Is every word that ends in -нный a past passive participle?

For example the word сезонный meaning seasonal? It has the double -нн ending that the past passive participles have but for the life of me I can't figure out which verb it comes from....

Another one that has me confused is сельскохозяйственный meaning argicultural. Is this a past passive participle? If so what verb is it made from?

Or are both of these just adjectives with a double -нн ending?

I'm totally confused because I though all words with the -нный endings were past passive participles but in these two cases I can't figure out what verb they would have come from.

Thanks for any help!