State and retention characteristics of water in hydrogel of water-absorbent slow release nitrogen fertilizer

The availability of water retained in water-absorbent fertilizer depends on not only absorbency, but also on state and retention characteristics of water in water-absorbent fertilizer. In the article, Differential Scanning Calorimetry(DSC) and moisture characteristics curve were firstly applied to measure state and retention characteristics of water in Water-Absorbent Slow Release Nitrogen Fertilizer(WASRNF) synthesized in the laboratory. The findings indicate that free and bound water content increased with the increments of equilibrium water content(EWC) of WASRNF, over 94% of EWC in WASRNF is free and bound water which has thermodynamic properties being equal or approximate to that of bulk water, and is equivalent to the sum of gravitation, capillary water and film-like water in soil, 90% of water retained in WASRNF is available for plant use. Less than 5% of the total water retained in WASRNF is non-freezing water which is equivalent to water which is inferior to permanent wilting point of soil, and difficult for plant use. The available water content(94％) in WASRNF measured by DSC is very close to that(91%) of WASRNF measured by water characteristics curve. Thus, DSC is suggested to be a rapid method to estimate the availability of water in WASRNF. Water-holding capacity of soil increased and evaporation reduced and retention and release water increased, the availability of water and nutrients in soil improves while applying WASRNF to soil. The heat preservation time of composite solutions to be polymerized during synthesis of WASRNF is an important process parameter.