Urea
phosphate (UP) is an acidic compound fertilizer, which significantly
improves the low efficiency of P application caused by high pH in
saline-alkali soil. In this study, urea phosphate potassium (UPK)
was prepared by adding potassium chloride (KCl) to modify urea phosphate
(UP) and the optimal combination of the synthetic process parameters
was obtained using the response surface methodology at a four-variable,
three-level experiment Box–Behnken design. Parameters such
as the reaction temperature, KCl molar number, reaction time, and
concentration of phosphoric acid were included for optimization. The
thermostability, crystal structure, and microscopic morphology of
UPK were measured by thermogravimetric analysis (TGA), X-ray diffraction
(XRD), and scanning electron microscopy (SEM), respectively. The fertilizer
efficiency was validated in an experiment on maize grown in pots containing
saline-alkali soil. The highest K
2
O content and UPK yield
were obtained by using the following parameters: reaction time of
60 min, KCl of 0.32 mol, reaction temperature of 78 °C, and phosphoric
acid concentration of 70%. Under optimal conditions, the predicted
K
2
O value content and UPK yield were 3.51% and 69.8%, respectively.
The experimental K
2
O content and UPK yield were 3.42 ±
0.35% and 67.58 ± 1.25%, respectively, which confirmed the strength
of the predicted model. This model can be used as an effective tool
to predict the K
2
O content and yield in UPK. Characterizations
showed that KCl was uniformly distributed in UPK and its fluidity
was effectively improved as observed in the angle-of-repose results.
Compared to a conventional phosphorus fertilizer diammonium phosphate
(DAP), the yield, total P use efficiency, soil available phosphorus
content, and soil acid phosphatase activity of UPK increased significantly
by 25.58, 174.5, 24.41, and 41.25%, respectively, and the soil pH
on UPK treatments decreased by 3.98% significantly. In conclusion,
this novel technology to modify UP by using KCl has an enormous potential
for large-scale applications to satisfy the increasing demand for
UP fertilizers on saline-alkali soil.
