Fine-Tuning Fertilizers to Boost Crop Yields: Lowering Fertilizer Ph Can Increase Solubility, Availability of Zinc

By Greg Basky

Worldwide, many agricultural soils are deficient in the nutrient zinc—despite the fact that farmers use fertilizers enriched with the element. This limits crop yields and reduces food quality. It's estimated that roughly a third of the global population consume foods low in zinc, which can increase sickness and death in early childhood, as well as impaired growth and cognition.

Using the Canadian Light Source (CLS) at the University of Saskatchewan (USask), researchers from the University of Adelaide in Australia investigated how to manufacture more efficient zinc-enriched fertilizers. The HXMA beamline at CLS enabled the researchers to examine at the molecular level what happens to the water solubility of zinc (its ability to dissolve in water) when it's added to ammonium phosphate fertilizer.

The findings are published in the Soil Science Society of America Journal.

"Going in to the project, our group thought the type of zinc compound would be a good predictor of a fertilizer's solubility" says Rodrigo da Silva, from the University of Adelaide.

"However, the CLS beamline enabled us to understand that the agronomic performance cannot be predicted based on what form of zinc is present in the fertilizer granules. Instead, the pH drives the fertilizer zinc solubility and availability to the crops."

Dr. da Silva and colleagues found that when zinc is added to phosphate fertilizer, it forms a range of different zinc phosphate compounds. However, its solubility was not related to the relative abundance of these compounds, but to fertilizer pH. This means that zinc added to more alkaline phosphate fertilizers such as diammonium phosphate will have very low solubility and hence low agronomic effectiveness for crop uptake.

The University of Adelaide team found that lowering fertilizer pH by spraying the granules with an acid solution further increases the solubility and availability of zinc compared to current commercial products.

The researchers also showed an additional method to increase Zn water solubility by putting a barrier coating on fertilizer granules, to separate the zinc from the phosphate. They showed that there was more uptake of zinc by plants treated with the barrier-coating fertilizer.