الرطوبة في التربة.

A careful pronunciation is:

ar-ruṭūbah fī at-turbah

A few notes:

• ر in الرطوبة makes the article sound like ar-, not al-
• ت in التربة makes the article sound like at-
• ط in رطوبة is an emphatic t sound, heavier than ordinary ت
• In normal pause, the final ة is usually heard as -a(h)

If you read full case endings in very formal MSA, it can be:

ar-ruṭūbatu fī at-turbati

Because both ر and ت are sun letters.

In Arabic, when الـ comes before a sun letter, the l sound assimilates to the next consonant.

So:

• الرطوبة is pronounced ar-ruṭūbah
• التربة is pronounced at-turbah

In writing, الـ stays the same, but the pronunciation changes.

في means in.

So the structure is:

• الرطوبة = the moisture
• في = in
• التربة = the soil

Together: the moisture in the soil

Grammatically, في is a preposition, so the noun after it is put in the genitive case.

After the preposition في, the following noun must be genitive.

So التربة is majrūr (genitive).

In fully vowelled formal Arabic, you would see:

في التربةِ

and in connected formal reading:

fī at-turbati

In everyday printed Arabic, the short vowel ending is usually not written, so you just see التربة.

Because both nouns are definite here:

• الرطوبة = the moisture
• التربة = the soil

Arabic often uses the definite article in places where English also uses the, but sometimes Arabic uses definiteness more broadly for general or known concepts.

So this phrase can mean something like:

• the moisture in the soil
• or, depending on context, soil moisture in a general sense

No, not in this form.

This phrase uses a preposition:

• الرطوبة في التربة = the moisture in the soil

An iḍāfa version would be:

• رطوبة التربة = soil moisture or the soil’s moisture

So the difference is:

• الرطوبة في التربة focuses on moisture located in the soil
• رطوبة التربة is a tighter noun-noun relationship, like soil moisture

Both are possible, but they are structured differently.

It can be understood either way, depending on context.

Most commonly by itself, it is read as a noun phrase:

• the moisture in the soil

But Arabic can also allow a nominal sentence with a prepositional phrase as the predicate, so in context it could also mean:

• The moisture is in the soil

If this appears as a heading, label, or topic title, the noun phrase reading is the most natural.

That letter is called tāʾ marbūṭa.

It often marks a feminine noun.

So:

• رطوبة is feminine
• تربة is feminine

In pronunciation:

• in pause, it usually sounds like -a
• when grammatical endings are pronounced, or in certain constructions, the t sound can appear

For example:

• paused: ruṭūbah
• with a full ending: ruṭūbatu

Yes, both are normally treated as feminine nouns, largely because they end in ة.

That matters for agreement with adjectives and pronouns.

For example:

• التربة الرطبة = the moist soil
• هذه التربة = this soil

The adjective or demonstrative must match the noun in gender, number, and definiteness.

They are related, but not identical.

• التربة usually means soil as a layer or growing medium
• التراب often means dust, earth, or dirt

So for agriculture, gardening, or science, التربة is usually the better word for soil.

That is why الرطوبة في التربة sounds natural for discussing soil moisture.