A PRACTICAL TEST TO ASSESS THE BIOLOGICAL STATUS OF AUSTRALIAN GRAIN-GROWING SOILS
- The PreDicta B DNA tests can measure pathogens and beneficial organisms in grain-growing soils.
- Measuring beneficial nematodes, in addition to pathogens, will help growers assess biological soil health and consider adopting practices that will improve the health of their soils, such as planting legumes as rotation crops to enhance nutrient cycling or retaining crop residues to improve the soil’s capacity to suppress to root pathogens.
- Some managements, particularly tillage and fertilisation practices, have detrimental effects on omnivorous and predatory nematodes.
Nematodes as a biological indicator
Agricultural soils contain a wide array of small animals and microbes that have many important functions. Some damage root systems and cause disease while others help maintain soil structure, fix nitrogen, store and cycle nutrients, and protect plants from soilborne pathogens. Growers wishing to maintain or improve productivity need to know something about the biological status of their soils. One way of obtaining that information is to collect a soil sample and subject it to nematode community analysis.
The Fact Sheets ‘The Nematode Community in Australian Grain-Growing Soils’ and ‘Nematodes as a Biological Indicator’ explain why the composition of the soil nematode community provides a good indication of a soil’s the biological status (figure 1). However, information on the nematodes present in soil is difficult to obtain because the methods available to extract these animals are laborious, and few people have the expertise required to identify nematodes. Therefore, manual nematode community analysis is not something that can be done on a routine basis.
Figure 1: Predatory (left) and omnivorous nematodes. Healthy soils have relatively high numbers of nematodes in these groups.
The PreDicta B DNA tests for nematodes
DNA assays provide a practical and affordable way of determining the composition of the soil nematode community. They have been used for many years to measure populations of plant-parasitic nematodes, but additional tests have recently been developed for free-living nematodes. These tests are available from the South Australian Research and Development Institute (SARDI) through its PreDicta B soil testing service which provides a quantitative assessment of the key pathogens and beneficial organisms in grain-growing soils.
DNA is extracted from a soil sample and measured, and the amount of DNA of all target organisms is presented as picograms of DNA/g soil. Since the pathogenic nematode population densities required to cause crop losses is known, the test is a useful risk management tool to decide whether it is safe to plant a susceptible crop, or whether management options such as tolerant varieties or resistant crops need to be considered.
The benefits of measuring other components of the nematode community
One problem with using pathogen diagnostic tests is that growers lose sight of the fact that many beneficial organisms also occur in soil. The advantage of tests which measure both pathogens and beneficial organisms is that it allows growers to think holistically about their soil biological community and consider adopting practices that will improve the health of their soils, for example:
- elimination of practices known to be detrimental to beneficial soil organisms (e.g. tillage and bare fallowing)
- planting a leguminous rotation crop to increase populations of bacterial-feeding nematodes, thereby enhancing nutrient cycling
- retaining crop residues and applying organic amendments to improve the complexity of the soil food web, thereby enhancing the soil’s capacity to suppress root pathogens.
Interpretation of test results
The free-living nematode component of PreDicta B quantifies nine groups of nematodes:
1–3. bacterial-feeding enrichment opportunists— Rhabditinae, Mesorhabditinae and Panagrolaimidae
4. bacterial-feeding nematodes in the family Cephalobidae
5–6. fungal feeders—Aphelenchidae and Aphelenchoididae
7. fungal-feeding and plant associates in the subfamily Tylenchinae
8. predators in the Order Mononchida
9. omnivores in the Order Dorylaimida.
A healthy soil will have the following characteristics:
- a nematode community that is dominated by free-living nematodes rather than plant parasites
- a low population of pest nematodes such Pratylenchus
- a diverse range of nematodes, and a good balance between bacterial and fungal feeders
- relatively high populations of omnivores and predators.
Although the above guidelines are useful, it is important to remember that results will be influenced by many factors, particularly soil type and rainfall. The nematode community in a sandy soil in Western Australia will be quite different to a brown earth from the South Australian mid north or a clay soil from the northern grain-growing region. Results of free-living nematode analyses need to be considered at a regional level, and must always be interpreted carefully. One way of doing this is to compare results from farms in similar soil types and environments and use the comparison to determine the management factors that are influencing the nematode community.
Management practices known to affect the nematode community
Research carried out over many years has shown that agricultural management has major effects on the nematode community. Active, diverse and stable nematode communities are most likely to occur under perennial pastures where rainfall is consistent and plant productivity is high. Soils under no-till cropping for at least 20 years also tend to have a relatively good biological status, particularly if yields have been consistently high and crop residues have been retained.
Crop rotation can affect the microbe-feeding component of the nematode community. For example, residues from legumes have a relatively low carbon to nitrogen ratio and tend to stimulate the bacterial-feeding nematode community. Fungi play a greater role in decomposing grass residues, and so populations of fungal-feeding nematodes will tend to increase when these residues are returned to soil (figure 2).
Figure 2: Nematode communities in relation to carbon to nitrogen ratios (C:N) of soil amendments.
The practice most likely to have a negative impact on the nematode community is tillage. Tillage is detrimental for two reasons: i) it increases losses of organic matter, the energy source that sustains the bacteria and fungi on which many nematodes depend; and ii) it kills many of the large omnivorous and predatory nematodes, and because they have long life cycles, they take a long time to return.
Fertilisation practices also have a negative impact on some soil nematodes. If high yielding crops are to be produced and adequate quantities of organic matter returned to the soil, nutrition must be adequate. However, it is important that fertilisers are not used excessively, as high nitrogen inputs are known to be detrimental to omnivorous and predatory nematodes.
Authors: Graham Stirling (Biological Crop Protection), Katherine Linsell (South Australian Research and Development Institute)
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.