The taking of heads has a pretty deep history in the Andes - "trophy heads" are seen in Chavin, Paracas, Moche and Wari iconography, as well as in the Nasca as you've mentioned in your question. Additionally the Inca would take skulls of defeated warriors (especially generals) and use them as chicha-drinking vessels.

David Proulx mentions the trophy heads in A Sourcebook of Nasca Ceramic Iconography (2006), as part of his discussion of many ceramic icons. He notes that many of the depictions of trophy head preparation for some form of ritual adornment show plants sprouting from the mouths of trophy heads. Also, among the contemporary Moche, there are elaborate depictions of decapitation, with the sacrificed individuals' blood collected in a cup for a priest to drink. Apparently (and he doesn't note where he got this interpretation) the Moche ritual was for agricultural fertility.

From all of this, I suppose I could stretch his argument to say that the Nasca expanded their territory through military campaigns with neighboring groups, took the heads of the defeated warriors, and in their sacrifice gained control over the new territory and promoted its future agricultural success. But I'm not really happy with that, and I'm betting that's not satisfactory to you either. So let's dig in a little deeper!

One of Proulx's figures (with the trophy heads) sticks out to me. There's been a lot of good work done on identifying plants in Andean iconography, especially by Patricia Knobloch. I would be curious if someone hasn't already identified some of these plants sprouting from the trophy heads' mouths; they may be agriculturally productive or hallucinogenic, reflecting the altered state of the shaman in the figure. I say this is a shaman and not a deity because it looks like he's wearing a jaguar "effigy", sashed on his back, to which the trophy heads are attached. I'm seeing a few parallels to jaguar effigies in depicted Moche Coca-Taking rites (Uceda, after McClelland, in Bourget and Jones 2008), which actually have archaeological examples (Bourget, in Bourget and Jones 2008) recovered. Jaguars in the Andes have been identified as liminal figures of power, moving between planes of earth and sky - often giving them associations with favorable (or unfavorable) weather. So obliquely I suppose they could be associated with agricultural success.

Honestly, all of this interpretation and discussion should tell you that while we've associated the trophy heads to rituals, and they're associated with plant icons, we aren't really sure what it is about the ritual that connects them to agricultural fertility. Moreover the plants could even be hallucinogenic, not agricultural. I wish I could give you a better answer; this is really just my thoughts from the modest reading I've done on the area.

References Cited (especially for images):

Proulx, David. 2006. A Sourcebook of Nasca Ceramic Iconography. University of Iowa Press.

Bourget, Steve, and Kimberly Jones. 2008. The Art and Archaeology of the Moche. University of Texas Press.