Vi vanner mer etter hvert som plantejorden blir tørr av undervanning.

Etter hvert som is a fixed expression meaning as, while, or gradually as.

It shows that two things develop alongside each other over time. In this sentence, the idea is:

• Vi vanner mer = we water more
• etter hvert som ... = as / as it happens that ...
• plantejorden blir tørr = the potting soil becomes dry

So this is not a literal after expression. You should learn etter hvert som as a set phrase.

No. Here som is not a relative pronoun.

In etter hvert som, som is part of the conjunction itself. The whole expression introduces a subordinate clause:

• etter hvert som plantejorden blir tørr av undervanning

So in this sentence, som does not mean who, which, or that.

Because Norwegian usually writes compound nouns as a single word.

• plante = plant
• jord = soil
• plantejord = plant soil / potting soil

Then the definite ending is added to the last part:

• plantejord = potting soil
• plantejorden = the potting soil

This one-word spelling is very normal in Norwegian.

Norwegian usually shows definiteness with an ending on the noun rather than a separate word in front.

So:

• jord = soil
• jorden = the soil
• plantejord = potting soil
• plantejorden = the potting soil

That is why you do not need a separate word like English the here.

Blir means becomes / gets, so it shows a change.

• blir tørr = becomes dry
• er tørr = is dry

So plantejorden blir tørr means the soil is changing into a dry state. If you said plantejorden er tørr, you would simply be describing its state, not the change.

Because plantejorden is not a neuter noun, so the adjective stays in the basic singular form tørr.

Compare:

• jorden blir tørr = the soil becomes dry
• gulvet blir tørt = the floor becomes dry

So tørt would be used with a neuter noun, but plantejorden takes tørr.

Here av means something like from, because of, or due to.

• undervanning = underwatering / insufficient watering

So:

• tørr av undervanning = dry because it has been watered too little

This is a cause expression: the soil becomes dry because of not getting enough water.

Vanner is the present tense of å vanne = to water.

Norwegian present tense can cover both:

• a general/habitual action: we water more
• an action happening now: we are watering more

The exact meaning depends on context. Norwegian does not normally need a special continuous form like English are watering.

Mer means more, and here it modifies the verb phrase vanner.

So:

• Vi vanner mer = We water more / We do more watering

It is talking about the amount or degree of watering, not describing the soil.

Normally, no.

In modern Norwegian, you usually do not put a comma before a subordinate clause when it comes after the main clause:

• Vi vanner mer etter hvert som plantejorden blir tørr av undervanning.

If the subordinate clause comes first, you normally put a comma after it:

• Etter hvert som plantejorden blir tørr av undervanning, vanner vi mer.

So the sentence as written is punctuated normally.