La composta mejora el sustrato para macetas.

Because composta is a feminine singular noun, so it normally takes the feminine singular article la.

• la composta = the compost
• una composta = a compost / some compost in context

In Spanish, nouns usually need an article more often than in English, especially when talking about something in a general or specific way.

Yes. Composta means compost.

A useful note for Latin American Spanish:

• la composta is common in many places
• You may also hear el compost in some regions or contexts

So if you learned compost elsewhere, that is not necessarily wrong, but la composta is a very natural Latin American form.

Because the subject is la composta, which is singular.

• la composta mejora = the compost improves
• las compostas mejoran = the composts improve

The verb has to agree with the subject in number:

• singular subject → mejora
• plural subject → mejoran

Here, la composta is doing the action, so the singular form mejora is required.

Mejora is the third-person singular present tense of mejorar.

That form can mean:

• he/she improves
• it improves
• you improve (formal *usted)*

In this sentence, it means it improves, because the subject is la composta.

Conjugation of mejorar in the present:

• yo mejoro
• tú mejoras
• él/ella/usted mejora
• nosotros/nosotras mejoramos
• ellos/ellas/ustedes mejoran

Not exactly. Sustrato often means growing medium or substrate, especially in gardening.

It is not always the same as ordinary ground soil. In plant and gardening contexts, sustrato can refer to the material plants grow in, especially in containers, and that material may include things like:

• peat
• compost
• coco coir
• perlite
• bark
• other blended materials

So sustrato is often more specific than English soil.

Here para means for.

So:

• sustrato para macetas = potting substrate / substrate for pots

This phrase tells you what kind of substrate it is. It is substrate intended for use in pots.

This is a very common Spanish pattern:

• agua para plantas = water for plants
• tierra para jardín = soil for the garden
• abono para flores = fertilizer for flowers

Because the phrase is talking about potting material for pots in general, not just one specific pot.

• para macetas = for pots / for potted plants
• para una maceta = for one pot

Spanish often uses the plural in this kind of general category expression, just like English often says:

• running shoes
• garden tools
• potting soil

So sustrato para macetas is a standard way to refer to the kind of substrate used in pots.

Sometimes, yes, but the meaning would shift a little.

• tierra usually means earth, dirt, soil
• sustrato is more technical or gardening-specific, often meaning growing medium

If you say:

• La composta mejora la tierra para macetas

it may sound understandable, but sustrato para macetas is often more precise if you mean a prepared potting mix rather than just plain soil.

So sustrato is a good word choice in gardening language.

It describes sustrato, not the verb.

The structure is:

• La composta = subject
• mejora = verb
• el sustrato para macetas = direct object

Within that object:

• el sustrato = the main noun
• para macetas = a phrase modifying sustrato

So the sentence means that compost improves the potting substrate, not that it improves something for pots as a separate adverbial idea.

Spanish often uses articles where English does not, so the original sentence is already a very natural way to say that general idea.

• La composta mejora el sustrato para macetas.

Even if English says Compost improves potting substrate, Spanish commonly keeps the articles:

• la composta
• el sustrato

If you remove them:

• Composta mejora sustrato para macetas

that sounds unnatural in standard Spanish.

So one important lesson is: do not copy English article usage directly into Spanish.

A simple Latin American approximation is:

• la composta → lah kom-POHS-tah

Syllables:

• com-pos-ta

Stress:

• comPOSta
  The stress falls on pos.

A few sound notes:

• c before o sounds like k
• o is a pure vowel, not a diphthong like English sometimes makes it
• a is a clear ah sound

So try to keep the vowels short and clean:

• kom-POHS-tah