K přesazování potřebujeme nejen nový substrát, ale i drenážní vrstvu.

K přesazování means for repotting or when repotting.

The preposition k normally takes the dative case, and one of its common uses is to express purpose or relation to an activity. In practical or instructional language, Czech often uses k + verbal noun this way.

So:

• k přesazování = for repotting
• literally something like for the activity of repotting

In English, we would often use an infinitive or a gerund, but Czech very naturally uses this noun phrase.

It is not a finite verb here. It is a verbal noun: a noun made from a verb.

It comes from přesazovat / přesadit, meaning to repot or to transplant.

So:

• přesazovat = to be repotting / to repot repeatedly or as a process
• přesadit = to repot successfully / as a completed action
• přesazování = repotting, the act/process of repotting

This is very common in Czech. English often uses -ing forms where Czech uses a verbal noun like this.

Because this noun happens to have the same form in several cases.

The preposition k does require the dative, so the form here really is dative. But with neuter verbal nouns like přesazování, the dative singular looks the same as the dictionary form.

So although it looks unchanged, it is still grammatically correct:

• nominative: přesazování
• dative: k přesazování

This is one of those places where the case is shown by the preposition more than by an obvious ending.

Potřebujeme is the 1st person plural form of potřebovat = to need.

So it means:

• we need

The ending -me tells you it is we. Because Czech verb endings already show the subject, the pronoun my is usually left out unless you want emphasis.

So:

• potřebujeme = we need
• my potřebujeme = we need, with extra emphasis on we

Both are direct objects of potřebujeme, so both are in the accusative case.

But they look different because they belong to different noun genders and declension patterns.

1. nový substrát

• substrát is masculine inanimate
• in the accusative singular, masculine inanimate nouns often look the same as the nominative
• so substrát stays substrát
• the adjective is also nový

2. drenážní vrstvu

• vrstva is feminine
• its accusative singular changes from vrstva to vrstvu
• the adjective drenážní stays drenážní here

So the difference is not because one phrase is doing something different. Both are objects; they just decline differently.

This is a very common Czech structure meaning:

• not only ..., but also ...

So:

• nejen nový substrát, ale i drenážní vrstvu = not only new potting mix, but also a drainage layer

It links two similar sentence parts, here two things that are needed.

You will also see close variants such as:

• nejen ..., ale také ...
• nejen ..., ale i ...

They mean essentially the same thing.

Here i means also or too.

So:

• ale i = but also

It strengthens the second part of the pair after nejen.

Compare:

• nejen A, ale i B = not only A, but also B
• nejen A, ale také B = not only A, but also B

Both are natural. I is short and very common.

Because ale is a coordinating conjunction, and in Czech it is normally preceded by a comma.

That remains true in the pattern:

• nejen ..., ale i ...

So the comma in:

• K přesazování potřebujeme nejen nový substrát, ale i drenážní vrstvu.

is standard Czech punctuation.

In gardening:

• substrát usually means potting mix, growing medium, or soil substrate
• drenážní vrstva means drainage layer

So although substrát may look like the English word substrate, in this context it is the normal word for the material you plant into.

And drenážní vrstva is the layer at the bottom of the pot that helps water drain properly.

Czech word order is flexible, and the beginning of the sentence often gives the topic or context.

Starting with K přesazování sets the scene:

• As for repotting...
• For repotting...

Then the sentence tells you what is needed.

A different order is possible, for example:

• Potřebujeme k přesazování nejen nový substrát, ale i drenážní vrstvu.

That is also grammatical, but the original wording sounds very natural in instructions or advice because it introduces the activity first.

Yes, you could, but the nuance is a little different.

• k přesazování focuses on repotting as an activity/process
• k přesazení can sound more like for the act of repotting / for a completed repotting

In many everyday contexts, both are understandable. But k přesazování is especially natural when talking generally about what you need for the task.

So in an instruction like this, k přesazování is a very good choice.

Because Czech has no articles like English a, an, and the.

So:

• nový substrát can mean a new potting mix or the new potting mix, depending on context
• drenážní vrstvu can mean a drainage layer or the drainage layer

Czech relies on context, word order, and shared knowledge instead of articles.