Farmers have traditionally been concerned to keep their soils in good condition because they understand that soil health has a direct impact on crop performance.
Managers need information on dynamic soil properties to test whether current systems of land use and management are sustainable or whether change is required.
The community, as well as farmers, are concerned that agriculture is sustainable and that the dynamic soil properties are not being degraded by current management practices.
Effects of Agricultural Management on Soils
Over a period of several years studies were undertaken across Tasmania to determine the impact of agricultural management on a variety of Tasmanian soil types. As detailed site management histories where not known a selection of sites for each land use on each soil type were investigated. This initial study was followed by further work investigating the links between selected soil properties and crop yield for two of the more widely grown crops in Tasmania - potatoes and poppies - on 130 paddocks on four soil types.
Site selection is important to ensure comparison of the same soil type in each study. Consequently the Tasmanian study was divided into looking at sites on distinctly different soil types as defined in the Australian Soil Classification. The types of management history selected have varied according to soil type.
All site locations were recorded using a GPS so that they can be revisited in the future, should this be desirable.
Soil properties tested
Land and profile description
Soil chemistry pH, organic C, permanganate- extractable C fractions, exchangeable cations, exchangeable aluminium and acidity, extractable P, total nitrogen
Soil biology microbial biomass C, earthworms, seed germination
Soil physics infiltration rate, vane shear strength, penetration resistance, bulk density, aggregate stability, Atterberg limits, field capacity, moisture release characteristics
Soil health via farmer survey (eg. tillage ease, hardness, decomposition of plant residues).
Dermosol (Cressy shaley loam) soil health
Management histories tested:
Results:
Soil organic carbon in the surface 75 mm was 7.0% under long term pasture compared to 4.3% and 4.2% in cropped paddocks
Microbial biomass carbon levels were 217 mg/kg, 161 mg/kg, and 139 mg/kg respectively. These differences were negatively correlated with the number of years cropped
Long term pasture paddocks showed stronger structural development and had smaller clods than cropped paddocks
Greater bulk densities were found in the surface layer of cropped paddocks but these were not associated with increased penetration resistance or decreased infiltration rate and are unlikely to impede root growth
Vane shear strength and penetration resistance were lower in cropped paddocks than under long term pasture
Many soil attributes showed no significant differences associated with management. Including potatoes in the rotation did not appear to affect these Dermosols which indicates a degree of robustness in these soils.
Ferrosol (Burnie clay loam) soil health
Management histories tested:
Low input pasture (beef/sheep)
High input pasture (irrigated dairy)
Intermittent cropping (2-5 years cropping in 10)
Continuous cropping (for at least 10 years)
Pyrethrum following continuous cropping
Results:
Among the most notable changes was soil organic carbon in the top 150 mm, which was about 30% less in cropping and pyrethrum paddocks than pasture paddocks, and microbial biomass C which was about 60% less. Organic carbon declined with increasing years cropped
Earthworm numbers showed even greater differences, with virtually no earthworms under pyrethrum
Pasture soils had higher shear strength and water contents at the liquid limit in the top 150 mm, and more water-stable aggregates in the subsurface soil
Despite these differences, the absolute values of most attributes in cropping paddocks were of a magnitude which suggests that Tasmanian Ferrosols, even under continuous cropping, are still in relatively good condition. This agreed with the views of the farmers, obtained by individual survey.
Sodosol (Brumby sandy loam) soil health
Management histories tested:
Long term pasture
Surface cropping (ie. cereals, peas & poppies)
Surface cropping plus potatoes ( ie, cereals, peas, poppies + potatoes) with more rigorous and deeper tillage including deep ripping and powered implements
Surface cropping plus potatoes harvested when the soil was wet.
Results:
Soil organic carbon in the top 150 mm was 2.7 % under long term pasture compared to 1.8 % in rigorously tilled cropping paddocks and microbial biomass C values were 194 and 129 mg/kg respectively
Readily oxidisable carbon 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 to 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 increased with all cropping, particularly after potatoes
Farmers identified more healthy than unhealthy soil attributes under all management histories but reported more unhealthy soil attributes when potatoes were included in their rotation
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.
Tenosol (Panshanger sand) soil health
Management histories tested:
Results:
Soil organic carbon in the surface 75mm was 2.6% under long term pasture compared to 1.1% in rigorously tilled cropping paddocks. Readily oxidisable carbon concentrations were 2.3 mg/g and 1.0 mg/g respectively. These differences were negatively correlated with the number of years cropped
Infiltration rate was greater and shear strength less in cropped paddocks compared to long term pasture
Dry bulk density was greater and total porosity and macroporosity were less in rigorously tilled paddocks
Subsoil compaction was apparent in paddocks which had grown potatoes
Cropping was not clearly associated with smaller or less stable aggregates
Farmers identified more healthy than unhealthy soil attributes under all management histories.
The effects of cropping are not associated with a broad range of degraded soil attributes on these Tasmanian Tenosols.
