der Sand
sand
A fine, grainy material made up of rock particles, found on beaches, in deserts, or used as a building material.
Die Kinder spielen am Strand im Sand.
The children are playing in the sand at the beach.

sand
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sand
A fine, grainy material made up of rock particles, found on beaches, in deserts, or used as a building material.
Die Kinder spielen am Strand im Sand.
The children are playing in the sand at the beach.

The word 'Sand-' is a very productive first part of compound nouns. If you see 'Sand-' at the beginning of a word, it usually refers to the material, as in 'Sandburg' (sandcastle), 'Sandkasten' (sandbox), or 'Sandstrand' (sandy beach).
Im Sommer gehen wir oft zum Sandstrand an der Küste.
In summer, we often go to the sandy beach on the coast.
In its main meaning, 'Sand' (sand) is an uncountable noun (mass noun). When talking about the material in general, it is often used without an article. The plural 'die Sande' is very rare and is only used in technical language for different types of sand.
Pass auf, du hast Sand in den Schuhen.
Watch out, you have sand in your shoes.
The idiom 'Sand im Getriebe haben/sein' (to have/be sand in the gears) figuratively describes that a process or collaboration is not running smoothly. You can say 'Es ist Sand im Getriebe' (there is sand in the gears) or 'Wir haben Sand im Getriebe' (we have sand in the gears).
In der Kommunikation zwischen den Abteilungen ist momentan Sand im Getriebe.
At the moment, there is a hitch in the communication between the departments.
The idiom 'den Kopf in den Sand stecken' (to bury one's head in the sand) is an active action. The subject of the sentence is the person who consciously ignores a problem or danger instead of dealing with it.
Wir können nicht länger den Kopf in den Sand stecken und müssen über unsere Finanzen reden.
We can no longer bury our heads in the sand and have to talk about our finances.
Earth / soil
'die Erde' and 'der Sand' can both refer to material on the ground. That makes them easy to confuse when talking about a garden, beach, or construction site. 'Erde' is used more for dark soil in which plants grow. 'Sand' is finer and grainier and is found, for example, on the beach, in the desert, or in a sandbox. In practical terms: you put plants in 'Erde', and children often play in 'Sand'.

Nach dem Strandtag war überall Sand in meinen Schuhen.
After the day at the beach, there was sand everywhere in my shoes.
Für die Tomaten braucht Luisa frische Erde im großen Topf.
For the tomatoes, Luisa needs fresh soil in the large pot.