Com més seca és la terra, més aigua necessita la planta.

It is the Catalan pattern for the more... the more... or the ...-er..., the more....

So:

• Com més seca és la terra = The drier the soil/earth is
• més aigua necessita la planta = the more water the plant needs

This is a very common structure in Catalan for showing a relationship between two things that increase together.

Because seca agrees with la terra.

• terra is a feminine singular noun
• the adjective must match it in gender and number
• so you get seca, not sec

Compare:

• el sòl sec = the dry soil masculine singular
• la terra seca = the dry earth/soil feminine singular

In the sentence, the adjective comes before the verb, but it still agrees with la terra.

Because in this kind of com més... structure, Catalan often puts the changing quality right after més.

So the sentence highlights seca first:

• Com més seca és la terra...

This is natural in Catalan. It is similar to how English says:

• The drier the soil is...

rather than:

• The soil is the drier...

So the word order is driven by the comparative pattern, not by a basic subject-verb-adjective order.

Because Catalan often uses the definite article where English uses a noun more generally.

So:

• la terra can mean the soil/earth in a general sense
• la planta can mean the plant as a general example, not necessarily one specific plant already mentioned

This is very normal in Catalan. English often leaves articles out in general statements, but Catalan usually does not.

Because més directly modifies the noun here.

• més aigua = more water

You do not need an article because aigua is being treated as an unspecified quantity of a mass noun.

You would only use an article if you meant a specific water already identified by context, which is not the case here.

Because the subject is la planta, which is singular.

• la planta necessita = the plant needs
• if it were plural, it would be les plantes necessiten = the plants need

So necessita agrees with la planta, not with aigua.

Yes, many speakers would naturally say:

• Com més seca està la terra, més aigua necessita la planta.

Why?

• estar seca often emphasizes a temporary condition or state
• ser seca can sound more general or descriptive

In a sentence like this, which expresses a general rule, és is understandable and grammatical, but està may sound more idiomatic to many speakers when talking about soil being dry at a given time.

Not really, at least not in the usual question sense.

In this sentence, com is part of the fixed structure com més... més.... You should learn it as a whole pattern meaning:

• the more... the more...
• the ...-er..., the more...

So it is better not to translate com by itself here.

The accent mark helps distinguish més from another word:

• més = more
• mes = month

So the accent is important for meaning, and it also reflects pronunciation and stress.

Yes. This version is a natural way to express a general relationship, but Catalan can also express the same idea in other ways, for example:

• Si la terra és més seca, la planta necessita més aigua.
• A mesura que la terra s’asseca, la planta necessita més aigua.

These are slightly different in tone:

• Com més... més... sounds like a general comparative rule
• Si... sounds more like a condition
• A mesura que... emphasizes a gradual process

So the original sentence is a very good model for this kind of the more... the more... idea.