Comparison of methods for estimating maximum soil water content for optimum workability

Tillage causes changes to soil structure, and if the soil is too wet when tillage is performed, the change to the soil structure will be detrimental. A methodology that could be used for estimating the maximum gravimetric soil water content for optimum tillage would be helpful to prevent soil structural damage. The objective of this study was to compare five methods for estimating the soil water content for tillage. These methods utilized existing data of water retention, Atterberg limits, and Proctor compaction tests. The database included 80 soils, 51 from Germany and 29 from north central USA. Additionally, on three sites in Germany, soil water content in the field was measured intensively and related to the estimates of soil water content for workability. Maximum soil water content for optimum tillage of cohesive soils was at a consistency index of 1.15 and 90% of the water content at the lower plastic limit. For both cohesive and non-cohesive soils, the maximum soil water content for optimum workability was equal to either the water content at maximum Proctor density or 70% of the water content at a tension of −5 kPa. At the water content of the inflection point of the water retention curve, in many cases the soil would be too wet for tillage. The results provide methods for estimation of the maximum water content for optimum tillage from databases with existing soil physical properties.