We Cotching

Effects of agricultural management on sodosols in northern Tasmania

Attributes of 25 Tasmanian sodosols were assessed using field and laboratory techniques to determine changes associated with 4 typical forms of agricultural management [long-term pasture, cropping with shallow tillage using discs and tines, cropping (including potatoes) with more rigorous and deeper tillage including deep ripping and powered implements, and cropping (including potatoes) where the potatoes were harvested when the soil was wet]. Soil organic carbon in the top 150 mm was 2.7% under long-term pasture compared with 1.8% in rigorously tilled cropping paddocks, and microbial biomass C values were 194 and 129 mg/kg, respectively. Readily oxidisable organic C concentrations were 1.8 mg/g and 1.3 mg/g, respectively. Infiltration rate was greater in paddocks with shallow tillage cropping than long-term pasture but was 43% less in paddocks which had grown potatoes and 70% less after a wet potato harvest. Dry aggregate-size showed no change under shallow tillage cropping compared with long-term pasture but decreased significantly in more rigorously tilled potato cropping paddocks. Aggregate stability in all cropped paddocks was nearly 50% less than in long-term pasture paddocks, with values in intensively tilled potato cropping paddocks approaching relatively low levels. Colwell extractable phosphorus (P) increased with all cropping, particularly after potatoes. Lower organic carbon and poorer physical properties were associated with paddocks which had grown potatoes, which adds weight to the view that cropping rotation and associated soil management practices are critical for sustainable management of Tasmanian sodosols. Farmers were surveyed about their views of the condition of their paddocks. They identified more healthy than unhealthy soil attributes under all management histories but reported more unhealthy soil attributes when potatoes were included in their rotation.

Determining the frequency, depth and velocity of preferential flow by high frequency soil moisture monitoring

Preferential flow in agricultural soils has been demonstrated to result in agrochemical mobilisation to shallow ground water. Land managers and environmental regulators need simple cost effective techniques for identifying soil - land use combinations in which preferential flow occurs. Existing techniques for identifying preferential flow have a range of limitations including; often being destructive, non in situ, small sampling volumes, or are subject to artificial boundary conditions. This study demonstrated that high frequency soil moisture monitoring using a multi-sensory capacitance probe mounted within a vertically rammed access tube, was able to determine the occurrence, depth, and wetting front velocity of preferential flow events following rainfall. Occurrence of preferential flow was not related to either rainfall intensity or rainfall amount, rather preferential flow occurred when antecedent soil moisture content was below 226 mm soil moisture storage (0-70 cm). Results indicate that high temporal frequency soil moisture monitoring may be used to identify soil type - land use combinations in which the presence of preferential flow increases the risk of shallow groundwater contamination by rapid transport of agrochemicals through the soil profile. However use of high frequency based soil moisture monitoring to determine agrochemical mobilisation risk may be limited by, inability to determine the volume of preferential flow, difficulty observing macropore flow at high antecedent soil moisture content, and creation of artificial voids during installation of access tubes in stony soils.

Dominant soil orders in Tasmania: distribution and selected properties

Dermosols (24%) and Organosols (14.8%) are the dominant soil orders in Tasmania, with the mapped occurrence of >985 000 ha of Organosols in Tasmania being the greatest in any Australian State. Tenosols and Rudosols are well represented in all 3 natural resource management (NRM) regions and Kurosols are more prevalent in the NRM North and South Regions. Tasmania has a greater proportion of Ferrosols (8.4%) than the whole of Australia (0.8%) and these soils are some of the most productive in Tasmania with >25 000 ha used for cropping. Hydrosols (3.7%) are probably underestimated. Chromosols (5.3%) and Sodosols (1.6%) are relatively minor soils in Tasmania, occurring predominantly in lower rainfall areas with <800 mm average annual rainfall. Parent material is a strong determinant of soil distribution in Tasmania but many Soil Orders occur on a wide range of parent materials. Brown suborders are predominant in several Soil Orders. A large part of Tasmania (2 658 000 ha) is mapped as being used for conservation, with one-third of this area being mapped as Organosols. The mean surface horizon soil carbon content (4.3%) is relatively high, likely due to Tasmania’s relatively high annual rainfall and cool temperatures. Most Soil Orders have moderately acid surface horizons but soils on calcareous parent materials are neutral to strongly alkaline (Tenosols and Calcarosols). The dataset covers the mainland extent of Tasmania, as well as all large islands around Tasmania’s coastline including King, Flinders, Hunter, Three Hummock, Robbins, Cape Barren, Clarke, and Maria Islands.

