Ek plant saad in die grond agter die huis.

Because saad can be used as a mass noun in Afrikaans, a bit like seed or seed material in English.

So Ek plant saad means something like:

• I am planting seed
• I am planting some seeds

If you wanted to be more specific, you could say:

• die saad = the seed / the seeds
• sade = seeds as a clear plural noun

So the version without an article sounds natural when talking generally about planting.

It is a verb here: to plant.

That can be confusing because English also has plant as a noun, and Afrikaans does too. In this sentence, though, Ek plant ... clearly means I plant / am planting ...

A useful thing to notice: in Afrikaans, the verb form usually stays the same with different subjects:

• Ek plant
• Jy plant
• Hy plant
• Ons plant

Unlike English, the verb does not usually change to plants with he/she/it.

Die is the Afrikaans word for the.

It is used for:

• singular nouns
• plural nouns
• all genders

So Afrikaans does not have separate words like the / der / die / das the way some other languages do.

In this sentence:

• die grond = the ground / the soil
• die huis = the house

Because in fits the idea of putting seeds into the soil.

• in die grond = in the ground / into the soil
• op die grond = on the ground

When you plant seeds, they normally go in the soil, not just on the surface.

Agter die huis means behind the house.

In this sentence, it most naturally tells you where the ground is or where the planting is happening:

• in die grond agter die huis = in the ground behind the house

So the phrase comes after grond, but in meaning it describes the location of the whole planting situation.

This is normal main-clause Afrikaans word order:

• Ek = subject
• plant = finite verb
• everything else follows

Afrikaans usually puts the finite verb in the second position in a main clause.

So:

• Ek plant saad in die grond agter die huis.

If you move another part to the front, the verb still stays second:

• Agter die huis plant ek saad in die grond.

That verb-second pattern is a very important feature of Afrikaans.

It is best understood here as a general or collective noun.

So it does not force you to think of exactly one seed or a clearly counted plural. It is more like:

• seed
• seed/seeds in general

If you want an explicit plural, you can use sade.

So:

• Ek plant saad = I plant seed / I am planting seeds
• Ek plant sade = I plant seeds

Grond is the normal word here because it means:

• ground
• soil
• earth in the sense of soil/land

That makes it the natural choice for gardening or planting.

Aarde often means:

• earth as the planet
• earth in a broader or more abstract sense

So for planting seeds, in die grond sounds much more natural than in die aarde.

Because Afrikaans verbs are much simpler than English verbs in the present tense.

Compare English:

• I plant
• he plants

In Afrikaans:

• ek plant
• hy plant

The verb usually stays the same. That is one reason Afrikaans verb grammar often feels easier for English speakers.

Yes. That would mean I am planting the seed / the seeds in the ground behind the house.

The difference is:

• saad = seed in a general or non-specific sense
• die saad = a specific seed or specific seeds already known from context

So both are grammatical, but they do not mean exactly the same thing.