the pH can be lower - 5.8 - 6.0. But if your pH really is 5.0 you might be seeing some blossom end rot in your tomatoes. Maybe not though if you added limestone and keep the beds well watered.