This feasibility study was conducted to gather and examine as much data as possible on suitable indicators that relate to soil health, to gain a better understanding of their impact on crop yields.
Soils were collected from many different cropped and reference sites in major production areas over two crop growing seasons in Queensland, New South Wales, Victoria and Tasmania.
Soil health is a complex web of many interrelated soil properties that are influenced by climate, soil type and management practices.
In general, this study highlighted the great potential of quantitative analytical measurements for determining soil factors that impact on crop productivity and for defining the status of a particular soil in relation to a healthy soil.
Impact of soil factors on crop productivity :
Capsicum and carrot crops were used as benchmark crops for this feasibility study.
The study indicated that the types of soil factors and management practices that have major influences on crop productivity are crop specific and can only be used as indications for those crops that were studied.
Carrot production is directly affected by a decline in soil health, with impacts on both carrot root growth (carrot shape) and carrot susceptibility to soilborne diseases.
Soil degradation, however, has less impact on capsicum production, because many of the adverse effects on root growth can be compensated for by intensive farm management practices that include soil fumigation, plastic mulching, multiple fertiliser applications and increased soil tillage.
As a result, this study also demonstrated that crop yields were not always influenced by soil properties that are closely related to soil health.
Many resulting adverse effects of soil degradation, such as carbon depletion, poor water retention, decline in soil structural properties, and decline in beneficial soil organisms, can be compensated for.
The use of crop yield as a measure of soil decline can also be misleading, as yields can also be increased through the introduction of high yielding new varieties, salt tolerant varieties, an improved range of fertilisers, better disease and pest control, and improved technology in farm management.
Therefore, when determining the long-term sustainability of crop production, we sometimes need to look beyond crop yield alone.
High crop yield that can only be obtained through high farm inputs is not sustainable when weighed against the high costs in labour, agrochemicals, and water, and the on-site and off-site environmental effects.
A major challenge to vegetable growers and researchers will be to develop management practices that will reduce reliance on chemical inputs and ensure the effective use of water, while also preserving environmentally friendly land use for plant, animal and human health.
It is conceivable that in evaluating a cost benefit ratio, growers may have to consider lower yields for a lower cost farm input production system.
These are the issues that can only be addressed though long-term evaluations to identify and develop alternative options to the current intensive and high input management systems.
Indicators of soil health :
- • The term “soil health” defines a soil’s resilience in sustainable productivity, maintaining environmental quality, and promoting plant, animal and human health.
Realistic benchmark values for a healthy soil in each region could be obtained from non-cropped reference sites.
- Potential soil health indicators can be broadly divided into two categories, in accordance with their functions.
One category is akin to a building (soil structure, aggregate stability, penetration resistance, soil structure score) and the other is akin to building materials that will influence the quality of the building (organic matter, air pores, total microbial activities, fungi, bacteria, nematodes).
This comparison highlights the importance of the latter in the overall health of a soil. • Soil microflora (bacteria and fungi) and microfauna (nematodes) are particularly sensitive to organic matter quality and quantity, soil disturbance and management practices.
Changes in other non-biological soil properties, such as total carbon, total nitrogen, soil aggregation, compaction, water holding capacity, soil pH and electrical conductivity will also affect these biological indicators.
Therefore, these dynamic biological indicators could serve as an early warning system for practices that can affect soil resilience, and may provide a better understanding of changes in organic matter, and conditions in the soil ecosystem.
- In general, higher levels of biological activities were recorded in the reference sites than in cropped sites, regardless of the different methodologies used.
Hence, the different methods for determining soil microbial population and activities result in comparable conclusions and are indicative of changes in the soil environment.
Useful methods identified in this study include nematode count, fluorescein diacetate hydrolysis, microbial biomass carbon, and PL-FAME analysis.
- Unfortunately, apart from the general impact of disturbed soils (cropped sites) versus undisturbed soils (non-cropped pasture reference sites), the impact on these microbiological indicators by various crop management practices could not be properly assessed in this survey study.
Long-term field trials of at least five years, involving replicated plots with different management practices, are required for comparative studies.
- Soil carbon was identified as the basic and most important building component for a healthy soil, irrespective of soil type, region, or climatic conditions.
Soil carbon impacts on many biological, chemical or physical soil properties.
Some cropped sites in Tasmania and Queensland showed similar or higher soil carbon values compared to reference sites.
This indicates that with good farm management practices, even with intensive land use for vegetable production, soil integrity and soil health can be sustainable.
- Unfortunately, with so many variables between the different sites in this short survey study, it was not possible to identify what constituted good farm practices and sustainable land use.
- This feasibility study established the potential of determining soil factors that impact on crop productivity and the use of potential soil health indicators.
The full benefit of this study to the vegetable industry can only be realised with some follow-on work as listed below.
- At the very least, the production of a booklet on soil health for growers and industry use is recommended.
Many of the concepts of soil health and explanations on how the various soil components influence soil structural integrity, as presented in this report, should be extended to the wider community in order to generate improved understanding of the relevance of the soil’s biological, chemical and structural properties.
- Conduct a review of available data on soils to produce a practical checklist or benchmark values and remedial steps that can be used on poor soils.
This would provide an invaluable source of reference for the vegetable industry.
- Long-term studies, of at least 5 years, are essential to gauge the impact of different crop management practices on soil health and soil resilience.
The identification of good farm management practices that conserve soil carbon, maintain soil health and crop productivity, even under intensive land use, will benefit many growers.
- Another major challenge will be to develop management practices that will reduce reliance on chemical inputs and increase the effective use of water, while also preserving environmentally friendly land use for plant, animal and human health.
This may require the development of an economic benefit method that accounts for the overall farm inputs, effects on soil health and the cost to the environment.
This project was funded by Australian vegetable grower levy funds through Horticulture Australia Limited.
The Australian Government provides matched funding for all HAL's R&D activities.
Support from participating capsicum and carrot growers in the project studies are also gratefully acknowledged.
We would also like to acknowledge Jason Olsen for his leadership in the project before leaving the team for another job in May 2002.
Larissa Bilston of R&D Extension Service is responsible for media releases and organising industry forums at Bowen and Bundaberg, and Doris Blaesing of Serve-Ag for facilitating the Devonport industry forum.
Researchers in Tasmania are also grateful to Susan Cross of Serve-Ag Research and Jamie Cooper of DPIWE for their assistance in field work.
We are also thankful to Mary Trebilco for proof-reading this report.