die Aussaat
sowing
The process of sowing; the spreading of seeds in fields or gardens.
Im Frühling beginnt für die Landwirte die Zeit der Aussaat.
In spring, the sowing season begins for farmers.

sowing / seed
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sowing
The process of sowing; the spreading of seeds in fields or gardens.
Im Frühling beginnt für die Landwirte die Zeit der Aussaat.
In spring, the sowing season begins for farmers.

'Die Aussaat' (sowing/seed) is the noun form of the separable verb 'aussäen' (to sow). It describes both the action of sowing and the material, meaning the seeds themselves.
Die Bauern säen im Frühjahr den Weizen aus.
The farmers sow the wheat in spring.
You can tell whether the action ('das Säen' — sowing) or the material ('die Samen' — the seeds) is meant from the verb. Verbs like 'beginnen' (to begin) or 'vorbereiten' (to prepare) refer to the action, while verbs like 'kaufen' (to buy) or 'lagern' (to store) refer to the seed.
Die Aussaat muss vor dem nächsten Regen abgeschlossen sein.
The sowing must be completed before the next rain.
'Aussaat' belongs to the word family around 'säen' (the verb) and 'die Saat' (the seed). These words are also often used figuratively, for example when someone 'sows' an idea or when a plan 'comes up' like seed.
Mit seinen kritischen Fragen hat er den Keim des Zweifels gesät.
With his critical questions, he sowed the seed of doubt.
order / reservation
'die Bestellung' and 'die Aussaat' can both have to do with work in the field in agriculture. 'Aussaat' refers specifically to the moment or process when seeds are put into the ground. 'Bestellung' is broader: it can include working the soil and preparing the field, so it means more than just sowing. If you are only talking about spreading the seeds, 'Aussaat' is more precise. If it is about all the field work before and during sowing, 'Bestellung' fits better.

Wegen des starken Regens musste die Aussaat von Weizen verschoben werden.
Because of the heavy rain, the sowing of wheat had to be postponed.
Die Bestellung der Felder beginnt im Frühling, sobald der Boden trocken genug ist.
The cultivation of the fields begins in spring as soon as the soil is dry enough.