Radishes can be grown intensively. I just scatter seeds usually. It's the same with Carrots, but it depends on how many seeds you want to use. With Radishes, you can space each seed a few inches a part, and you don't need 1 foot between each row. If it's just for looks, you can do it that way.

For the Carrots, you can just space them a few inches a part also. It depends on how big you want the Carrots to be in diameter. When I planted too closely, I just allowed it to grow until I saw a tiny carrot starting to grow, and then pulled and ate it. You may be surprised how big they get when spaced closely together.

Could I apply the layout of square foot gardening to in ground planting? Would it really save space? Would it overcrowd the plants?

Sure you can. That is what Sq Foot gardening is - gardening in the ground. The only difference is the Mel's mix they use supposedly allows for more intensive planting vs. dirt or raised bed fillings. So what is your 20x20' plot made of? It isn't clear from your post. Is it a raised bed? Filled with what? Or is it tilled dirt, you amended it with what?

Dave

The 20' x 20' plot I have is at a community garden; there is the native soil, which is horrible for gardening (w/out amendment); I know, I tried growing stuff in the ground there (@ the community garden plot) just using the native soil and the plants didn't do well at all.

The soil has been amended with container gardening soil mix that I just dumped into the plot (some of the containers were too heavy too move because of how big they were, so the soil was just dumped into the plot to amend it)

The container gardening mix consisted of top soil, pine bark mulch, Osmocote (a fertilizer), compost, vermiculite, and sphagnum peat moss.

However if I was SFG layout in the ground, I am concerned that because there is no row spacing, the plants would end up overcrowded; so if I were to use the SFG layout in the ground, since the layout will be in the ground, will I have to do row spacing? If so, could I reduce the row spacing of the traditional row spacing measurements that I posted above?

As I said I want to stretch my space out, but I don't want to overcrowd stuff.

No you don't have to use rows. Many folks use Sq Foot gardening guidelines but don't use Mel's Mix in the bed. You can plant just as Sq foot does and work more nutrients into each square or you can reduce the number you put in each square. It is up to you. But either way you will have to add more nutrients to what you have so far.

And if you are direct seeding then all you have to do if it looks overcrowded after they germinate is just thin the plants out. Even as they grow if it gets over crowded remove a couple. Don't over-complicate the process. Nothing says once planted it has to stay there if it is creating problems. Pull it so the others can do well.

Dave

Planting in native ground is very hit and miss. For example, a few years back I started trying to grow vegetables for the first time.I planted Radishes and they were growing and getting leafy. The problem is that they weren't bulbing up ever. A year later, I tried again with another area, and the Radishes did really well! So native soil can be hit and miss.