No abstract

Shallow mechanical loosening of soil to 22 cm deep (aeration) was investigated as a method for ameliorating soil compaction caused by dairy cattle treading. Soil physical and pasture measurements taken over 46 weeks compared plots grazed under normal grazing practice (non-aerated) with plots under normal grazing practice where soil was mechanically loosened (aerated). Aerated soil initially showed reduced (P < 0.05) penetration resistance, degree of packing, and bulk density, and increased (P < 0.05) hydraulic conductivity, total porosity, macroporosity, and proportion of small aggregates, compared with non-aerated soil. However, after 40 weeks aerated soil showed some reversion back to a non-aerated state, and only the most sensitive measurements (penetration resistance, degree of packing, soil structure, and *Author for correspondence A99038 Received 6 August 1999; accepted 13 April 2000 macroporosity) showed significant (P < 0.05) treatment differences. Pasture herbage yield, botanical composition, and root length were unaffected (P < 0.05) by aeration, but aeration increased (P < 0.05) root dry weight and decreased bare ground. This work suggests that timing of aeration with regard to soil moisture and atmospheric conditions is vital for optimal soil and pasture responses. The need to use methods which sample large volumes of soil and pasture to detect soil physical and pasture changes due to loosening is also stressed.

Like time domain reflectometers, cheaper CS615 water content reflectometers (WCRs) also measure dielectric properties of the soil to determine its volumetric water content (VWC), but are more affected by environmental factors. Quadratic equations described the laboratory data from 12 horizons of 4 soils of vastly differing properties slightly better than linear equations. Root mean squared errors (RMSE) averaged over the 3 horizons increased slightly in the order Gley (av. 1.0%), Pumice (av. 1.3%), Recent (av. 1.6%), and Allophanic soil (av. 1.9%). Using the manufacturer’s standard calibration resulted in significantly higher RMSE (av. 6.2–25.3%) and mean errors (av. –5.5% to +21.5%), with the VWC of the 2 soils of volcanic origin being underestimated. An atypical dielectric response of water stored in volcanic soils has been attributed to their low bulk density, high porosity, and large specific surface area. The in-situ verification was hampered by the variability observed between the data from duplicate WCRs. Measuring the inter-sensor variability in air and water indicated that this could account for a significant part of this variability, while small-scale variation of VWC in-situ was also observed. Nevertheless, the laboratory calibrations were usually better, or at least similarly suited, to describe the in-situ data than the manufacturer’s calibrations.

In a 6-month laboratory incubation study, we compared the net C and N mineralisation of the soil organic matter (SOM) of 3 pasture soils and the mineralisation of glucose-C in intact versus sieved&sol;refilled soil cores. The main questions were whether the net C and N mineralisation differed between intact and sieved&sol;refilled soil cores after a conditioning period of 4 weeks, and how much the C and N mineralisation of SOM differed among the similarly managed pasture soils. Apart from the net nitrogen mineralisation in one soil, there were no significant differences in cumulated mineralisation of C or N from SOM between the core types. In a fine-textured soil, net mineralisation of glucose-C differed significantly between core types, which was attributed to the different distribution of the amended glucose in intact and sieved&sol;refilled cores. Net C and N mineralisation of SOM were closely correlated in the sieved&sol;refilled cores, whereas no significant correlation was found in the intact cores. Expressing net C and N mineralisation as percentages of total soil C and N showed a more than 2-fold maximum difference between the soils in spite of similar long-term organic matter input. Subsequent studies should be done using more replicates and wider diameter, better controllable cores on ceramic plates. CO2, net nitrogen mineralisation (NNM), soil microbial biomass.