Há uma relação entre adubo e colheita.

In this sentence, há is the verb haver used in an impersonal way to mean there is or there are.

So:

• Há uma relação... = There is a relationship...

It does not mean English has here.

This use of haver is very common in Portuguese, especially in more standard language. In European Portuguese, há is a very normal way to say that something exists.

Examples:

• Há um problema. = There is a problem.
• Há muitas pessoas aqui. = There are many people here.

Notice that Portuguese uses há where English uses there is / there are.

Because the sentence is expressing existence, not identity or location.

• há = there is / there are
• é = is (identity, definition)
• está = is (location or temporary state)

So:

• Há uma relação entre adubo e colheita. = There is a relationship between fertilizer and harvest.

You would not use é here, because you are not saying what something is.
You would not use está, because you are not saying where something is.

The accent helps distinguish há from a.

Compare:

• há = there is / there are; also ago in time expressions
• a = to / at / the feminine article in some contexts

Examples:

• Há uma relação. = There is a relationship.
• Daqui a uma semana. = In a week.

For a learner, it is useful to treat há as a separate word with its own meaning, not just as a with an accent added randomly.

Yes. Relação is a feminine noun, so it takes the feminine article uma.

• uma relação = a relationship

If the noun were masculine, you would use um instead.

Examples:

• um problema
• uma relação

A lot of Portuguese nouns ending in -ção are feminine:

• a relação
• a informação
• a situação
• a decisão

So this is a very useful pattern to remember.

Because here they are being used in a general sense.

• entre adubo e colheita = between fertilizer and harvest

This sounds like a general statement about the connection between the two ideas.

If you wanted to talk about specific things, articles could be used:

• Há uma relação entre o adubo e a colheita.
  = There is a relationship between the fertilizer and the harvest.

That sounds more specific: a particular fertilizer and a particular harvest.

So:

• no articles = more general / conceptual
• with articles = more specific

Entre ... e ... means between ... and ....

In your sentence:

• entre adubo e colheita = between fertilizer and harvest

This is a very common structure.

Examples:

• entre Lisboa e Porto = between Lisbon and Porto
• entre o trabalho e a família = between work and family
• entre duas opções = between two options

So the pattern is:

• entre X e Y

Yes. That is perfectly possible.

• Há uma relação...
• Existe uma relação...

Both are correct and natural.

Very roughly:

• há is the usual existential form: there is
• existe can sound slightly more explicit, like exists

In many contexts they are interchangeable.

Examples:

• Há uma solução.
• Existe uma solução.

Both mean There is a solution.

In this sentence, há is probably the most neutral and natural choice.

Yes, and this is important: when haver means there is / there are, it stays in the singular.

So you say:

• Há uma relação. = There is a relationship.
• Há várias relações. = There are several relationships.

Not:

• Hão várias relações for this meaning

This is a common point for learners, because English changes from there is to there are, but Portuguese uses há for both.

A key point is the lh.

In European Portuguese, lh is a palatal sound, similar to the lli sound in some pronunciations of English million, though not exactly the same.

So:

• colheita

The tricky part is:

• lh = a soft ly-like sound, not a normal English l
• ei usually sounds like ay in say (but in European Portuguese it is often shorter and less open than in English)

A rough learner-friendly approximation might be:

• colh-AY-ta

But remember this is only approximate.

Also, European Portuguese often reduces unstressed vowels, so the real pronunciation can sound more compressed than you might expect from the spelling.

A useful approximation is:

• a-DU-bo

The stress is on the du syllable.

In European Portuguese, unstressed vowels are often reduced, so the first and last vowels may sound weaker than an English speaker expects.

So even though it is written clearly as a-du-bo, the spoken form in Portugal may sound more compressed.

This is the most natural neutral order.

• Há uma relação entre adubo e colheita.

Portuguese does allow some variation, but changing the order often changes the emphasis or makes the sentence sound less neutral.

For example:

• Entre adubo e colheita, há uma relação.

This is possible, but it gives extra emphasis to entre adubo e colheita.

For a learner, the original order is the best pattern to remember:

• Há + noun + entre X e Y

Often yes, but it depends on context.

Adubo is a general word for fertilizer or manure-like material used to help plants grow.

Depending on context, Portuguese may also use:

• fertilizante = fertilizer
• adubo = fertilizer / manure / plant feed, often the more everyday word

So in an agricultural or general-growing context, adubo is very natural.

Colheita most commonly means harvest or the act/result of harvesting.

Depending on context, it can refer to:

• the harvest itself
• the season or process of gathering crops
• the yield collected

So in this sentence, colheita is best understood as harvest.

It is not usually the exact same thing as crop in every context, although English translations can overlap sometimes.

For a learner, the safest basic meaning is:

• colheita = harvest