Plantejorden er fuktig.

In Norwegian, the definite article is usually added to the end of the noun.

So:

• en plantejord = a potting soil / a planting soil
• plantejorden = the potting soil

This is very different from English, where the comes before the noun.

In this sentence, plantejorden is the definite singular form.

Plantejorden is a compound noun:

• plante = plant
• jord = soil / earth

Together, plantejord means something like potting soil or planting soil, depending on context.

Then the ending -en makes it definite:

• plantejord = potting soil
• plantejorden = the potting soil

Compound nouns are very common in Norwegian.

Because plantejord is a common gender noun, also called an en-word.

So the pattern is:

• en plantejord = a potting soil
• plantejorden = the potting soil

If it were a neuter noun (et-word), the definite form would usually end in -et instead.

Er means is / are / am, depending on the subject. It is the present tense of å være = to be.

In this sentence:

• Plantejorden er fuktig = The potting soil is moist

Some useful forms of å være:

• er = am / is / are
• var = was / were
• vært = been

Here fuktig is a predicate adjective, because it comes after er.

In Norwegian, predicate adjectives often stay in their basic form with a singular common-gender noun:

• Plantejorden er fuktig = The potting soil is moist

If the subject were a singular neuter noun, you would often add -t:

• Huset er fuktigt would look logical by pattern, but in actual Norwegian the correct form is Huset er fuktig? No—this adjective is a bit special in usage, and many speakers still use fuktig in practice. However, the general rule for predicate adjectives is that neuter singular often takes -t.

The most important thing for this sentence is simply:

• with plantejorden, fuktig is correct

Not exactly.

• fuktig = moist / damp
• våt = wet

So fuktig suggests some moisture, but not soaking wet. For soil, fuktig is often a very natural choice.

Compare:

• Jorden er fuktig = The soil is moist
• Jorden er våt = The soil is wet

Yes, but the meaning changes.

• Plantejorden er fuktig = The potting soil is moist
  (a specific soil you are talking about)

• Plantejord er fuktig = Potting soil is moist
  (more general, like talking about the material in general)

So the version with -en is definite and specific.

This sentence follows the basic Norwegian pattern:

• Subject + verb + adjective
• Plantejorden + er + fuktig

That is very similar to English:

• The potting soil + is + moist

Norwegian is a verb-second language, so in many sentences the finite verb comes early. In this simple statement, the subject comes first, then the verb.

A rough guide is:

• Plantejorden ≈ PLAHN-teh-yor-den
• er ≈ air or a short ehr, depending on accent
• fuktig ≈ FOOK-tig or FOOK-tihg

A few helpful notes:

• j in Norwegian is usually like English y
• jord sounds roughly like yor
• u in fuktig is not exactly like English oo, but oo is a decent approximation for beginners

Pronunciation varies somewhat by region, so don’t worry if you hear slightly different versions.

Yes. It is a completely natural sentence.

It could be used when talking about:

• gardening
• houseplants
• watering plants
• checking whether soil needs more water

For example:

• Plantejorden er fuktig, så du trenger ikke vanne ennå.
  The potting soil is moist, so you don’t need to water yet.

So both the grammar and the vocabulary are very normal.