Siembra was the second of four collaborations between trombonist/bandleader Willie Colón and singer/songwriter Rubén Blades, and an artistic peak for both men. Colón’s complex and multifaceted arrangements (the album opener, “Plástico,” begins as string-laden disco before becoming a funky salsa anthem) frame Blades’s incisive lyrics, which are full of social critique and vivid character portraits, as on “Pedro Navaja,” inspired by Bertolt Brecht’s “Threepenny Opera.” The production is astonishing; every sound, from the trombones to the tiniest percussion instrument, gets its own space in what’s almost a surround-sound mix. “María Lionza” is particularly beautiful, opening with booming low-end piano chords and adding lushly romantic horns, castanets, and a male chorus. But there are also moments of silly fun, like the vocalist’s “mouth solo,” which sounds like he’s making cartoon sound effects, on “Buscando Guayaba.”