baiyoudo ni huyoudo wo mazeru.

• 培養土 = ばいようど
• に = に
• 腐葉土 = ふようど
• を = を (pronounced お in modern Japanese)
• 混ぜる = まぜる

So the whole sentence is read:

ばいようど に ふようど を まぜる。

Here, に marks the thing that something is being added to.

So in:

培養土に腐葉土を混ぜる

the 腐葉土 is being mixed into the 培養土.

A useful way to think about it is:

• A に B を 混ぜる = mix B into A

So に does not mean a general location here. It shows the target/base material.

を marks the direct object of the verb.

The verb 混ぜる means to mix. The thing being actively put in or mixed is 腐葉土, so that gets を.

Structure:

• 培養土に = into the potting soil
• 腐葉土を = leaf mold / leaf compost
• 混ぜる = mix

So the sentence is built like:

• mix 腐葉土 into 培養土

The particles tell you.

In this sentence:

• 培養土に = the base/target material
• 腐葉土を = the thing added

So it means:

• take 腐葉土
• mix it into 培養土

If the particles were reversed, the meaning would change.

Both can appear with 混ぜる, but they give slightly different nuances.

• A に B を混ぜる = mix B into A
  • This highlights a base material and something added to it.
• A と B を混ぜる = mix A and B together
  • This treats them more symmetrically.

So:

• 培養土に腐葉土を混ぜる = mix leaf mold into potting soil
• 培養土と腐葉土を混ぜる = mix potting soil and leaf mold together

In gardening instructions, に is very natural when one material is the main medium and the other is being added.

混ぜる is the plain/dictionary form of the verb.

That means it is the basic form you see in dictionaries. In a sentence like this, it often sounds like:

• a neutral statement
• a manual instruction
• a recipe/gardening instruction
• a heading or step in a process

In context, it can feel like Mix leaf mold into the potting soil.

Yes, the subject is omitted, which is very normal in Japanese.

Depending on context, this could mean:

• I mix leaf mold into the potting soil
• You should mix leaf mold into the potting soil
• Mix leaf mold into the potting soil
• Leaf mold is mixed into the potting soil (in a very loose instructional sense)

Japanese often leaves out subjects when they are obvious from context.

Yes. It is especially natural in instructions, labels, manuals, and notes.

Japanese often uses short plain-form sentences like this in practical contexts. For example, in gardening instructions, it is normal to see something concise like:

• 培養土に腐葉土を混ぜる。
• 水を入れる。
• 種をまく。

In conversational English, you might expect more wording, but Japanese often does not need it.

Yes, to some extent, because the particles show each word’s role.

For example, these are still understandable:

• 腐葉土を培養土に混ぜる。
• 培養土に腐葉土をよく混ぜる。

The most neutral order is often:

• place/context first
• object next
• verb last

So 培養土に腐葉土を混ぜる is very natural.

The important thing is that the verb usually stays at the end.

Correct: normal Japanese usually does not use spaces like this.

A standard written version would be:

培養土に腐葉土を混ぜる。

The spaced version is often used in learning materials to make the structure easier to see.

Yes. That would make it a more explicit polite instruction.

Compare:

• 培養土に腐葉土を混ぜる。
  plain, concise, instructional/manual style
• 培養土に腐葉土を混ぜてください。
  polite request: Please mix leaf mold into the potting soil

Both are correct; the version you have is just shorter and more matter-of-fact.

Yes. 混ぜる is transitive here, because it takes a direct object marked by を.

The direct object is:

• 腐葉土を

So the sentence tells you what is being mixed.

This is why を appears naturally with 混ぜる.

A very useful pattern is:

• A に B を 混ぜる

Meaning:

• mix B into A

In this sentence:

• A = 培養土
• B = 腐葉土

So:

• 培養土に腐葉土を混ぜる
• mix leaf mold into potting soil

This is a good pattern to remember because it works with many nouns.