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High N-NH3 losses are expected when conventional urea is applied to the soil surface. In order to reduce it, urea granules could be coated with different materials to decrease fertilizer dissolution rate or to stabilize N-NH4+ by acidification. In this study, we investigated the effect of a polymer-coated urea and powdered S0 added to urea, in the presence or absence of a S-oxidizing bacterium (Acidithiobacillus thiooxidans), on soil pH, SO42- availability, NH4+, and NH3 volatilization. Applying S0 before urea and the inoculation with bacteria have promoted the highest S0 oxidation rates. The greater decrease in soil pH occurred when S0 was applied before urea at a higher dose, which also decreased NH3 volatilization by 83% up to 4 days after urea application. However, the decrease in soil pH did not increase the concentration of NH4+, nor did it decrease the accumulated amount of volatilized NH3 over time. The inoculation of A. thiooxidans accelerates S0 oxidation process, but it was insufficient to counteract the H+ consumption by urea hydrolysis. Therefore, the S0 application with urea did not offer chemical protection against NH3 loss, but a physical barrier in the controlled-release urea had less dissolved urea in soil and reduced NH3 losses.

Irrigation water with high electrical conductivity (EC) compromises the sustainability of agricultural soils. Calcium sulfate (CS) or gypsum is commonly used on removal of soluble ions such as sodium (Na), however, large applications of CS can affect soil pH, EC, and nutrient availability to plants. The objective of this study was to investigate the effects of CS and ammonium sulfate (AS) rates on the soil pH, EC, and exchangeable cations in a salt-affected agricultural soil. Samples from the 0-20 cm soil depth layer were collected from an agricultural soil reported to have low potato yield due to high EC irrigation water. Soil was incubated with rates ranging from 0 to 4000 kg ha-1 of CS and 0 to 600 kg ha-1 of nitrogen (N) using AS. The treated soil was incubated for 60 d at 25 ºC and moisture was maintained at 60% of soil field capacity. After incubation, the soil was analyzed for pH, EC, Na, manganese (Mn), and zinc (Zn). Increasing rates of CS resulted in a small decrease in soil pH and a significant linear increase in soil EC, while the application of AS linearly reduced the soil pH and quadratically increased soil EC. The application rate of 200 kg ha-1 of N as AS resulted in a decrease of soil pH from 5.9 to 5.2, while the EC increased from 1.3 to 3.0 dS m-1. Extractable Na increased linearly with the application of AS due to its effect on the soil pH. The soil extractable Mn and Zn were not affected by the application of CS. Applications of AS resulted in a linear increase in soil extractable Mn and Zn concentrations, respectively. Results from this incubation study suggest that the use of large rates of CS for consecutive years may further impair soil conditions for cropping in areas with high EC in the irrigation water.

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.

The efficiency of phosphate fertilization has been a major challenge for tropical agriculture. Lately, cover plants better adapted to tropical conditions have allowed non-tillage in these areas. Our aim was to investigate the effects of soil and plant (millet) on the efficiency of different P sources combined with N sources in an oxisol with medium and low initial P availability. For this, rhizotubes filled with oxisol under different combinations of P and N source were exposed to the absence and presence of millet plant (Pennisetum glaucum L., Leeke). We characterized the soil before and after, as well the fertilizer sources and plants. There was no effect of medium or low initial P availability in the soil on the P recovery rate (PRR). NH4+ was advantageous mainly combined with a soluble P source, increasing up to 40% the P recovery rate in relation to NO3−. Bayóvar phosphate rock (PR) had higher solubilization than Araxá PR, as well as a significant soil pH increase. Without plants, there was no significant correlation among the variables evaluated, evidencing the plant’s role in dissolving less soluble P sources in an oxisol. Our findings suggest no effect of previous medium P availability on the P recovery rate regardless of phosphorus sources. In addition, an improvement in the P recovery rate when an ammoniacal source is used is observed when soluble phosphate fertilizers are used. Cover plants on oxisols are crucial for increasing the efficiency of phosphorus fertilizers and their plant availability over time.