Phosphogypsum and Machinery Traffic Affecting Aggregate Tensile Strength in an Oxisol Under No-till System.

Tensile strength (TS) is a soil structural property that may affect the physical environment of the soil seedbed for germination and initial plant development. Machinery traffic can increase TS values, causing difficulties to germination, emergence and establishment of the crops. Phosphogypsum (PG) has been used to manage soil chemical fertility, but improvements in soil structural quality has also been found. The aim of the study was to evaluate the impacts of PG application rates and machinery traffic in aggregate TS under no-till system. The experiment was setup in an Oxisol from Guarapuava, Parana State, Brazil, using a split-splot design with four replicates. The PG rates were 3, 6, 9 and 12 Mg ha^-1 plus a control (without PG) and we evaluate three machinery traffic intensities (wheel track, between wheel track and no traffic). Soil samples have taken 36 and 6 months after PG application and machinery traffic, respectively. The aggregate TS was determined by an electronically controlled loading frame. The elemental analysis has done using a X-ray energy-dispersive spectrometer attached to a scanning electronic microscope (EDS/SEM). At 0.0-0.1 m layer, the aggregate TS was linearly decreased by PG rates on wheel track zones, having a negative correlation with calcium (Ca) levels. The correlation between Ca²⁺ and carbon (C) was also increased by gypsum at this layer, being confirmed by the kappa index (K) for the EDS/SEM aggregate images. Machinery traffic significantly increased aggregate TS up to 0.2 m soil layer, also raising the values in the area between the wheels. Rates of PG increased soil Ca²⁺ concentration, improving C chemical protection and reducing aggregate TS. PG application was efficient to reduce the negative impacts of the machinery traffic on TS. However, even without influence of PG, the no traffic zone had the smallest values of aggregate TS.