По сравнению с кактусом фикус требует больше воды и удобрения.

По сравнению с is a fixed expression meaning compared with or in comparison with.

It is built from:

• по
• сравнению — the dative singular of сравнение (comparison)
• с — with

So literally, it is something like by comparison with. In normal English, you would just translate it as compared with/to.

Because after по сравнению с, the noun after с goes in the instrumental case.

So:

• кактус → nominative
• кактусом → instrumental

That is why you get с кактусом here.

Literally, yes, с often means with, but in this sentence it is part of the fixed expression по сравнению с.

So you should understand the whole phrase together:

• по сравнению с кактусом = compared with a cactus

It is not saying that the ficus is together with the cactus in any physical sense.

Because фикус is the subject of the sentence.

It is the thing doing the verbal action of requiring, so it stays in the nominative case:

• фикус требует... = the ficus requires...

Требует is the 3rd person singular present tense form of требовать (to require).

So:

• я требую = I require
• ты требуешь = you require
• он / она / фикус требует = he / she / the ficus requires

It matches фикус, which is singular.

Because требовать normally takes the genitive case in standard Russian:

• требовать чего? = to require what?

So:

• вода → воды
• удобрение → удобрения

Also, больше usually goes with the genitive too:

• больше воды = more water

So the genitive here makes perfect sense.

Here it is best understood as genitive singular of удобрение.

That gives the meaning fertilizer as a substance or material, like English water or fertilizer as mass nouns.

So:

• удобрение = fertilizer
• удобрения = of fertilizer / fertilizer (in genitive singular)

Russian often treats things like this as substances rather than countable individual items.

In this sentence, it is usually understood as applying to both:

• more water and fertilizer
• in natural English, often more water and more fertilizer

Russian often allows one quantity word to cover a small list like this.

If you wanted to make it completely explicit, you could say:

• фикус требует больше воды и больше удобрения

But the original sentence is normal.

Because по сравнению с кактусом is a short introductory comparison phrase, and in Russian such short adverbial phrases are often written without a comma.

So:

• По сравнению с кактусом фикус требует...

looks natural and standard.

A comma is not needed here.

Yes. Russian word order is much more flexible than English word order.

For example, you could also say:

• Фикус по сравнению с кактусом требует больше воды и удобрения.

The original version puts the comparison first, which highlights the contrast from the start:

• Compared with a cactus, ...

So the chosen word order is natural and helps set the frame of comparison.

Because Russian has no articles.

So:

• фикус can mean a ficus or the ficus
• кактусом can mean with a cactus or with the cactus, depending on context

Russian leaves that distinction to context, while English usually has to make it explicit.

Yes, в сравнении с also exists and can mean something similar, but по сравнению с is the more common and standard neutral expression for compared with.

So for most learners, по сравнению с is the main phrase to remember.