mizuyari no ato, ekitaihiryou wo tukau.

It is read:

みずやり の あと、えきたいひりょう を つかう。

More naturally without spaces:

水やりの後、液体肥料を使う。
みずやりのあと、えきたいひりょうをつかう。

水やり means watering, especially watering plants.

It comes from:

• 水 = water
• やり = giving / doing

In this expression, it does not just mean water by itself. It means the act of giving water to plants.

So 水やりの後 means after watering.

Here, 後 is a noun meaning after / later / the time after.

Japanese often connects one noun to another with の. So:

• 水やり = watering
• 後 = after
• 水やりの後 = the time after watering / after watering

This is a very common pattern:

• 食事の後 = after the meal
• 仕事の後 = after work
• 運動の後 = after exercise

Both readings exist, but あと is the normal, natural reading in everyday sentences like this.

• 後（あと） is common in ordinary speech and writing.
• 後（のち） is more formal/literary in many cases, or used in certain fixed expressions.

So in this sentence, みずやりのあと is what most learners should expect.

In full sentences, you will often see:

• 水やりの後に、液体肥料を使う。
• 水やりの後で、液体肥料を使う。

Both are natural.

In short instructions, notes, labels, or concise written style, Japanese often drops particles that are understood from context. So:

• 水やりの後、液体肥料を使う。

sounds like a brief instruction: After watering, use liquid fertilizer.

So nothing is wrong here; it is just a compact style.

液体肥料 is read:

えきたいひりょう

It means liquid fertilizer.

Breakdown:

• 液体 = liquid
• 肥料 = fertilizer

Together they form one noun: liquid fertilizer.

を marks the direct object of the verb 使う.

So in:

• 液体肥料を使う

the thing being used is 液体肥料.

That means:

• 液体肥料 = liquid fertilizer
• を = marks it as the object
• 使う = use

So: use liquid fertilizer

使う is the plain/dictionary form of the verb to use.

Japanese often uses the plain form in:

• instructions
• manuals
• labels
• general statements
• short written guidance

So this sentence has a neutral, concise instructional feel.

If you wanted a more polite version, you could say:

• 水やりの後、液体肥料を使います。

If you wanted a direct instruction to someone, you might say:

• 水やりの後、液体肥料を使ってください。

Japanese often leaves out the subject when it is obvious from context.

In this kind of sentence, the understood subject is usually something like:

• you
• one
• the gardener / plant owner
• or just a general instruction with no explicit subject

So the sentence works like an instruction: After watering, use liquid fertilizer.

Yes. That is very natural too.

For example:

• 水をやった後、液体肥料を使う。
• 水やりした後、液体肥料を使う。

These versions focus more explicitly on the action after having watered.

The original 水やりの後 uses a noun phrase, which is short and common in instructional writing.

The comma marks a pause and makes the structure easier to read.

It separates:

• the time expression: 水やりの後
• the main action: 液体肥料を使う

So it works like:

After watering, use liquid fertilizer.

Japanese commas are often used quite freely, especially to show a pause or make a sentence clearer.

Not exactly a strong command. It is more like a neutral instruction or procedural statement.

Compare:

• 水やりの後、液体肥料を使う。 = concise instruction / general rule
• 水やりの後、液体肥料を使ってください。 = please use liquid fertilizer after watering
• 水やりの後、液体肥料を使いなさい。 = use liquid fertilizer after watering (stronger command)

So the original sounds matter-of-fact rather than forceful.

Usually, yes: 水やり is a very common way to write it.

You may also sometimes see related expressions like:

• 水をやる = to water plants
• 水やりをする = to do watering

But 水やり by itself is a standard noun meaning watering.

For learners, it is useful to remember that Japanese often turns common actions into noun-like expressions like this, especially in practical contexts such as gardening instructions.