Abstract:
The work describes a novel strategy for controlled and sustained release of plant nutrients
nitrogen (N), phosphorous (P) and potassium (K) into soil. In the study two nano systems,
(a) inorganic inner nano-core consisting of macronutrient nanoparticles (b) a natural
cellulose based outer core containing micro / nano porous cavities were used in order to
obtain slow and sustained release of nutrients. Hydroxyapatite (HA) nanoparticles were
synthesized, surface modified using urea and characterized using PXRD, SEM / EDX, AFM,
FTIR and TGA / DTA. Urea modified HA nanoparticles dispersion and saturated potassium
chloride solution were separately pressurized into the cavities present in Gliricidia sepium, a
soft wood stem, under a pressure of 9 bar. N, P and K release behavior ofthe nanofertilizer
composition was studied using soil from three elevations in Sri Lanka (pH 4.2, 5.2 and 7)
and the release properties were compared with that of a commercial fertilizer composition.
The release properties ofthe nanofertilizer show a slow and sustained release. In general, at
all pH values, even on day 32, the remaining N to be released in the nanofertilizer was about
20% and there was no more N to be released in the commercial fertilizer. A similar trend
shows for K release and at all pH values the remaining K to be released in nanofertilizer
after 40 days was about 15% while there was no more K to be released in the commercial
fertilizer. The solubility of P of in the nanofertilizer system was higher than the P release of
the commercial fertilizer.
A model was developed for the solubility of bulk HA / HA nanoparticles. The solubility of
HA in terms ofsoluble phosphate (P043') can be expressed by an equation,
3
4 7 10-if
vjcw;
p0*3"L 5
total
2.503Kfl2+fx0.509Xv7] 8
3
5*e
From pH 3 to 4 and pH 4.5 to 6 the decrement of log value of total soluble phosphate
concentration per unit pH is 11.8 and 3.03, respectively, when the ionic strength of the soil
solution is between 12 to 110 mol kg'1 according to the Ostwald and Freundlich equation, the
solubility of phosphate can be increased by reducing the size of HA, that is, by using HA
nanoparticles.
