Buying soil, seeds, and fertilizer to plant in the 8th year

Answer

In a nutshell: The problem is not the materials listed above; it is marit ayin, appearing to do a forbidden action during shemitah.  To ensure it is obvious you will be using the soil, fertilizer, and seeds next year: Store them indoors until after Rosh Hashanah. Alternatively, you can store them outside if in closed bags.

Expanded answer

The issue with buying seeds, potted soil, fertilizer, and such materials is not so much with the items themselves. As noted earlier, most seeds are imports or have long shelf life; potted soil is a mix of ground-up organic matter. 

The issue we need to avoid is that it will appear that you are bringing these items to your garden for use during shemitah.  Even if you are performing a permitted action, such as straightening out soil to lay artificial grass (which is permitted), others will see this and may conclude that you are preparing the soil for planting. It is important that it is obvious that you are doing a permissible action, which is halachically termed heker (for artificial grass, you would lay out the roll of grass on the ground before starting work, for instance).

For this situation, a heker would be to store them indoors or outside in a closed bag until after Rosh Hashanah.

Note, too, that while it is forbidden to sell gardening tools and such to those who are "suspicious regarding shemitah observance," for fear they will use it in a forbidden fashion during shemitah, there is no such problem for a religious person to buy such items at a nursery when intending to use them properly.

For this reason, it is permissible to buy tools, plastic planters, seeds, fertilizer, potting soil, seeds produced in a permissible fashion, flowers grown in a permissible fashion, etc., and store them until after Rosh Hashanah.