El sustrato está húmedo.

Because Spanish usually uses estar for a state or condition, and húmedo here describes the current condition of the substrate.

So:

• El sustrato está húmedo = the substrate is damp/moist right now.
• El sustrato es húmedo would sound more like you are describing it as inherently or characteristically moist, which is much less natural in this context.

For things like temperature, physical condition, wetness, dryness, cleanliness, etc., Spanish often uses estar.

El is the masculine singular definite article, meaning the.

So:

• el sustrato = the substrate

Yes, in this sentence you normally do need it, unless the noun has already been introduced in a structure where Spanish allows omission. In a simple standalone sentence, El sustrato está húmedo is the natural form.

Because húmedo has to agree with sustrato, and sustrato is a masculine singular noun.

Agreement works like this:

• masculine singular: húmedo
• feminine singular: húmeda
• masculine plural: húmedos
• feminine plural: húmedas

Examples:

• El sustrato está húmedo.
• La tierra está húmeda.
• Los sustratos están húmedos.

Sustrato is a fairly specific word. In everyday practical contexts, especially gardening or plants, it usually refers to a growing medium or substrate rather than ordinary soil in the broadest sense.

Depending on context, it could mean:

• potting mix
• growing medium
• substrate
• base layer/material

If you are talking about garden soil in a simpler everyday way, Spanish often uses words like tierra or suelo instead.

So sustrato sounds more technical or specialized than just soil.

A rough pronunciation for an English speaker is:

el soos-TRAH-toh es-TA OO-meh-doh

A few key points:

• h is silent in Spanish, so húmedo starts with an oo sound.
• está is stressed on the last syllable: es-TA
• húmedo is stressed on the first syllable: OO-meh-doh
• In Spain, the s sound is clear and crisp, and the vowels are usually pure and short.

They have accent marks for different reasons.

• está has a written accent because the stress falls on the last syllable, and Spanish spelling rules require the accent mark there.
• húmedo has a written accent because the stress falls on the third-to-last syllable: HÚ-me-do. Words stressed that far back always need an accent mark in Spanish.

The accent marks show stress, not a different vowel quality like in English.

Normally, no. That sounds incomplete or unnatural in standard Spanish.

You would usually say:

• El sustrato está húmedo.

Spanish often uses articles with nouns where English might sometimes be more flexible. Unless you are using sustrato in a special headline, note, label, or telegraphic style, the article is expected.

Not exactly.

• húmedo = moist, damp
• mojado = wet, often more strongly wet

For plant substrate, húmedo is usually the better choice if you mean it contains moisture but is not soaked.

Compare:

• El sustrato está húmedo. = The substrate is moist/damp.
• El sustrato está mojado. = The substrate is wet.

So mojado can sound stronger than húmedo.

The plural is:

Los sustratos están húmedos.

Everything changes to plural:

• el → los
• sustrato → sustratos
• está → están
• húmedo → húmedos

Spanish is very consistent about this kind of agreement.

Yes, but El sustrato está húmedo is the most neutral and natural order.

You might also see:

• Está húmedo el sustrato.

This can sound more marked, contrastive, or context-dependent, as if you are emphasizing the condition first.

For a learner, the normal order to use is:

subject + estar + adjective

So stick with El sustrato está húmedo.

Yes. It is a perfectly normal, complete sentence.

It has:

• a subject: El sustrato
• a verb: está
• a complement/adjective: húmedo

It is simple, natural, and exactly the kind of sentence you would use in gardening, plant care, agriculture, or technical descriptions.