jīntiān búyòng shīféi.

This is because of tone sandhi.

• 不 is normally fourth tone: bù
• But when 不 comes before another fourth-tone syllable, it changes to second tone

Since 用 yòng is fourth tone, 不用 is pronounced:

• búyòng

This is a pronunciation change, not a meaning change.

Here, 不用 + verb means there is no need to ... or you don’t need to ...

So 不用施肥 means:

• no need to fertilize
• don’t need to apply fertilizer

In other contexts, 不用 can also mean do not use, but in this sentence it clearly means no need to, because it is followed by the action 施肥.

In Mandarin, time words often come near the beginning of the sentence, before the main action.

So:

• 今天不用施肥。
• literally: Today, no need to fertilize.

This is very natural in Chinese. A common pattern is:

• time + verb phrase

You can think of 今天 as setting the time frame first.

Mandarin often omits the subject when it is already clear from context.

So 今天不用施肥 could mean:

• You don’t need to fertilize today
• We don’t need to fertilize today
• There’s no need to fertilize today

The exact subject depends on the situation. Chinese often sounds more natural than English when the subject is left unstated.

施肥 means to apply fertilizer or to fertilize.

It is made up of:

• 施 = to apply / carry out
• 肥 = fertilizer / manure

So 施肥 is a fairly standard, somewhat formal verb used in farming, gardening, and plant care.

In everyday speech, people may also say things like:

• 给植物施肥 = apply fertilizer to plants
• 给花施肥 = fertilize the flowers

In this sentence, 施肥 is a verb phrase meaning to fertilize.

So in:

• 今天不用施肥

不用 works like don’t need to, and 施肥 is the action that follows.

Chinese words can sometimes behave flexibly depending on context, but here it is clearly verbal.

These are important differences:

• 不用施肥 = no need to fertilize
• 不要施肥 = don’t fertilize
• 不能施肥 = must not / cannot fertilize

So:

1. 不用 focuses on lack of necessity

   • It isn’t required.

2. 不要 is usually a command or instruction

   • Someone is telling you not to do it.

3. 不能 means it is not allowed, not possible, or not appropriate

   • There is some restriction or impossibility.

So 今天不用施肥 is softer than 今天不要施肥. It means fertilizing today is unnecessary, not forbidden.

Yes.

• 不用施肥。 = No need to fertilize.

That is also correct. Adding 今天 simply makes the time specific:

• 今天不用施肥。 = No need to fertilize today.

Yes, but the most natural placement is near the beginning.

Common patterns include:

• 今天不用施肥。
• 你今天不用施肥。

Both are natural.

The version with a subject makes the target clearer:

• 你今天不用施肥。 = You don’t need to fertilize today.

But putting 今天 after 不用, as in 不用今天施肥, would usually sound unnatural unless you are contrasting times in a special way.

The spaces are only there to help learners.

Normal Chinese writing is usually written without spaces:

• 今天不用施肥。

Teaching materials often add spaces or pinyin to show word boundaries more clearly:

• 今天　不用　施肥。

So the standard written sentence is still:

• 今天不用施肥。

It is generally neutral, but 施肥 itself is a bit more technical or practical because it is used in agriculture, gardening, or plant care.

So the whole sentence sounds natural in contexts like:

• farming
• gardening
• plant instructions
• care schedules

It is not unusually formal, but it is more specific than everyday verbs used for ordinary actions like eating or going.

A useful way to see it is:

• 时间 + 不用 + 动作

In this sentence:

• 今天 = time
• 不用 = no need to
• 施肥 = action

So the full pattern is:

• 今天 + 不用 + 施肥
• Today + no need to + fertilize

This is a very common Chinese structure. You can make similar sentences like:

• 今天不用浇水。 = No need to water today.
• 明天不用上班。 = No need to work tomorrow.
• 现在不用担心。 = No need to worry now.