Mi metas sterkon en la substraton.

They have different reasons:

• sterkon has -n because it is the direct object of metas. It is the thing being put.
• substraton has -n because en can take the accusative to show movement into something.

So:

• Mi metas sterkon = I put manure/fertilizer
• en la substraton = into the substrate

If there were no movement, you would use en la substrato instead.

This is a very common Esperanto pattern:

• en la substrato = in the substrate / inside it, with no idea of movement
• en la substraton = into the substrate, showing movement from outside to inside

So in this sentence, en la substraton is used because something is being moved into the substrate.

Compare:

• La sterko estas en la substrato. = The manure/fertilizer is in the substrate.
• Mi metas sterkon en la substraton. = I put manure/fertilizer into the substrate.

Because al means to, toward, but it does not mean into.

• al la substrato would mean movement toward the substrate
• en la substraton means movement into the inside of the substrate

Since the idea here is that the manure/fertilizer is being mixed or placed inside the growing medium, en la substraton is the natural choice.

metas is the present-tense form of meti, which means to put, to place, or sometimes to set.

So:

• meti = to put
• mi metas = I put / I am putting

Esperanto present tense can cover both simple present and present progressive, depending on context.

Yes. Esperanto does not build the present progressive the way English does.

• Mi metas can mean:
  • I put
  • I am putting

The exact nuance comes from context. Esperanto usually does not need a separate word for am in this kind of construction.

la is the definite article, meaning the.

So la substraton means the substrate — a specific substrate that the speaker and listener can identify from context.

Without la, substraton would mean a substrate or substrate in a less specific sense.

Esperanto has no indefinite article. There is no separate word for a or an.

So:

• sterko can mean manure, fertilizer, or some manure/fertilizer, depending on context
• la sterko means the manure/fertilizer

That is why sterkon appears without any article.

sterko usually means manure or fertilizer, depending on context.

In practical use, it can refer to:

• animal manure
• fertilizer more generally
• a substance used to enrich soil or growing medium

So the exact English word depends on the situation. If the meaning has already been given to the learner, that tells you which sense is intended here.

substrato is a fairly technical or specialized word. It means substrate — the material or medium in which something grows or rests.

In gardening or plant-related contexts, it often means a growing medium, not necessarily ordinary soil.

It is an international-style word, so it looks similar to English substrate.

Yes, that is possible.

• meti = to put
• enmeti = to put in / insert

So Mi enmetas sterkon en la substraton emphasizes the idea of putting something in. However, Mi metas sterkon en la substraton is already perfectly correct and natural.

Esperanto often allows both:

• a basic verb plus a prepositional phrase
• a prefixed verb with a similar meaning

Yes. Esperanto word order is more flexible than English because the endings show the grammatical roles.

The neutral order is:

• Mi metas sterkon en la substraton.

But you could also say things like:

• Sterkon mi metas en la substraton.
• En la substraton mi metas sterkon.

These alternatives change the emphasis, not the basic meaning.

You know from the combination of grammar and meaning:

• sterkon is the direct object of metas
• en la substraton is a prepositional phrase showing direction

Even though both words end in -n, their roles are different because one stands alone as the object, while the other is introduced by en.

So the sentence structure tells you:

• thing being moved: sterkon
• destination: en la substraton

It could be either. Mi metas does not by itself force one interpretation.

Depending on context, it can mean:

• I am putting manure/fertilizer into the substrate right now
• I put manure/fertilizer into the substrate as a regular practice

If you need to make the timing clearer, context or extra words can help.