A debris-free, narrow-bandwidth, single-line laser-plasma soft x-ray source has been developed suitable for water-window microscopy using zone-plate optics. The 10 Hz table-top source utilizes microscopic ammonium hydroxide droplets as target. The N VI emission at λ=2.88 nm is ∼1×1012 photons/(sr pulse). As an alternative to ammonium hydroxide, it is demonstrated that urea dissolved in water also produces strong emission from nitrogen ions, thereby extending the applicability of the droplet x-ray source to a new range of target materials.