In this paper we describe a strategy for sustained release of nitrogen into the soil. Specifically, ureamodified hydroxyapatite nanoparticles were encapsulated under pressure into cavities of the soft wood of Gliricidia sepium. Nitrogen release of the nanofertilizer composition was studied using soil from three elevations in Sri Lanka (pH 4.2, 5.2 and 7) and compared with that of a commercial fertilizer. The nanofertilizer showed an initial burst and a subsequent slow-release even on day 60 compared to the commercial fertilizer,
which released heavily early followed by the release of low and non-uniform quantities until around day 30.

http://www.ias.ac.in/currsci/10jul2011/73.pdf