It is because "blow" is a verb, and you need an adjective. "Blown" is a past participle, and that allows it to behave as an adjective. All past participles can behave as adjectives. Here are examples: "blown dust" "broken car" "poured water" "flown flag" Verbs cannot act as adjectives. An easy ways to turn a verb into an adjective is to use its past participle. You can also use its present participle: "blowing dust" "breaking car" "pouring water" "flying flag" "Dust, blowing from the land, contains nutrients that fertilize the surrounding waters."

To form a question ask ‘Why can’t I use …’ or just ‘Can I use ….?’ The ‘why’ is usually implied by the context. It’s short for ‘Dust that has been blown from the land contains nutrients …’ Similar would be ‘Dust blows from the land and contains nutrients …’ because it seems like the author is talking about a general relationship. But the original sounds better.

Because blown is the past participle as it’s the past tense you want to use here. The dust was blown. If it’s still being blown as you speak, you’d use blowing.