Zinc Diffusion and Availability Affected by Different Sources in Soils of Contrasting Textures

Trends in new fertilizer technologies should balance the nutrient release rate from fertilizers with plant demands over time, while developing suitable physical characteristics of the fertilizer’s particles. The objective of this study was to evaluate the performance of three zinc fertilizers [ZnO, ZnSO4, and a commercial co-granulated ZnO+S0 fertilizer (ES_Zn)] on Zn diffusion in soil and their agronomic performances. A Petri dish trial was carried out in order to evaluate the diffusion of Zn in the soil. The experiment was designed as a factorial scheme (3 + 1) × 2 × 3, with three Zn sources, one control treatment (without Zn application), two soils of contrasting textures (sandy and clay), and three incubation times (1, 30 and 60 d). The experiment was carried out under a completely randomized design with four replications. Zinc diffusion was assessed according to the method proposed by Degryse et al. (2015) along of incubation times. For that, a ZnSO4 solution or ZnO suspension was applied by pipetting 15 μL of solution or suspension into a small hole (~0.5-cm deep) in the center of the Petri dish. A single pastille of ES_Zn fertilizer (30±0.5 mg) was placed in the center of the Petri dish, at the same depth. Soil was watered to 80% of field capacity. Filter papers (Whatman) were impregnated with CaCO3 and placed on the soil surface. After 2 h of reaction, the CaCO3-impregnated filter papers were collected, and the precipitated Zn in the papers was colored with dithizone, giving a pink color. The performance of Zn sources was evaluated in a greenhouse through a successive maize-soybean-millet crop. The trial was designed as a 2 × (3 × 3 +1) factorial scheme, being two soils (sandy and clay), three Zn sources (ZnSO4, ZnO, and ES_Zn), three Zn doses (1.5, 3.0, and 6.0 mg dm-3 Zn), and a control treatment. The experiment was a randomized block design with four replications, being the experimental unit composed of a pot with 4 dm3 of soil. Pastille ES_Zn, ZnO (as suspension), and ZnSO4 (as solution) were applied at five equidistant points, at 5 cm below the soil surface. After 30, 60 and 60 days of planting, shoot of maize soybean and millet were harvest, oven-dried at 70 °C for 72 h (until constant weight), weighed and milled for chemical analysis. ES_Zn fertilizer promoted a delay Zn release in the soil, being effective as a fertilizer only in the last crop (millet), as well as ZnO. Zinc oxide and ZnSO4 had similar performances for increasing Zn availability in the inner soil portion, but its diffusion in soil was superior when the source was sulfate. The highly soluble ZnSO4 was more effective than ZnO-based fertilizers in terms of plant nutrition, especially for the two first crops. Our results also suggest that ZnO is solubilized in soil at high pH (6.6), its dispersion in soil being a key factor for the dissolution rate.