Pământul de flori este fertil.

Because Romanian usually puts the definite article at the end of the noun.

• pământ = soil, earth
• pământul = the soil, the earth

So pământul de flori is a definite noun phrase. In context, it means a specific or identified kind of soil. Romanian often uses this attached article where English would use the as a separate word.

It is a neuter noun.

That matters because Romanian neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular you get forms like:

• pământul
• fertil

Both follow the masculine/neuter singular pattern.

de flori describes the type or purpose of the soil.

In Romanian, de + noun is very common for this kind of modifier. It can show things like:

• purpose
• type
• category
• material

Here, de flori means something like for flowers or flower- as in flower soil / soil for flowers.

So pământul de flori is a fixed-type phrase, not just any soil that happens to be near flowers.

Because Romanian often uses the plural in expressions like this when speaking about a general category.

So de flori means soil intended for flowers in general, not just one flower.

This is similar to how English often uses a noun modifier without worrying about singular versus plural in the same way. Romanian simply uses the form that sounds natural in this construction.

Because de flori and florilor do different jobs.

• de flori = indicates type, purpose, or category
• florilor = genitive/dative plural, meaning something like of the flowers or to the flowers

So:

• pământ de flori = flower soil / soil for flowers
• pământul florilor = the soil of the flowers

The second version sounds more like the soil belonging to some flowers, not the normal label for this kind of product.

Because the adjective has to agree with pământul, and pământ is neuter singular.

In the singular, neuter nouns take the same adjective form as masculine singular nouns, so:

• masculine/neuter singular: fertil
• feminine singular: fertilă

That is why pământul de flori este fertil is correct.

este is the 3rd person singular of a fi, meaning is.

So it is the verb that links the subject to the adjective:

• Pământul de flori este fertil.
• The soil is fertile.

In normal Romanian, you usually do not omit this verb in a sentence like this.

In everyday speech, people often shorten este to e:

• Pământul de flori e fertil.

That is very common and natural.

The given order is the most neutral and standard one:

• Pământul de flori este fertil.

Romanian does allow some flexibility, but changing the order usually adds emphasis or makes the sentence sound more literary or marked.

For example, other orders are possible in special contexts, but the version you have is the one a learner should treat as the default.

A few helpful points:

• ă in pământul is like the a in about
• â in pământul has no exact English equivalent; it is a central Romanian vowel
• este is pronounced roughly YES-te
• fertil is roughly fer-TEEL

A beginner-friendly approximation of the whole sentence is:

puh-MUHN-tul de FLORI YES-te fer-TEEL

A couple of notes:

• the stress in pământul falls on mân
• the final i in flori is often very light in natural speech

Yes. pământ de flori is also possible, but it is less definite.

Compare:

• pământ de flori = flower soil / some flower soil
• pământul de flori = the flower soil / the soil for flowers

Which one sounds best depends on context. The sentence you were given uses the definite form, so it is talking about a specific soil or a known item/category in the conversation.