Field studies to assess the workable range of soils in the tropical zone of Veracruz, Mexico

In the tropical area of Veracruz (Mexico) the decision of when and how to carry out tillage operations is based on qualitative criteria. It often results in excessive and unnecessary work, energy waste, operational delay, soil exposure to water erosion and soil structural damage. Objective criteria are needed in this area for selecting when and how to do cultivation in order to meet crop and conservation requirements. The workable range of typical soils of the area (Haplic Pheaozems) was quantified by assessing in-field implement effects on the soil structure and measuring the specific energy applied by the tractor–implement combination. This was done over a range of soil moisture contents inside and outside the theoretical friable consistency state of the soils, determined by the shrinkage and plastic limits. Empirical relationships between initial moisture content and the technological result of tillage showed that these results for moist loam and clay soils shifted from optimum to poor at soil water potentials that coincides quite well with the plastic limit. However, as soil was drying out, the implement effect changed from optimum to sub-optimum at soil moisture contents well above the shrinkage limit, so the actual field workable range was smaller than the theoretical friable status of the soils. The minimum input of specific energy to obtain optimum results, was close to the soil water potential where results changed to sub-optimum as soils were drying out. To support decisions for tillage planning and operation, valuable information on workable periods can be obtained by making use of quantified workability thresholds.