Soil Blocking vs. Plug Trays

When starting or expanding your gardening adventure, rather for veggies, flowers or both, you may have come across soil blockers and had no idea what they were, why they’re needed, do you need them too? Why can’t you just use good, ole fashion seed trays that are easily available?

I’m here to try and answer those questions for you and hopefully clear up what soil blocker are, if you need them for your individual situation or maybe you just need them for certain things.

What is Soil Blocking & is it right for you?

Soil blockers come in multiple sizes, though usually you will be able to use the smaller size. Any bigger and I just go to trays. You pack in your soil mix, push from the blocker using the top handle and you get these tiny little blocks. Since they are so small, they take up little to no space. Once formed you sit and plant 1-2 seeds per block. You can fill up a segment with one plant or mix and match. I often separate the little blocks and just plant what I need of each annual.

You do have to put them onto a tray or pan. I use little foam plates that can be put on heating mats if your starting things super early in the season or just as a space saver. Can be easily put under grow lights or even on a well lit window sill. One small foam pad can hold at least 36 blocks. That’s 36 seeds started in a space of less than a square foot!! If you’re working in a serious space restriction, that is likely your entire grow season put together in a tiny little area to sprout. This gives you time to prepare beds, rows, containers, or whatever else you plant to grow in.

These are best kept inside because of their small size. If rain were to get to them, it could completely destroy the little guys! Even a good gust of wind can blow them from whatever surface you are keeping them on. However, if you’re in an apartment or other place with little outdoor space, or if you’re planting super early to extend your season, you are likely keeping them indoors anyway so this is a perfect system! It could be worth it for you to invest in 2 -3 sizes of blocks and start them this way every year or multiple times per year for things like lettuce and spinach that need to be succession planted throughout the year or season.

The really cool thing about soil blocks is the seedlings will not get root bound. Since there is no container edge for the root to hit and circle around, they just see the end of the soil and stop. Plants are super cool! However, this means they must be planted or put in larger blocks or into trays promptly or you can start losing your little babies.

One downside is that since they are self contained little blocks, you have to use a more specific soil and/or compost mix so that they can hold their form wet or dry with out simply falling apart.

Soil Mix recipe for blocking!

This will hold together nicely & you can still spray with supplements if you wish. This is from Lisa Mason Ziegler of Gardener’s Workshop. Her information is aimed at flower farming but there is quite a bit of crossover, as they farm organically as well. I just love her and her approach.

Click here for a printable soil blocking recipe!

Overall, soil blocking a great way to get going and plant seeds for your annual and succession planting every year!

So should you use soil blocking?

Positives	Negatives
Great for small seeds like tomatoes & zinnias	Must buy multiple sizes for different seeds or use tray on larger seeds
Will not get root bound	Many have to transplant into larger blocks or tray
Allows more seeds to be planted in a much smaller amount of space	Many use on heat trays and/or under grow lights meaning more products to buy
Allows to be used on heat mats if put onto foam trays for earlier planting!	Using outsides can cause damage to the blocks due to wind and rain.

Seed trays aka Plug trays

These guys are tried and true. You’ll find them in most garden stores when spring is near, making them easy to access. They will fit any size seeds or even multiple of the smaller seeds. Seeds like green beans that are not only large but prefer to be planted with a grow buddy will always do well in a seeds tray.

Because they are not self contained, unlike the soil blocks, you can put whatever soil mix you prefer and the individual sections will just do their thing. Another good thing about the tray and it’s form is that it is great to use indoors or out! It’s easy to move around if outside gets a bit chilly early in the season and can handle a bit of rain as long as the seeds are deep or the seedling has sprouted.

The down size is that they tend to be made from plastic and that will break down over time. You will have to replace them every 2 or 4 seasons depending on how much use and sun they get over your grow season. They also take up more space. Each section is much larger than a soil block and each tray has at least 30 sections.

Because each section has walls, the seedlings can become root bound much more easily. This will happen quick if out doors in in warmer weather where the little babies can grow at their own will. Root bound seedlings will have a hard time reviving transplanting and can because sick.

One last tidbit, Many trays come with a large self watering tray beneath. These are a great resource for the busy or part-time gardener as they will need less of your attention. Plus, these bottom trays can double as micro trays saving you from purchasing yet another supply for the year!

Overall seed trays are a great resource that most us us will have easy access to.

Are seed tray the better solution?

Positives	Negatives
Great for larger seeds such as green beans	Plants can become root bound if left in the tray too long.
Widely available in most garden stores	Takes up more space cause of the base and divided sections
Soil mix doesn’t need to pack well or hold it’s form.	Will need to replace more often as plastic get brittle and starts cracking after a couple of seasons
Bottom section can double as micro trays when not in use
Great for indoor and outdoor use

Which is better for you?

The truth is that it’s really situational. I prefer to use a combination of both soil blocks and seed trays, depending on time of year and seed size. It also depends on my schedule with work and other tasks. If I cannot guarantee to get to them to plant in a timely manner, seed tray are the way to go as they are more forgiving if a few days pass after the original planned plant date.

Once the season is in full swing and the weather is warmer, it’s really seed trays all the way in my garden as I’m almost 100% outdoors at this point.

It really seems to be a personal preference on what people like and/or the type of plants and space that they are working with. Here is some basic information to hep you decide if soil blocks and plug tray should be a part of your regular planting routine.

**Tips and tricks

Use a toothpick and dampen in to grab super small seeds and push them into your soil. Especially important in blocks which will break up if you try to use your finger.
Use the bottom section of seed tray for grow trays for things like micro greens