How is the human attempt to control a theme in this chapter? How is the sureness of uncertainty a theme?
You must be taking about Chapter Four: the Potato! The subtitle of this chapter, "Control," addresses the attempts by humans to utilize the potential of potatoes as a food crop by modifying them via genetic engineering. These methods can produce potatoes which are resistant to common pests, and thus make them more resilient and prolific, as well as more profitable.
But there is an historical lesson discussed in this chapter as well: Pollan explores the legacy of the Irish potato famine, which, ironically, was not caused because too few potatoes were grown, but because the ones that did grow were infected with blight, and caused illness and death in those who ate them. The lack of diversity in Ireland's agriculture was one of the factors that caused this to occur: the overwhelming dependency upon potatoes as a source of food led to widespread hunger and even starvation. By controlling the structure of agriculture to make it focused upon this one important crop, farmers created a precarious situation that led to disaster. But control is also inherent in the desire to create new strains of potatoes that wouldn't be vulnerable in this way.
Botanists used this event as a starting point to find ways to make the potato a more successful food crop, and to study other cultures where there was a lack of diversity in agriculture. The idea of a "sureness of uncertainty" refers, perhaps, to Pollan's finding the genetic modification of potatoes to be a "solution" that may be causing its own problems. Even if certain pests or diseases are banished, are we inviting a whole new set of problems by genetically modifying plants?
In this chapter, Pollan also explores the idea of control via the concept of human nature's desire to have Apollonian order; while nature prefers a sort of Dionysian chaos. So even as humans attempt to impose order, chaos may still be able to reign.