Physical Properties of Soils under 14-year Conservation and Standard Tillage

Long-term changes in soil physical characteristics in a cropping systems study in California under conservation tillage were measured to provide greater understanding of the viability of conservation tillage systems in California. This study determined effects of tillage and of winter cover crops (CC, NO) on soil physical properties: bulk density (compliant cavity and known volume), water stable aggregates, slaking, infiltration and post-drainage volumetric water content. Measurements were conducted in a tomato-cotton rotation, on a long-term (1999-2014) 20 hectare field experiment in Five Points California. Tillage treatments included conservation tillage (CT): using drill seeding, minimal disturbance and maintained residues; and standard tillage (ST).;Bulk density values were greater under CT than ST treatments from depths of 0-15cm and 15-30cm using both compliant cavity and known volume methods and cover crop presence/ absence did not impact bulk density values. Water stable aggregate percentages increased 23% in the presence of cover crop and 25% in the more clayey soil type (clay loam vs sandy clay loam), but were not affected by tillage treatments. Aggregate resistance to slaking was 27% greater under conservation tillage after 5 minutes and 28% greater following 5 submersions. Cover crop and conservation tillage treatments increased infiltration rates relative to the alternatives for the first acre-inch of applied water (CC 0.44 min; NO 1.32 min) (CT 0.5 min; ST 1.19 min), but tillage maintained a larger relative impact for the second acre-inch, with CT more than halving infiltration times relative to ST (CC 4.03 min; NO 6.46 min) (CT 3.15 min; ST 7.33 min). Post-drainage volumetric water content was 2.5 percentage points lower (relatively 6.4% lower between treatments) in conservation tillage (CT 37.78 gH2O cmsoil-3; ST 40.37 gH2O cmsoil-3), which may have been caused by increased macro-porosity of the treatment, increasing gravimetric water drainage.;When examined in light of yields (cotton was 10% less in CT, while tomato was 9.5% more), these data suggest that the numbers measured under conservation tillage, i.e. higher bulk densities and lower volumetric water contents, do not necessarily impact crops as might be expected from compacted soils or less water. This ambiguity supports the idea that the nature of CT is fundamentally different from tilled systems and that some crops (such as tomatoes) will fare better under a CT regime despite physical conditions that might otherwise be expected to produce negative results (i.e. increased bulk densities, lower water holding capacity). In addition, the data reinforce the link between a soil's high resistance to slaking and the high infiltration rates observed under CT. The need to conduct further studies in conditions specific to California is evident in light of the limiting factors that may be more intrinsic to irrigated crops in a xeric moisture regime relative to those observed in rain-fed udic or ustic cropping systems, and is underlined by the significant variation in physical responses of the two relatively similar soil types to different treatments.