Greetings,

Recommended soil depth for most vegetables is between 6 to 12 inches depending on the crop. Depending on the variety of carrot, 8 to 10 inches of potting mix should be sufficient.

Is your planter box on legs to bring it to waist height, or is it ~3 feet deep?

If you are working with a very deep container, you can add filler material to the bottom, lay a piece of landscape fabric on top of the filler, and then just fill the remaining foot or so of space at the top with soil. For example, I have several very deep pots that I grow in, so I fill the bottom with empty soda cans as filler, and plant in the top 12 inches of the pot.

Using lightweight filler will help with both drainage and expense, not to mention keeping the containers light enough to move if there is a need to.   

Please let me know if there is anything else I can help with. 

Thank you, Gina.

The soda cans are an excellent idea!  Our planter is full depth, not on legs.  So I will do a filler.

Another question, if I may.  The planter is made of non-treated plywood.  I want to seal the plywood to preserve it but am concerned about the kind of sealer to use.  It needs to be strong enough to be durable when in constant contact with damp soil, but also needs to be non-toxic and safe for food growth.  Any recommendations?  Any knowledge of FlexSeal for use in food planter boxes?

Thanks again,
Alder

Greetings Alder, 

I normally don't recommend using a wood sealer, though they do have some that are rated for garden bed use. No matter if they are deemed non-toxic, there will be some leaching that occurs, which could effect flavor and/or the balance of microbes within the soil. 

Instead, I recommend using two layers of plastic landscape material to line the bed. My mentor has used this method exclusively and has had the same untreated wooden raised bed for 12 years now.  Just remember to drill out some holes through both the wood and the plastic once the liner is in place.