Crop yields and soil properties on eroded slopes of red ferrosols in north-west Tasmania

The differences in soil properties and crop yield on slopes of varying steepness in intensively cropped paddocks with red ferrosols (Humic Eutrodox), which had evidence of erosion over many years, were determined at 5 sites in north-west Tasmania. We found that soils on the steeper slopes, which have had greater soil loss, had significantly lower topsoil carbon concentrations than soils on flatter slopes. We found that significant redistribution of soil had occurred on the sites in this study. Areas with concave slope positions had over-thickened topsoils. However, the effect of erosion on the topsoil thickness of eroded slope positions was masked by the uniform depth of cultivation across paddocks. Penetration resistance increased gradually with depth on all transects, indicating no compacted layers or plough pans on these intensively cropped sites. Soil physical properties did not significantly change from non-eroded, level areas to steep, eroded parts of the paddocks, even although considerable soil loss had occurred from the steep slopes. We attribute this to the gradational nature of these red ferrosols, which characteristically have strongly structured subsoils. Crop yield was significantly correlated with slope but was not significantly different on land up to 18% slope. We anticipate that continued erosion at current rates on this less steep land will inevitably reduce crop yields. We found a significant correlation between organic carbon, total exchangeable bases, extractable K, and crop yield, but no correlation between available water content and crop yield. We conclude that soil organic matter contributes to productivity through its effect on nutrient concentrations. Total exchangeable bases appeared to provide the best and simplest explanatory model to predict crop yield. We propose a set of 4 tests for future assessment of soil condition on these red ferrosols.

Subsurface Lateral Flow in Texture-Contrast (Duplex) Soils and Catchments with Shallow Bedrock

Development-perched watertables and subsurface lateral flows in texture-contrast soils (duplex) are commonly believed to occur as a consequence of the hydraulic discontinuity between the A and B soil horizons. However, in catchments containing shallow bedrock, subsurface lateral flows result from a combination of preferential flow from the soil surface to the soil—bedrock interface, undulations in the bedrock topography, lateral flow through macropore networks at the soil—bedrock interface, and the influence of antecedent soil moisture on macropore connectivity. Review of literature indicates that some of these processes may also be involved in the development of subsurface lateral flow in texture contrast soils. However, the extent to which these mechanisms can be applied to texture contrast soils requires further field studies. Improved process understanding is required for modelling subsurface lateral flows in order to improve the management of waterlogging, drainage, salinity, and offsite agrochemicals movement.

Effects of agricultural management on Vertosols in Tasmania

Attributes of 21 Vertosols in 2 different regions of Tasmania were assessed using field and laboratory techniques to determine differences associated with 3 local forms of agricultural management (long-term pasture, rain-fed cropping, and irrigated cropping). Vertosols in the northern Midlands had better physical properties (lesser bulk density and penetration resistance, and greater porosities and water holding capacities), poorer nutrient status (lower pH, exchangeable bases, and extractable P), and better biological properties (greater organic carbon (OC), carbon fractions F1 and F3, and more worms) than south-eastern Vertosols. When adjusted for clay content, cropped sites had less soil OC than pasture sites at 0-75 mm depth. Readily oxidisable (fraction F1) carbon in the surface 75 mm was 3.6 mg/g and 6.9 mg/g under long-term pasture compared with 2.5 mg/g and 3.9 mg/g in irrigated cropped paddocks on south-eastern and Midlands sites, respectively. Soil organic carbon values were positively correlated with physical and chemical soil properties. Long-term pasture paddocks showed stronger structural development and had smaller aggregates than cropped paddocks, which had more larger clods. Vane shear strength and penetration resistance were less in rainfed cropped paddocks compared with long-term pasture but this effect was not apparent on irrigated cropped paddocks. Farmers considered that a majority of their soil attributes were healthy under all management histories but strategies for maintaining organic matter levels and minimising clod formation by tillage are essential for long-term sustainable use of these Vertosols.

Carbon stocks in Tasmanian soils

Soil carbon (C) stocks were calculated for Tasmanian soil orders to 0.3 and 1.0 m depth from existing datasets. Tasmanian soils have C stocks of 49–117 Mg C/ha in the upper 0.3 m, with Ferrosols having the largest soil C stocks. Mean soil C stocks in agricultural soils were significantly lower under intensive cropping than under irrigated pasture. The range in soil C within soil orders indicates that it is critical to determine initial soil C stocks at individual sites and farms for C accounting and trading purposes, because the initial soil C content will determine if current or changed management practices are likely to result in soil C sequestration or emission. The distribution of C within the profile was significantly different between agricultural and forested land, with agricultural soils having two-thirds of their soil C in the upper 0.3 m, compared with half for forested soils. The difference in this proportion between agricultural and forested land was largest in Dermosols (0.72 v. 0.47). The total amount of soil C in a soil to 1.0 m depth may not change with a change in land use, but the distribution can and any change in soil C deeper in the profile might affect how soil C can be managed for sequestration. Tasmanian soil C stocks are significantly greater than those in mainland states of Australia, reflecting the lower mean annual temperature and higher precipitation in Tasmania, which result in less oxidation of soil organic matter.

