Is it the potting soil?

What am I doing differently this year such that my seedlings are not growing well? The tomatoes (left) and peppers (right) that I started on April 3rd have germinated but have not developed as they should. Admittedly I started them later than I wanted to but these little plants represent seedlings that emerged four weeks ago. The same is true of most of my other seedlings. The lettuce I transplanted this week are the smallest little lettuces I have ever planted out. That is also true of the broccoli, zen and kale that I have now transplanted.

My suspicion is that the potting soil I used after I finished up last year's bag is the cause. I know that a lot of sources on starting seedlings say to not use commercial potting soil but I have never had a problem. What I have been using the last several years is Miracle-Gro potting soil (the bag also says that it feeds plants up to six months). When that ran out this year I bought what I thought was the same - well, the color of the bag was the same - but I think it was Scotts Premium Potting Soil, which, according to their website, is intended for container gardening. I knew that it was different when I opened it because it was fairly coarse and full of little chips of mulch and woody stems.

The seedlings I grew thorough the winter to try out in the cold frames did not have any problem but they would have been grown in last year's potting soil. The most recently planted seeds also seem to be doing well and would have been planted in potting soil from a new bag of Miracle Grow. It is what I started in between - that is, using the coarse potting soil - that has not done well.

Since the soil around these seedlings, when I knock them out to plant, is very wet I suspect that this potting soil is a type created to retain moisture. On the other hand I have also changed the way I water the seedlings, using a little watering can and watering from the top as opposed to my prior method of pouring water into the bottom of the tray in which the pots are sitting and then draining it out a short time later. But I don't really think this would change the amount of moisture retained by the soil in the pots.

As a result of the slow growth, I have been planting seedlings into the garden that are much smaller than what I would normally plant. Time will tell if they will thrive. They may have been in too much water too long and can't recover in the garden soil. [The tiny lettuce plants seem to be doing okay, however.]

Also today I transplanted the tomatoes and peppers into larger containers using the newer potting soil. Hopefully they will soon be smiling back at me - and growing.