Get your soild tested. You can get all kinds of guesses and "might be's" but the only answer to your question lies in testing the soil.

My seedlings started great but now growth is really slow and older leaves look yellow (on all plants including the peas) which might be a sign of nitrogen deficiency.

While I agree that having a professional soil test done can be quite beneficial, the yellowing of older leaves is almost always due to over-watering. Most of us tend to over=water anyway, forgetting that air and oxygen is just as important to plant roots as water. Wet soil prevents root access to that oxygen.

Nutrient deficiencies will show up in new, not old growth on plants so I'll bet that if you will cut back on your plant watering substantially you'll soon see an improvement. ;)

Dave

I agree that kitchen compost is the only one with any oomph (manure, too). Yard compost really does not have a lot of nutrients. It will improve nutrient absorption, but if it is less than perfectly finished, it will in fact tie up nitrogen some. It is almost certainly not the watering. Give them some 10/10/10.

Next year, after earthworms have mixed the top soil with the regular soil somewhat, you could test it. This way you will test it only once. But typically vegetables are heavy feeders, specially the greens. Unless you can topdress with 1-2 inches of fresh manure every year, you have to fertilize eventually. I do the same: fertilize everywhere except where I topdress with manure or kitchen compost.

In my experience, only carrots, beans and peas can go a season without any nitrogen boost. And even they like a few pounds of wood ash per row, to get the other minerals.

Since your seedlings started with healthy growth it rules out any Nitrogen deficiency.Yard waste contains grass clippings and leaves that will supply enough enough nitrogen to begin with.Nitrogen deficiency will slow down the growth.Excessive Nitrogen will put the plant in vegetative growth.
Yellowing of the leaves in this case is more water supply to the plants which are in early stage of growth.
You may hill the plants that will drain some water before reaching to the roots.

With all due respect to glib, you'll find very few composters that will agree that "kitchen compost is the only one with oomph" or that yard waste compost lacks nutrients. If that were true there would be many compost-starved gardens in this world. ;)

As chaman said, yard waste compost contains ample nutrients and in a readily useable form for the plants, and millions of composters who only compost yard wastes will attest to that.

Further, the use of "fresh manures" in any food plot, much less top dressing with it, is strongly discouraged and has been for years because of the problems of manure pathogens and food contact. The standard recommendation is that only well-composted (4-6 months minimum) manures be used in food plots. You'll find numerous discussions on this issue over on the Soil & Compost forum.

Lastly, as both chaman and I have already said, yellowing of older leaves on plants that started out healthy is due to excessive water NOT nitrogen deficiency. Please do a search of the forum using the term "yellowing leaves" and you will find literally hundreds of discussions about this issue and many suggestions on how the problem can be eliminated.

All of this is not to say that your plants may need some fertilizer/supplement feeding if only because over-watering dilutes and washes out the nutrients. But nutrient insufficiency is not the primary cause of your problems as you have identified them.

Good luck and I sincerely hope this is helpful to you.

Dave

My own 2 cents from using the same mix you are using including using municipal compost from the same basic ingredients is that you probably do have an N deficiency.

If you aren't opposed to using a synthetic fertilizer you can find out in roughly 7-10 days if it is a N deficiency. Use a high N Miracle Grow fertilizer one time and wait a week or so.

Compost is great stuff, but unless manure or high N ingredients are used in the compost and it is used quickly, there isn't usually enough N in it to sustain high N plants.

For the first 2 years with this mix I had to add N to get good growth all season long. As I said, Miracle Grow will do the trick quickly. After that feel free to supplement N with anything you like. I like soybean meal, but there are many choices.

I really doubt you have an overwatering situation with a raised bed using Mel's mix. Yes, it retains water, but it also drains well and allows a lot of oxygen into the root zone which makes root rot induced problems rare. Even in my Wisconsin springs where I do not water because the rains seem like they will never end I don't have any rot/overwatering problems in this mix.

Since we have a disagreement, try both ways, half plot, and see what works. The type of soil will make a difference, and all of my gardening has been in sandy soil, where nutrients are poorly retained, but where water dissipates in seconds.

If you are watering about one inch a week, and water dissipates quickly, I find it hard to believe it is overwatering. I can appreciate that other people with richer, denser soils may be able to garden based only on yard waste as fertilizer, but those cases are fairly rare.
At least at my site, greens will not grow well on grass clippings and dead leaves, and my beds are made entirely of compost ( probably two feet of stuff total, compacted now to about 6 inches).

Assuming that in your case water does not cause yellowing of leaves I will recommend something from my experience and that is the use of Epsom salt. Epsom salt helps greening of leaves.Epsom salt is Magnesium Sulphate which is soluble in water.Spraying water solution of Epsom salt will help chlorophyl build up in leaves.Chlorophyl gives green color to leaves.It also produces sugar for plant food with the help of sun light

that is a lot of vermiculite for an outdoor bed.

that's more of a non-commercial container-type mix.

People -- Mel's method is used in Square Foot Gardening. Basic mix is as was described by OP -- "1/3 to 1/2 compost, 1/3 to 1/4 each of peat and vermiculite "

Lots of posts about gardening with such a mix can be found in the Square Foot Gardening Forum.

Beyond that, Yes, the plants are short of nitrogen

Here is a link that might be useful: Square Foot Gardening Forum

If there wasn't so much vermiculite in the soil it might not need so much peat.

You can probably grow some killer root veggies in that and it's not like it's going to hurt anything, though

I just find it an odd soil mixture for outdoor field planting.

Yes, as Jean says: it can not be overwatering. Original poster says the soil is crumbly, and the bed is raised anyway.

Just to add a couple other thoughts to the mix (no pun intended).

I gardened in San Diego for many years, on what was then NAS Miramar. While the ground never freezes, the nights can occasionally get to near freezing in December, with light frost some years. The chlorosis you are experiencing could be due to colder temperatures, in which case the plants will green up again when the weather warms next month.

The city compost is made with mostly brown material, sometimes even shredded wood... so if you are using a soil less mix, some amount of N is probably necessary. If you added manure, however (composted, I assume) then you probably do not have a nitrogen deficiency.

You could, however, have a problem with the pH; I would recommend testing it. Should the mix test out on the acid side, you would probably only need to mix in some of the underlying soil; my garden plot had a pH of just over 8.

All that aside... if your plants are even alive right now, they will probably make it.

I've had the same problem after putting in a new raised bed. The problem could be too much organic matter that has not been broken down enough and therefore has some of the nutrients tied up. My problem took care of itself in a couple months and I had a great bed. I bet yours will turn around by the Spring growing season.
I did add some green sand to help with iron uptake, but then again I have alkaline soil.
A soil test is the only way to find out the real problem.

Happy Gardening,

Pete