Al's gritty mix IS creating your own potting mix, it's just that the recipe is already there. If you are curious why a lot of people use it, read the "Container soils - Water movement & retention" thread on this forum (click the link). It actually is quite interesting!

Here is a link that might be useful: click me!

Congratulations on your choice of studies. And working in the industry will be a learning experience of at least as much value as your studies. On the job training may often seem to be just a job, doing the same thing over and over. The real value is the experienced people you work with. Take full advantage of the knowledgeable people you work with.
As for the "gritty mix" its value is its longevity, and simplicity,try it, you will like it. Al

Isn't it easier to just create your own potting mix?

Yes it is although it isn't as easy as buying a brand named bagged soil a costume mix soil is more reliable than a name brand bagged soil used in container cultivation

why spend money on it?
Dollar for dollar, pound for pound, square foot for square foot most of us find the better value to be a custom mix

AGM in it's three forms provide less margin of error basic soils providing exelent drainage with good to very good moisture retention and allows very good air for root growth for many plant types.

AGM in all three recipes and parts can also be compared to many other soil types from Bosnia soil to Cati mix. You may find AGM to hold the same ingredients as the compared soils making AGM a very good read for the beginner to adventurous gardener.

I think fertilizer programs for container plants would be a separate question but is also explained by many on the forums including fert choices to some times good debates all I can add is it's true we do need to feed our plants in containers.

Hi pastlife,

Zone 14, where are you studying?
Lot's of different ways to construct mixes.
For myself I've focused on only using inputs from my yard:
leftover mix, leaves, and logs.
It has worked quite well in my upstairs deck. Not a long term mix though.

The link below shows some of my experiments with mixes.

http://lowcostvegetablegarden.blogspot.com

Here is a link that might be useful: Stump and branch pot comparison