Topsoil structure in no-tilled soils in the Rolling Pampa, Argentina

Some topsoil physical properties evolve unfavourably under continuous, no-till farming. On the Pampa, loam soils under no-till sometimes have lower infiltration rates than those conventionally tilled; this is due to the occurrence of platy and massive structures. In this study, we aimed to identify the soil management practices that promote platy structure formation, and explain the soil physical behaviour linked to the thickness of platy structures in relation to infiltration rate, bulk density and shear strength. Six fields with different numbers of years under agriculture and diverse previous crops (maize or wheat-soybean double crop) were sampled, distinguishing within each field headlands (areas with higher traffic) and centre (lower traffic). Twenty samples were taken at random along a 200-m transect to characterise soil structure (platy, granular or massive) and the thickness of the platy structure. Principal component analysis revealed linkages between previous crop and location in each field and type of structure. ANOVA showed a significant (P < 0.05) interaction of previous crop x location. The frequency and thickness of the platy structures were lower, and those of granular structures higher, under wheat-soybean double cropping and in the centre of the field. Greater thickness of the platy structure determined lower water infiltration rate (r = -0.337; P < 0.01) and greater soil shear strength (r = 0.297, P < 0.01). Micromorphological analysis indicated the dominance of massive and platy structure in the headlands and bioturbation in the centre of the fields with wheat-soybean double cropping. These results suggest bioturbation, crop-root binding and low machinery traffic as the main factors minimising soil evolution towards unfavourable structural types under no-till farming in the area.