There is an introductory article to the series found here
What does it mean to be EARTH?
Earth monsters are thematically often sturdy, hidden, or mundane. Sturdy monsters can be shown to have high defense or be resistant to being killed by battle. A card like Fortress Warrior would not feel right in any other attribute. The hidden aspect often comes as flip monsters or monsters similar to flip monsters such as Medusa Worm. Then there's the mundane. The vast majority of earth monsters have been given the attribute as a way to show they aren't special. Kind of depressing really.
Double any battle damage your opponent doesn't take when they attack this monster
Defensive
So the first mechanical manifestation of what it means to be earth is to be better on the defensive than on the offensive. We see this with cards like Stone Statue of the Aztecs. It asks you to set it and punish your opponent for attacking it. Not only is this horribly dated, but being reactive rather than proactive has never been good. You want to be able to win despite your opponent doing practically nothing. This is accomplished by having a proactive gameplan, anything you do to get towards winning. This is the main reason why defensive cards rarely work out in the meta.
Konami have tried to fix this while keeping the theming. The main example is Superheavy Samurais that use their high defense in place of their attack. This makes for some odd interactions with cards meant to protect you becoming usable to go on the offensive. This has not become a real issue though since Superheavy Samurais have never been meta. That, combined with their inherent xenophobia, keeps them in check.
FLIP
The FLIP mechanic is more prevalent in earth than any other attribute. The flavor of hidden or buried monsters works and it also plays into being defensive. However, just like I mentioned with defensive cards, FLIP monsters are inherently reactive. Once again Konami has recognized this is a problem and given FLIP archetypes ways to circumvent the issue. The two main ways is to let you FLIP them with card effects and/or punishing your opponent for removing them by other means.
Flipping them with card effects may sound like a good idea, but it kind of just erases the identity of them being FLIP monsters anymore since they become essentially normal effects. In Subterrors, this has led to The Hidden City becoming mandatory to get anywhere with the archetype. This in turn leads to the effects needing to be balanced around being used on your own first turn as well, putting us back to square one. Making them float means your opponent gets caught between a rock and a hard place when it comes to dealing with your board. Krawlers and perhaps more notably Shaddolls make it so neither battle nor card effects are efficient ways to remove your board. I feel like this is the way to go when designing FLIP effects for the modern game since it keeps the integral identity of the mechanic while mitigating one of its more notable weaknesses.
Mundane
Hmm... No flavor
Okay, this is cheating. Being mundane is not a mechanic, but it does have mechanical impact. Marauding Captain is an earth monster because he is just an army man. He is not a heroic paladin. His sword is just made out of steel, not flames or the souls of the damned. That makes him earth attribute. But how do you create a compelling support card for someone who is just some guy?
This has led to there being next to no good generic earth support since there just isn't anything fun you can do with the flavor. Designing the "Steel Sword of Some Guy" equip card isn't as fun as designing Salamandra. For cards that are mundane, being in specific zones of play doesn't mean anything. Water cards can be salvaged from the graveyard. A sacrifice of dark monsters can be The Beginning of the End. Light monsters can have their honesty rewarded with protection from a guardian angel descending from the hand. Earth monsters don't really have this. The field is just the field and the graveyard is just where monsters go when they are destroyed.
Throwaway Attribute
Finally, Earth often gets used as a sort of filler attribute. This means that many monsters made Earth are that way because it doesn't conflict with their typing. Marauding Captain is a warrior and that's all he is. So as to not "distract" from that design, Konami made him Earth. Big Koala is a huge beast but the flavour isn't that it was corrupted and grew more dangerous as a result like Mad Dog of Darkness or a time traveling dog that fights evil like Ryko, Lightsworn Hunter. He is just a really big koala! The focus lies solely on him being big and a koala. Inherent to his design is that he's a beast, not earth.
This doesn't really have an easy fix other than sitting down and forcing yourself to design some decent earth support, even if it is harder than designing flavorful cards for other attributes. I really hope we can see some of this in the future, since mixing and matching semi-generic support has always been a fun aspect of the game in my opinion.
Join me next week for a deep dive into the water attribute!