Dissolution rates of mineral nitrogen fertilisers : effects of moisture and precipitation

Sammanfattning: Hygroscopicity and deliquescence refer to the ability of a solid to absorb air moisture in which it dissolves and have long been recognised as properties having negative impacts on the product quality of explosives, pharmaceuticals, and fertilisers. Contrastingly, this thesis reconsiders and reappraises the hygroscopic effects of common mineral fertilisers, proposing hygroscopicity to beneficially enable quick dissolution under humid air conditions in absence of precipitation. Although precision fertilisation has improved, little is known about the dissolution dynamics of mineral nitrogen fertilisers in the field when exposed to either moist air or precipitation. In this thesis, dissolution rates of fertilisers based on ammonium nitrate (Axan™) and calcium nitrate (Kalksalpeter™), and of comparable pure salts, were recorded in a hygroscopicity test (≥90 % relative humidity at 25°C). Furthermore, dissolution rates of the fertilisers were also studied in a rain simulation (moderate intensity). In the hygroscopicity test, the pure reference salts had a higher hygroscopicity and shorter dissolution time relative the fertiliser products. The ammonium nitrate-based fertiliser and the pure ammonium nitrate salt moreover had higher dissolution rates (completely dissolved after 5.0 and 2.3 hours) compared to the corresponding calcium nitrate fertiliser and salt (21.1 and 4.3 hours respectively). All compounds tested dissolved within 24 hours. Endothermic properties, i.e. heat uptake during dissolution, seemed strongly correlated to the hygroscopic rates recorded. According to a thermodynamic calculation made, strongly endothermic salts had higher hygroscopicity compared to those with moderate or weak endothermic properties. In the rain simulation, the ammonium nitrate and calcium nitrate-based fertilisers required 5 and 6.5 mm of simulated precipitation, respectively. However, a theoretic estimation suggested less water to be sufficient at optimal application. For highly soluble fertilisers, the time during which the fertiliser is exposed to water may be more important from a dissolution point of view than the solubility. Considering the dissolution rates of fertilisers reported in this work, regardless if driven by hygroscopicity or precipitation, the limiting factor in the fertilisation system does not seem to be the dissolution but the dilution to concentrations non-toxic for crops. Future research should focus on the transport of dissolved nitrogen in the soil, at different soil moisture levels. Thereby, the time for a dry solid fertiliser to dissolve and reach the root zone of a crop could be better predicted.