Slow Release Characteristics of Crystal Green Struvite Quantified in Situ Using X-Ray Computed Tomography.

Struvite (magnesium ammonium phosphate hexahydrate) can be used as a slow release fertilizer containing plant-available phosphorus, magnesium and nitrogen. Crystal Green® is the trade name for a granular struvite fertilizer produced using the patented Pearl® process for nutrient recovery; the recovered Crystal Green (CG) granules are naturally slow release and have no coating or additives. The combination of low water solubility and high citrate solubility results in unique release characteristics that can be challenging to asses using standard methods. Time resolved, non-destructive, 3D micro-focus X-ray computer tomography (CT) imaging was used to visualize, assess and quantify the interactions of CG with plant roots in situ. Crystal Green and triple superphosphate (TSP) were used in 3 treatments where the equivalent of 80 kg P₂O₅/ha was provided by (1) CG only, (2) TSP only, and (3) CG + TSP (50:50). The fertilizer granules were placed in large soil filled tubes, which were seeded with spring wheat. All other nutrients were supplied by a modified Hoagland solution. Weekly CT scans of the roots (60 µm spatial resolution) were taken over 12 weeks. Roots came within several millimeters of the fertilizer granules, close enough for root hairs to access diffused P from the fertilizer. The influence of the soil and roots was quantified by measuring volume changes of the individual fertilizer granules over time. CG granules showed negligible volume change in the absence of roots, however, measureable volume loss was seen after roots grew close to the granules.  This type of “plant activated” release could have positive implications for phosphorus use efficiency and for reducing the environmental risks associated with phosphorus fertilizer applications.