How Much Compost Is Too Much? (For Seedlings)

Recently at Learn Dirt I've been experimenting with seed–starting mix recipes and ratios.

I've noticed that using too much compost in my seedling mix has a tendency to burn my seedlings, and cause various nutrient toxicities, which can kill the seedlings or severely stunt them.

As much as I would like to use 100% homemade compost for all my seedlings (compost is free), I've had to come to the realization that the compost I'm making is far to nutrient-rich to plant most seedlings directly into.

This was a difficult truth to except, because it means I'll need to continue to buy coco peat to dilute my compost for seed starting.

On the other hand, this is a fantastic revelation - that my compost is so nutrient-rich that I can dilute it, and use it in smaller amounts (let's be real, there's NEVER enough finished compost anyway).

But what exactly is the right amount of compost to use in seed starting mix anyway? I'm here to answer that very question.

The Experiment

For this experiment, I began with 10 cucumber seedlings, which have been started in 2 inch soil blocks.

All of them were started in identical soil mixtures, containing a majority coco peat, a small amount of homegrown compost, and a few pinches of native sand.

the cucumber seedlings were raised up for one week indoors, and all had healthy root development.

I then created the following seedling mixes:

• 20% compost (80% coco peat), 1 pinch native sand
• 40% compost (60% coco peat), 1 pinch native sand
• 60% compost (40% coco peat), 1 pinch native sand
• 80% compost (20% coco peat), 1 pinch native sand
• 100% compost, 1 pinch native sand

I transplanted 2 pairs of cucumber plants into each of the 5 mixes.

The Conditions

• All 10 pairs of plants or watered with plain tap water
• All 10 cucumber pairs were grown indoors under LED lighting @ ~78F
• A ceiling fan was blowing lightly on the seedlings during growth
• The tray was rotated every few days for even exposure to light and wind

The Results

23 days later, we've got our results:

The results actually surprised me! I assumed that too much compost would be the worse problem, causing plants to burn.

As the results show, however, it looks like not giving the plants enough compost & nutrients is the real issue.

From here on out I'll be using about 2/3 compost and 1/3 coco peat (plus a pinch of sand) in my seed-starting mix recipe for seedlings in 4" cubes.

Drawing Conclusions

Of course this is an extremely small sample size, and I'd encourage you to replicate this experiment yourself. See what results you come up with and if they match mine here.

It does highlight one thing, however, which is that your seedling mix does matter. Your ratios matter. Having a good soil mix recipe for seedlings at various stages of growth will set them on the right path for vigorous growth.

Do yourself a favor and run some seedlings tests with various different soil mixes.

Find the right amount of compost and other nutrients for each seedling stage through experimentation. Seedling mixes can evolve just like recipes or species, through trial-and-error and doubling down on what works.

The more you can get this dialed in, the better your success will be each and every season from here on out.

Final Thoughts...

Good luck, and have some fun running your own soil experiments! Nothing beats testing gardening ideas yourself and seeing the results first-hand. Don't take my word for it (or anybody else's), just test it for yourself and see what happens.

And then let us know in the comments if your results match mine. We'd love to know if you've learned something from your experiment that's worth sharing!