El humus de lombriz mejora el sistema de raíces.

In Spanish, it is very common to use the definite article with general nouns, especially when talking about something as a category or substance.

So El humus de lombriz means worm humus / worm castings / vermicompost as a general thing, not necessarily one specific bag of it in front of you.

English often leaves out the in these cases, but Spanish usually keeps it:

• El café es popular. = Coffee is popular.
• La leche es buena para... = Milk is good for...

So El humus de lombriz mejora... sounds natural in Spanish.

Literally, humus de lombriz means humus of worm or worm humus.

In natural English, this usually refers to:

• worm castings
• vermicompost
• worm humus

Here:

• humus = humus, compost-like organic matter
• de = of / from
• lombriz = worm, especially an earthworm

So de lombriz tells you what kind of humus it is: humus produced by worms.

Spanish often uses the singular after de when naming a type or category.

So humus de lombriz is like saying worm humus as a product name or class of material. The singular lombriz works almost like an adjective here.

Compare similar patterns:

• jugo de naranja = orange juice
• zapatos de cuero = leather shoes
• cepillo de dientes = toothbrush

Even if many worms are involved, the singular can still be used because it names the type, not the number of worms.

Because the subject is singular: El humus de lombriz.

The full subject is one unit:

• El humus de lombriz = singular

So the verb must also be singular:

• El humus de lombriz mejora... = Worm humus improves...

If the subject were plural, then you would use mejoran:

• Los fertilizantes mejoran el sistema de raíces.

A common learner mistake is to look at raíces and think the verb should be plural, but raíces is not the subject. It is part of the object: el sistema de raíces.

Mejora is the third-person singular present tense of mejorar.

It can mean:

• he/she improves
• it improves
• you improve (formal usted)

In this sentence it means it improves, referring to el humus de lombriz.

Conjugation of mejorar in the present:

• yo mejoro
• tú mejoras
• él/ella/usted mejora
• nosotros/nosotras mejoramos
• ustedes/ellos/ellas mejoran

El sistema de raíces means the root system, which is a more technical or complete expression than simply the roots.

Compare:

• las raíces = the roots
• el sistema de raíces = the root system

Spanish often uses this kind of noun phrase in scientific, agricultural, or formal language. It sounds more precise because it refers to the whole structure or network of roots, not just individual roots.

The singular is raíz and the plural is raíces.

The accent mark helps show the correct stress:

• ra-ÍZ
• ra-Í-ces

Without the accent, the pronunciation rules would suggest a different stress pattern. So the accent is necessary.

This is also a spelling change that happens in some words ending in -z:

• luz → luces
• voz → voces
• raíz → raíces

Notice that z changes to c before e in the plural.

In many contexts, lombriz can mean worm, but in gardening, composting, and agriculture it very often refers to an earthworm.

So in humus de lombriz, learners should usually understand lombriz as earthworm, not just any worm in general.

That is why this phrase is often associated with vermicompost and soil improvement.

Yes. Mejorar can work both ways.

In this sentence it is transitive, because it has a direct object:

• El humus de lombriz mejora el sistema de raíces.
• Worm humus improves the root system.

But it can also be intransitive:

• Las plantas mejoran con este producto.
• The plants improve with this product.

So:

• mejorar algo = to improve something
• mejorar = to get better / improve

Yes, that would be grammatically correct, but the meaning is slightly less specific.

• mejora las raíces = improves the roots
• mejora el sistema de raíces = improves the root system

The original sentence sounds more technical and precise. It is the kind of phrasing you might see on product packaging, in agriculture, or in gardening advice.

In standard Spanish pronunciation, the h is silent.

So humus is pronounced approximately like OO-moos.

A few notes:

• The h is not pronounced.
• The u sounds like oo.
• The stress is usually on the first syllable: HU-mus.

Also, be careful not to confuse Spanish humus with English pronunciations you may have heard for related words. In Spanish, just remember: silent h.

Yes. The present tense here is being used to state a general fact or typical effect.

So El humus de lombriz mejora el sistema de raíces means something like:

• Worm humus improves the root system
• Worm castings improve root development

This is a very common use of the Spanish present tense: expressing facts, habits, or general truths.