How did Paul 'sterilize' ninety planets, and why?

Question

It is mentioned in Dune Messiah that Muad'Dib's jihad resulted in the "sterilization" of a number of planets.

“Very good, Stil.” Paul glanced at the reels in Korba’s hands. Korba stood with them as though he wished he could drop them and flee. “Statistics: at a conservative estimate, I’ve killed sixty-one billion, sterilized ninety planets, completely demoralized five hundred others. I’ve wiped out the followers of forty religions which had existed since—”

The question of why his followers felt the need to wage a Jihad is addressed here but I'd like to know how this particular feat (sterlising planets) was accomplished and what these poor souls did to deserve such treatment.

Answer 1

The how is addressed in Paul of Dune. When Muad'dib wishes to destroy a world utterly, he waves his ringed hand and his armies make it happen using all the tools at their disposal. This would appear to include conventional explosives, chemical and biological weapons , but not nuclear weapons.

A Heighliner carried one hundred of the largest and most powerful Atreides vessels, each loaded to capacity with weapons, explosives, highly toxic chemical bombs, defoliants, and wide-dispersal incendiaries.

Paul had never given such a frightening command before: Sterilize the world. Memnon Thorvald's people had to be more than defeated, more than exterminated. They must be... gone.

The Atreides ships gave no warning, engaged in no negotiations, gave no quarter to the people of Ipyr. They switched off all but their battle communications systems, so no one would hear the wails of terror, the cries for mercy or, afterward, the resounding silence. The heavily armed vessels circled down, calling up charts of every single planetary settlement, and the annihilation began.

_{Paul of Dune on Google Books}

Answer 2

Valorum's answer based on the non-Frank Herbert novels is certainly correct. However, I would suggest that it points out one of the many flaws in those sequel/prequels: they take things mentioned in the core books way too literally.

I doubt FH imagined Paul literally giving the order to sterilize a planet (this would be out of character even for Leto, who is much more bloody-minded). I take the passage mentioning sterilized planets to refer to Paul reflecting ruefully that the damage done by his Fremen legions pushed these planets into ecological collapse so complete they became sterile (imagine the destruction of critical terraforming equipment, or the global winter resulting from continent-wide uncontrolled fires). This would be in keeping with FH's overarching concern for ecological themes-- but admittedly does not have other textual support in the books.