How do I plant plants in sand? Do I just burry the roots underneath the sand? Will it grow that way? Do aquarium plants need a special fertilizer to keep growing? Can I place the plants in tiny pots and then burry the pot in sand?
I'd love to help you but to do so I need to know what plants you have. Some plants do best when floating, others like to be placed on drift wood or rocks, still others like to be planted, each type will require different types of fertilizer.
If the plants are rooted you can plant them in the sand just like any other substrate. I would put plant tabs by each plant and use Flourish supplement ( The way it says in the directions on the bottle. If the plants are rhizome plants like Java fern and Anubias then do not plant these plants and bury them in the sand just plant the roots and leave the rhizome above the substrate. Or leave these plants float. (With the Java fern you could yse a plant weight to hold it in place on the top of the sand and let the roots grow into the sand on their own.
And yes you could plant the potted plants leaving them in their pots but I really don't recommend it. The plants roots in a pot have to grow and expand just like a flower or a plant you would plant out side.
With the right water parameters, PH, lighting, and Ferts You will find the plants do very well in the sand.
All of my tanks have sand and are heavily planted. The plants thrive with the right care.
Hornwort you do not want to plant as it has no real viable roots....I found that out the hard way. Plant the roots only on the Anubias and Java Fern; not the rhizomes. I either tie mine down, float or hold down with plant weights and let them self-root.
I like my stem plants like Anacharis, Hornwort, etc., in bunches so I use plant weights instead of planting individually.
Make sure you turn the Marimo Moss Balls on occasion or they can rot. They are not actually plants but a specific form of algae.
Do not plant the crown (where the leaves emerge) of the plant. I use planting tongs, push the plant into the sand and then gently lift until the crown is level with or above the substrate. I believe most plant substrate fertilizer directions say to plant ferts in a grid. Usually the directions will instruct how far apart, etc.
The exception to the above, at least for me, is a Sword. They are very heavy feeders.
I am using and like Niloc fertilizers. Cost a bit more but they are complete so you don't need trace elements like to you with other general ferts.