Sampling soils used for growing pastures, field and fodder crops
Note Number: AG0375
Published: May 1998
Updated: May 2009
The usefulness of soil analysis depends on representative samples being supplied to the laboratories. Even in small areas and apparently uniform paddocks, soil properties can vary. The standard sampling procedures outlined below are designed to minimise the effects of soil variation and help you collect a representative sample.
Soils are generally sampled by driving a sampling tube into the soil, usually to 100 mm depth, and extracting a soil core. To obtain a representative sample, soil cores are combined from a large number of places within the sample area. The more cores that are combined, the more representative the final sample will be and the better the advice that can be given.
Where a coring device is not available the soil can be sampled using a spade. Simply dig a hole with a vertical side and cut out a column of soil to the required depth, usually 100 mm. Ensure that the column is uniform all the way up and that the same amount of soil is taken from each hole. Combine these columns in the same way as the cores would be combined to make your sample.
Depth of sampling
In sampling soils it is important that the standard sampling depths are used. In the case of surface soil samples this depth is 100 mm. Shallower sampling will often result in higher values for chemical tests because the amount of nutrients is usually greater near the soil surface.
Sampling areas prior to planting a new pasture or crop
When sampling soil proposed for use in establishing a new pasture or crop it is recommended to sample at a time when the seedbed is as close to final preparation for sowing as practical; allowing time for soil analysis and reporting and the ordering and delivery of fertiliser.
Selecting areas for sampling
Soils are tested for many reasons and there is no single rule for determining which areas should be sampled. When seeking a total farm fertiliser program it is often best to sample every paddock on your property, but this can be both time-consuming and costly. A good alternative is to select paddocks or sections of paddocks that represent the major range of conditions present, for example, good - poor pasture, high - low stocking rate, flats - slopes, black - red soil, sandy - clay soil or granite - basalt. In subsequent years different areas can be sampled and the results combined to provide a profile of the entire property.
An alternative strategy is to sample a set portion of a farm annually, for example, 30%. As paddocks are resampled the effects of land management on soil fertility can be assessed.
Sometimes soil testing is used to investigate specific problems. In these cases it is often helpful to contrast the situation by taking a sample from a non-problem area as well, for example good and poor sections of a paddock.
General sampling guidelines
1. A. For paddocks or paddock sections less than 20 ha (50 ac)
Take cores from at least 30 sites evenly distributed over the paddock or paddock section. For small areas less than 20 ha (50 ac), a grid pattern should be used to determine the site locations (see Figure 1)
|Figure 1. Recommended sampling sites for areas less than 20 ha (50 ac) - regular shaped paddocks||Figure 2. Recommended sampling sites for areas less than 20 ha (50 ac) - irregular shaped paddocks|
1. B. For paddocks or paddock sections more than 20 ha (50 ac)
Take cores from at least 30 sites evenly spaced along one or more straight line paths that are representative of the sampling area. Permanently mark the fence posts opposite the end of each path so that future sampling can be carried out along the same line. In irregularly shaped paddocks or sections a landmark such as a tree, fence corner or dam can be used to identify where sampling lines cross (see Figure 3)
|Figure 3: Recommended sampling sites for areas greater than 20 ha (50 ac) - regular shaped paddocks||Figure 4. Recommended sampling sites for areas greater than 20 ha (50 ac) - irregular shaped paddocks|
2. Cores should be taken from spots of average growth or poorer spots if they are numerous. Bare ground should only be sampled if that constitutes a significant part of the paddock. Remember that you are trying to get an "average" sample that is representative of the entire area. Avoid patches of very good growth, obvious dung or urine patches, stock camps, stock tracks, fertiliser dump sites, recently grazed strips or where hay or lucerne are stored. It is best not to sample within 10 m (30 ft) of fence lines, gates, troughs or trees. Fertiliser takes a little while to be incorporated into soil so it is also best not to sample soils which have been fertilised or limed within the past six weeks.
3. When taking cores, growing plant material should be avoided by inserting the sampler tip between leaves and stems. However it is important not to remove any soil or dead plant litter from the surface before taking the core. Take the cores to a depth of 100 mm (4 in). Remove each core carefully from the sampler using a clean screw driver or similar tool and place it in a clean plastic sample bag or other suitable container. If only part of the core comes out with the sampler discard that core and re-sample the site.
4. At least 30 cores should be taken from the area being sampled. The more cores that are taken the more representative the sample will be. In small intensively grazed and regularly fertilised paddocks, such as dairy pastures and house paddocks, it is particularly important to collect as many cores as possible.
5. If you have more than the amount of sample required by the laboratory it will be necessary to sub-sample. Place the sample in a clean container, break up the clods and spread the total sample evenly on a clean bench. Divide the sample into four quarters, discard the two diagonal quarters and place the remaining sample in a clean container.
6. Transfer the cores or the subsample to a clean sample bag if you have not already done so and seal the bag. Mark the bag with the paddock or section name, the number of cores taken and the depth of the sample.
7. Your samples are now ready for dispatch to the laboratory. Be sure to provide all the information requested by the laboratory processing your samples to ensure that the best possible recommendations can be made.
Modifications to guidelines for sampling cultivated land or new pasture
When sampling cultivated land or new pasture the soil may be "fluffed-up". Ensure that you get cores that are a full 100 mm (4 in) depth by firming the soil with your foot prior to sampling.
Guidelines for sampling subsoils
Sampling of the sub-surface soil may be required where:
- a deep-rooted species such as lucerne is being established;
- deep-rooted plants are not growing well; or
- salt is suspected of being a problem.
It is recommended that at least two subsoil depths are always sampled. Examples of the depths to sample are presented in Figure 5. Each composite sample should consist of soil cores from at least 15 sites evenly distributed across the paddock.
Where the depth to the clay layer in a soil varies, sample to the top of the clay layer and then sample the clay layer as a separate sample.
When collecting subsoil samples be careful not to contaminate them with scrapings from other soil layers. Because the chemical properties of soils vary dramatically with depth, even minor contamination of subsoil samples can make the interpretation of chemical results very difficult.
Figure 5: Recommended subsoil sampling depths - soils showing a marked and soils showing a gradual change in colour or texture.
It is recommended that soils are sent to a certified laboratory for analysis. A list of certified laboratories is available from the Australasian Soil and Plant Analysis Council (ASPAC).
Contact us for more information on soil analyses and sampling.
This Agnote was developed by David Nash and Austin Brown, May 1998.
It was reviewed by David Nash, May 2009.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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