if you want to be as efficient as possible use the Thanalan soil on all of them EXCEPT the first one if you're starting with an empty garden. Crossbreeding is figured out when you plant the seed, so if you don't have anything next to it, it's an instant "no" for crossbreeding results. This way you use 7 instead of 8 grade 3's Thanalan soil, and if you use say the yield booster on the first one (which doesn't work on things like minion seeds and Thanalan onions BTW) you can get some benefit from the first seed at least.
There are a lot of gardening sites out there that will tell you the order and optimal layouts for certain crops. http://www.ffxivgardening.com/ is my favorite. Certain combinations require you plant in a certain order as well. EG: A seed then B seed will yield an entirely different crop than B seed then A seed.