Not all soil is created equal.

When you're building large planter boxes, you have to think about what kind of soil to put in it.

Using soil from the garden is likely a bad idea. You'll be transplanting weeds and possibly diseases into your boxes. And you may have to augment the soil to get the nutrition levels up. But what should you put in those shiny new boxes?

I did my first couple of boxes with the soil mix recommended in the All New Square Foot Gardening book, by Mel Bartholomew; 1 part compost, 1 part peat moss, and 1 part vermiculite or perlite. 

Compost: be careful where you get this. If you're buying it in the bag, read the labels carefully, and make sure it's organic. If you don't, it's possible you will get a compost that has been treated with a chemical that inhibits seed germination or herbicides. Keeps the weeds down, but it makes it hard for your own seed to germinate. Not exactly what you want for your garden. I bought 1.5 cubic yards of compost from a local nursery. It looked and smelled like fantastic compost. But I didn't ask if it was organic before ordering it, and many of my seeds didn't germinate (arugula, peas and beans), and my tomato plants developed a weird leaf curl that doesn't look like anything associated with diseases (the leaves themselves curled under and down, making only the leaves look like a shepherd's crook.)

Peat moss vs Coconut Coir: Skip the peat moss. It's not renewable and it doesn't wick moisture very well. It may hold moisture in a traditional top down irrigated bed, but for the sub-irrigated planters where I used peat, I have inconsistent moisture levels in the bed. The later wicking beds that I made with Coconut coir are far superior; moist without being damp, consistent moisture throughout the day, even when it's 104F outside (like today). The plants in coconut coir/compost mixture don't wilt in the hot of the day, and the peat-mix plants do. Coconut coir is by far superior.

Vermiculite/perlite: Slightly different products which do about the same thing. They are lightweight puffy stone bits the supposedly aerate the soil. I'm kind of at a loss why this would be important in a sub irrigated planter. It seems like these airy bits would interfere with the wicking process. In fact, I used the 1:1:1 mixture in the first bed. This may be part of the reason that my first bed isn't quite as healthy as later beds. In later beds, the mixture was more like 2 parts compost: 1 part coir: 1/8 part perlite. In the future, I'm not going to add perlite for aeration. I'm going to do it a better way. Worms.

Worms: RobBob talks a fair bit about worms. He makes sure he's got a mix of them in each bed. As you mulch the top of the beds, the worms nibble it down and poop nutrition for the plants. Their burrowing also keeps the soil light and aerated. It's going to be worms and not perlite for me from now on. Just remember to keep feeding your worms by mulching your beds. You can also make worm feeding stations, and fill them with ground up veggies.