Linking Tasmanian potato and poppy yields to selected soil physical and chemical properties

Selected soil properties and paddock management characteristics were measured for 121 potato and poppy crops in north and northwest Tasmania to see if variation in these characteristics explain variation in crop yield. The soil properties we selected were those that previous work found had changed the most as a result of cropping and, therefore, may be affecting yield on the particular soil type. The soil properties and management characteristics that were significantly correlated with crop yield varied with crop and soil type. None of the soil characters had correlation coefficients greater than 0.63, probably reflecting the capacity of individual farmers to overcome particular soil limitations through their management of tillage, nutrition, irrigation, weeds and pathogens. On ferrosols, a visual score of soil structure was significantly correlated with potato yield (r = 0.57) and exchangeable aluminium was significantly correlated with poppy yield (r = 0.63). Exchangeable calcium (r = 0.54) and penetration resistance (r = 0.38) correlated positively and topdressed nitrogen (r = –0.49) correlated negatively with poppy alkaloid assay, an important determinant of overall poppy yield. On dermosols, depth to 2000 kPa penetration resistance (r = 0.60) and fertiliser P (r = –0.67) were correlated with potato yield, structure score correlated with poppy yield (r = 0.59), and penetration resistance with poppy assay (r = 0.52). On sodosols, fertiliser K (r = –0.41 and r = 0.55) and N (r = –0.45 and 0.42) correlated negatively with poppy yield and positively with poppy assay. On clay loam soils such as dermosols and ferrosols, increased topsoil cloddiness appears to be having a deleterious effect on crop yield. Cloddiness is readily assessed on these soils using the structure scorecard, which could therefore become a practical diagnostic test for farmers and advisers.

Land use and management influences on surface soil organic carbon in Tasmania

The effects of environmental parameters, land-use history, and management practices on soil organic carbon (SOC) concentrations, nitrogen, and bulk density were determined in agricultural soils of four soil types in Tasmania. The sites sampled were Dermosols, Vertosols, Ferrosols, and a group of texture-contrast soils (Chromosol and Sodosol) each with a 10-year management history ranging from permanent perennial pasture to continuous cropping. Rainfall, Soil Order, and land use were all strong explanatory variables for differences in SOC, soil carbon stock, total nitrogen, and bulk density. Cropping sites had 29–35% less SOC in surface soils (0–0.1 m) than pasture sites as well as greater bulk densities. Clay-rich soils contained the greatest carbon stocks to 0.3 m depth under pasture, with Ferrosols containing a mean of 158 Mg C ha–1, Vertosols 112 Mg C ha–1, and Dermosols 107 Mg C ha–1. Texture-contrast soils with sandier textured topsoils under pasture had a mean of 69 Mg C ha–1. The range of values in soil carbon stocks indicates considerable uncertainty in baseline values for use in soil carbon accounting. Farmers can influence SOC more by their choice of land use than their day-to-day soil management. Although the influence of management is not as great as other inherent site variables, farmers can still select practices for their ability to retain more SOC.

Integrating Farm Production and Natural Resource Management in Tasmania, Australia

Abstract This paper reports on the social learning from a project aimed to increase the knowledge and capacity of a group of farmers in Tasmania, Australia, to reduce the impacts of intensive agriculture on soil health and waterways, and to optimise the efficient use of on-farm inputs. The plan-do-check-review cycle adopted in this project required the farmers to assess current management practices, identify where to make changes, implement changes and monitor for improvements. The success of the project was due to careful attention to social processes as well as technical input. The combination of group activities with individual mentoring and one-to-one advice was key to the success of this project in enabling farmers to undertake on-farm action. There is value in social learning that included developing relationships, using one-to-one contact and group workshops together with expert input when working with farmers to tackle some difficult and complex interrelated natural resource management and production issues. Sufficient time must be allowed for the process of facilitating good practice in natural resource management, particularly when addressing systemic environmental impacts. Practical operational recommendations are presented on communication, feedback, focus of activities and meeting content, as these will be useful to other project officers and facilitators working with farmer groups.