Vertosol (Canola and Chruchill clay) soil health
Management histories tested:
Long-term pasture
Rain-fed cropping
Irrigated cropping.
Results:
When adjusted for clay content, cropped sites had lower soil organic carbon than pasture sites at 0-75 mm depth
Readily oxidisable carbon in the surface 75 mm was 3.6 mg/g and 6.9 mg/g under long term pasture compared to 2.5 mg/g and 3.9 mg/g in irrigated cropped paddocks on southeastern 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 and larger clods
Vane shear strength and penetration resistance were lower in rain fed-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
Vertosols in the northern Midlands had better physical properties (lower 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 (higher organic carbon, readily oxidisable carbon, and more worms) than southeastern Vertosols.
Linking soil health to crop yield
What we did
We sampled 130 paddocks on 4 soil types in north and northwest Tasmania during the 2001-02 growing season and tested each paddock for 4 or 5 key soil properties that depended on soil type. Farmers supplied information on agronomic inputs including irrigation, fertilisers and pesticides used. Soil properties and agronomic information were analysed for correlation with commercial potato or poppy crop yields.
What we found
The majority of paddocks on Ferrosols and Dermosols (Cressy shaley loam) were in good condition. However, more than one third of cropped paddocks on each of the four soil types showed signs of soil structure associated with long term cropping, ie. increased cloddiness, increased resistance to root penetration, reduced infiltration, increased density and reduced organic carbon content.
The links to crop yield
The soil properties and agronomic variables found to be significantly correlated with crop yield and assay varied depending on crop and soil type.
The analysis of yield and soil factors in our data set suggests that:
- Soil structure (as assessed by visual score) may influence potato yield on Ferrosols and poppy yield on Dermosols
- Exchangeable Al may influence poppy yield and exchangeable Ca may influence poppy assay on Ferrosols
- Penetration resistance may influence potato yield on Dermosols and poppy assay on Dermosols and Ferrosols
- Tenosols - too few sites to show significant results.
Implications for farmers
If your clay loam textured topsoils look cloddy, this is likely to be decreasing crop yield
Chemical fertility, such as Exchangeable Al on Ferrosols, can influence crop yield if these factors become growth limiting
These soil chemical properties are more easily managed than the physical properties through additions of appropriate quantities of lime and fertiliser. However, degraded soil physical properties are often harder to remedy and require beneficial management practices to be applied over the long term. Prevention of soil structure degradation is usually the best way to manage this problem.
Our results show that different soil properties and management factors may be influencing plant production on different soils. Consequently, interpretation of data on soil properties, as well as any advice given to farmers to maximise production, needs to be made with respect to the soil type and crop under consideration. If signs of degradation are apparent, then this information can be used to advise on appropriate soil management practices such as timing and depth of tillage, rotations to include a rejuvenating pasture phase, and prevention of erosion.
Soil health is not an end in itself
The ultimate purpose of assessing soil health by research is not to achieve high aggregate stability, biological activity, or some other soil property. The purpose is to protect and improve long-term agricultural productivity, water quality, and habitats of all organisms, including people. We use soil characteristics as indicators of soil health, but in the end, soil health must be identified by how it performs all of its functions.
Managing for soil health
Each combination of soil type and land use calls for a different set of practices to enhance soil health. Yet, several principles apply in most situations.
Add organic matter. Regular additions of organic matter are linked to many aspects of soil health. Organic matter may come from crop residues at the surface, roots of cover crops, animal manure, green manure, compost, and other sources. Organic matter, and the organisms that eat it, can improve water holding capacity, nutrient availability, and can help protect against erosion.
Avoid excessive tillage. Tillage has positive effects, but it also triggers excessive organic matter degradation, disrupts soil structure, and can cause compaction. For more information about tillage, visit this tillage link.
Carefully manage fertilizer and pesticide use. Over the past 60 years pesticides and chemical fertilizers have revolutionized agriculture. In addition to their desired effects, they can harm non-target organisms and pollute water and air if they are mismanaged. Manure and other organic matter also can become pollutants when misapplied or over-applied. On the positive side, fertilizer can increase plant growth and the amount of organic matter returned to the soil.
Increase ground cover. Bare soil is susceptible to wind and water erosion, and to drying and crusting. Ground cover protects soil, provides habitats for larger soil organisms, such as insects and earthworms, and can improve water availability. Cover crops, perennials, and surface residue increase the amount of time that the soil surface is covered each year.
Increase plant diversity through crop rotation. Diversity is beneficial for several reasons. Each crop contributes a unique root structure and type of residue to the soil. A diversity of soil organisms can help control pest populations, and a diversity of cultural practices can reduce weed and disease pressures. Diversity across the landscape and over time can be increased by using buffer strips, small paddocks, crop rotations, and by varying tillage practices. Changing vegetation across the landscape or over time increases plant diversity, and the types of insects, microorganisms and wildlife that live on your farm.