SEATTLE — The last time Bill Gates played an active role at Microsoft, as chief software architect, he witnessed the company muffing its earliest efforts to become a major player in search, smartphones and tablet computers.

In the six years since then, he has watched as the technology industry changed without him. The personal computer era that Microsoft so ably dominated during and after Mr. Gates’s heyday as Microsoft’s chief executive has started to fade away. Now, the focus is on social media and mobile devices that run in large part on cloud computing.

And what technologies has Mr. Gates, who turned to philanthropy, been most vocal about during all that change? Things like self-contained toilets and vaccines.

“I am a little obsessed with fertilizer,” reads the first sentence of a column he wrote for a recent issue of Wired, the technology magazine.