My son tested types of bagged soil for germination and seedling growth. He used plain topsoil, Scott's brand potting soil, and Miracle-Gro brand potting soil. Both the Scott's and MG have built in fertilizer. He planted broccoli seeds.
Anyway, the seedlings in the Miracle-Gro did absolutely horrible. Fewer seedlings emerged, and ALL of the ones that did where (and continue to be) withered, yellow, and stunted. The new hypothesis is that the Miracle-Gro simply has too much nutrients (probably nitrogen) for the young seedlings.
I've used the stuff before to start seeds, and I remember having bad results. In my mind, all of the bagged "soils" were about the same. Now I know better.
Edit: Final results: Germination was highest in the Scott's, and initial growth was the same in the Scott's and regular top soil. Now that true leaves have formed, the plants in the Scott's are growing better than the ones top soil. The plants in Miracle-Gro are still pathetic.
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ButterflyLady29
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Interesting. I do use MG potting soil for transplants, but I normally do not use it for seed starting. I find it holds too much water unless I add more perlite to it and I only use it if I don't have enough to make my own potting mix. Unfortunately, I usually run out of perlite. When I have used it, in pure form, I did notice that it stayed wetter longer. I added osmocote to the potting mix, because I always add it to my potting mixes. My seeds did sprout for the most part as long as I did not water every day. Sometimes I have failures but that was usually due to using a fast release fertilizer instead of a slow release. I have to try an experiment with my potting mix and MG soil and see if there is a difference in germination. It would be an interesting experiment to see if I see similar results.
This thread scared me, because I JUST bought a bag of MG for starting my seeds. 
My local hardware store is very small with very little selection. I just ran downstairs to see which product I purchased. It's the seed starting mix, not the potting soil. 0:0.01:0 Phew!
An excellent follow-up lesson for your son (and the rest of us
) is to discuss the importance of the conclusions we draw from our experiments. It's tempting to conclude that MG potting mix is trash. What we actually learned was that broccoli doesn't like germinating in MG potting mix.
Further follow-up, if you're up to it for fun: try squash seeds or another heavy feeder in the potting mix. Also, if you need to up-pot seedlings like your tomatoes or peppers, try them in the MG potting mix. Teach your son (and yourself ) to keep notes, because you won't remember next year.
Gardening is one long, long experiment where you often don't get to re-try your failures for another year.
My local hardware store is very small with very little selection. I just ran downstairs to see which product I purchased. It's the seed starting mix, not the potting soil. 0:0.01:0 Phew!An excellent follow-up lesson for your son (and the rest of us
) is to discuss the importance of the conclusions we draw from our experiments. It's tempting to conclude that MG potting mix is trash. What we actually learned was that broccoli doesn't like germinating in MG potting mix.Further follow-up, if you're up to it for fun: try squash seeds or another heavy feeder in the potting mix. Also, if you need to up-pot seedlings like your tomatoes or peppers, try them in the MG potting mix. Teach your son (and yourself
) to keep notes, because you won't remember next year. Gardening is one long, long experiment where you often don't get to re-try your failures for another year.
It is an interesting thread and you are right about drawing conclusions from a single experiment. Usually before conclusions can be drawn, the experiment has to be repeated to see if the results can be duplicated and it may be something else that was the problem like the type of seed, soil, moisture, temperature, or even if the seed requires darkness or light to germinate. Sometimes even position matters. All variables need to be taken into consideration before a conclusion can be drawn.
It is also one of the failings of testing when a researcher's bias, draws them to see what they want to see. That is why tests should be designed so that the experiment can be duplicated independently and see if the results can be duplicated.
I could get baby's breath to sprout in my seed starting mix, but because it is so acidic with peat moss, it does not live very long. That same potting mix works for practically everything else I grow except the succulents, because most of my other plants are living in the acidic soil of my yard. For baby's breath, I practically have to grow it in sand and my succulents are all in cinder for the drainage. I have paph orchids and they will only grow well on a particular part of my orchid bench, anywhere else they die.
The nutrient levels in the potting mix doesn't seem like they are high enough to cause yellowing and stunting though.
I add osmocote to my planting mixes and they have an NPK of 14-14-14, but it is slow release over three months, but still much higher than what is in the MG soil and my seeds start fine in peatlite which drains very well. When I make a mistake or get desperate and use fast release fertilizer, I have very poor germination. So, I think your conclusion that it is the fertilizer in the potting soil may need to be further investigated. I know sometimes when I have bought some potting soil that had been sitting around in the rain awhile they have fungus growing in the bag and it did not smell good. So, usually, I either returned those or put it through the steam sterilizer again. I try not to get potting mix after it rains because the bags are a lot heavier too. You could have just gotten a bad bag too.
It is also one of the failings of testing when a researcher's bias, draws them to see what they want to see. That is why tests should be designed so that the experiment can be duplicated independently and see if the results can be duplicated.
I could get baby's breath to sprout in my seed starting mix, but because it is so acidic with peat moss, it does not live very long. That same potting mix works for practically everything else I grow except the succulents, because most of my other plants are living in the acidic soil of my yard. For baby's breath, I practically have to grow it in sand and my succulents are all in cinder for the drainage. I have paph orchids and they will only grow well on a particular part of my orchid bench, anywhere else they die.
The nutrient levels in the potting mix doesn't seem like they are high enough to cause yellowing and stunting though.
I add osmocote to my planting mixes and they have an NPK of 14-14-14, but it is slow release over three months, but still much higher than what is in the MG soil and my seeds start fine in peatlite which drains very well. When I make a mistake or get desperate and use fast release fertilizer, I have very poor germination. So, I think your conclusion that it is the fertilizer in the potting soil may need to be further investigated. I know sometimes when I have bought some potting soil that had been sitting around in the rain awhile they have fungus growing in the bag and it did not smell good. So, usually, I either returned those or put it through the steam sterilizer again. I try not to get potting mix after it rains because the bags are a lot heavier too. You could have just gotten a bad bag too.