Aggregate strength and mechanical behaviour of a sandy loam soil under long-term fertilization treatments

Current concern for soil quality has stimulated research on soil biological and chemical properties. In contrast, the mechanical behaviour of soil is somewhat neglected. We have examined the effects on soil mechanical properties of more than 100 years of contrasting fertilization employing three treatments from the Askov long-term experiment: UNF (unfertilized), NPK (mineral fertilized) and AM (animal manured. We have measured tensile strength of aggregates when air-dry and when adjusted to -10, -30 and -100 kPa pressure potential. Four aggregate size classes were investigated (1-2, 2-4, 4-8 and 8-16 mm diameter). Soil fragmentation was characterized in the field using a drop-shatter test. Bulk soil strength was determined in the field using a shear vane and a torsional shear box. Soil texture, pH, cation exchange capacity and microbial biomass were measured. The unfertilized soil has little soil organic matter and microbial biomass and is dense. Its aggregates were strong when dry and weak when wet. In contrast, the manured soil had strong aggregates when wet and rather weak ones when dry. The NPK soil generally had intermediate properties. The differences between the soils when dry seem to be related to differences in dispersible clay content, whereas the differences when wet are related to differences in the amount of organic binding and bonding material. The optimal water content for tillage as well as the tolerable range in water content was largest in the manured soil and smallest in the unfertilized soil. Our results indicate that soil mechanical properties should be measured over a range of water regimes to determine the effects of various long-term fertilization treatments.