Vi bytter plantejorden, ikke minst fordi sørgemyggen kommer tilbake.

Because Norwegian normally writes compound nouns as a single word.

• plante = plant
• jord = soil/earth

So plantejord is one compound noun, and plantejorden is its definite form.

This is very common in Norwegian:

• blomsterpotte = flowerpot
• hagejord = garden soil
• stueplante = houseplant

English often uses separate words where Norwegian uses one compound.

Plantejorden is the definite singular form: the potting soil / the plant soil.

That often sounds natural in Norwegian when you mean a specific, already understood soil, for example the soil in the plants you are talking about.

Compare:

• Vi bytter plantejord. = We are changing potting soil / some potting soil.
• Vi bytter plantejorden. = We are changing the potting soil.

Norwegian uses the definite form in situations where English might sound fine without the, so this can take some getting used to.

Yes, in Bokmål, plantejorda can also be correct.

That is because jord can be treated as a feminine noun, so you may see both:

• plantejorden
• plantejorda

Both are acceptable in Bokmål. The -en form is often more neutral/widely used in writing, while -a can sound a bit more colloquial or reflect a speaker’s style or dialect.

Here bytter means something like replace or change out.

The verb is å bytte, and in this sentence it means that the soil is being removed and new soil is being put in.

So in context:

• å bytte klær = change clothes
• å bytte plass = switch places
• å bytte plantejorden = replace the potting soil

A very natural English translation here is We’re replacing the potting soil.

Ikke minst is an idiomatic expression meaning not least, or more naturally in many cases, especially.

It highlights one important reason among several possible reasons.

So the sentence suggests:

• We are changing the soil for several reasons,
• and one especially important reason is that the fungus gnats keep coming back.

You can think of it as:

• ikke minst fordi ... = not least because ...
• or especially because ...

Because fordi starts a subordinate clause, and subordinate clauses normally keep the basic order:

subject + verb

So:

• sørgemyggen = subject
• kommer = verb
• tilbake = adverb/particle

That gives:

• fordi sørgemyggen kommer tilbake

A learner often expects some kind of inversion, but after fordi, you usually do not invert the verb and subject.

Compare:

• Main clause: Sørgemyggen kommer tilbake.
• Subordinate clause: ... fordi sørgemyggen kommer tilbake.

If you move the subordinate clause to the front, the following main clause shows normal Norwegian V2 word order:

• Fordi sørgemyggen kommer tilbake, bytter vi plantejorden.

It is the definite singular form of sørgemygg.

Structure:

• sørgemygg = fungus gnat
• sørgemyggen = the fungus gnat

It is also a compound noun:

• sørge
  • mygg

In real usage, the definite singular can refer to the pest problem in a general way, not just one single insect. That is similar to how languages sometimes use a singular noun for a whole type or species.

Possible forms:

• en sørgemygg = a fungus gnat
• sørgemyggen = the fungus gnat
• sørgemygg = fungus gnats
• sørgemyggene = the fungus gnats

Komme tilbake means come back or return.

It is a very common Norwegian verb expression:

• Han kommer tilbake i morgen. = He is coming back tomorrow.
• Problemet kommer tilbake. = The problem comes back / returns.

So in your sentence, sørgemyggen kommer tilbake means the fungus gnats return or keep reappearing.

The word tilbake is a separate word, just like back in English come back.

The comma is there because ikke minst fordi ... is being added as an extra explanatory comment.

Importantly, the comma is not there simply because of fordi. In Norwegian, a subordinate clause at the end of a sentence does not automatically need a comma.

So this sentence is punctuated that way because the writer is setting off the phrase ikke minst and the reason that follows as a kind of added emphasis:

• Vi bytter plantejorden, ikke minst fordi sørgemyggen kommer tilbake.

This is natural and clear Norwegian punctuation.

The present tense in Norwegian is often used for:

• something happening now
• something habitual or recurring
• something planned or decided

So here:

• Vi bytter plantejorden can mean We are replacing the potting soil
• sørgemyggen kommer tilbake can mean the fungus gnats keep coming back / are coming back

This sounds very natural in Norwegian. English often chooses a progressive form, but Norwegian uses the plain present tense much more often.

Yes, absolutely.

Bytte ut often makes the idea of replacing even clearer than plain bytte.

So these are both natural:

• Vi bytter plantejorden.
• Vi bytter ut plantejorden.

The version with ut can sound a bit more explicit, as in take the old one out and put a new one in.

You might also hear skifte in similar contexts, but bytte (ut) is very natural here.