Why are my plants turning brownish

[maxxal22]

hiya, my plants have been turning brownish lately. Not sure what is wrong, as there is a layer of nutrient soil present underneath the sand layer.

Thanks for the help

Link to the photos (they were too large to upload from my phone)

https://drive.google.com/drive/folders/1--Ky1FqNqFIreaslnuvlypH1YjORV9aj