Guideline Index

Chapter 8: Assessing Soil Nutrients

8.3 Suggested soil sampling guidelines

The standard sampling procedures outlined below are designed to minimise the effects of soil variation and to help you collect a representative sample.

8.3.1 Timing of soil sampling

As soil nutrient availability can vary throughout the year due to changes in temperature and moisture, it is best to:

  • sample at the same time each year,
  • sample at least 6 weeks prior to planting, and
  • avoid sampling within 3 months of liming or 2 months of applying fertiliser.

There is not one particular time of the year to soil sample, as it will vary depending on location, seasonal conditions, the type of crop or pasture being planted, growth flushes, and the nutrient that it is intended to apply.

8.3.2 Selecting areas for sampling

In recent years, dairy farmers have been changing their choice of fertilisers, with an increasing use of organic and potassium fertilisers. Efficient use of fertilisers requires an awareness of environmental and soil conditions. In order to achieve increased nutrient use efficiency, dairy farmers are placing a greater reliance on independent consultants to assist with interpreting soil test results and prepare nutrient budgets. Deciding which field to sample for the nutrient budget should be carefully considered so as to:

  • provide a strategy for the management of fields including future rotations, and
  • deliver objectives and optimal soil fertility.

A common strategy is to sample a representative paddock from a farm management zone (FMZ) see Chapter 15 .4.1.

From these zones, select a number of representative ‘monitor’ paddocks to sample. These monitor paddocks can be sampled regularly over time, following the same transect, to determine if the farm soil fertility is changing. Initially, these areas may need to be sampled every 1 to 2 years while in the development stage of soil fertility. Once the maintenance stage is reached and you are confident that your fertiliser strategies are meeting the maintenance requirements of your farm, the sampling interval could increase to every 2 to 3 years (See Chapter 1.7 ). It may then be possible to rotate the areas tested each year so that the soil fertility is monitored on other parts of the farm.

Soil samples can vary greatly in their ability to truly represent the area being tested, even when taken and analysed correctly. For example, a very wide variation in nutrient levels can exist between paddocks and within paddocks. Figure 8.1 shows the variation of Olsen P (estimated from Colwell P data) within one paddock that was soil tested on a gridline basis, with 20 samples taken randomly around each grid point. Each number in the figure indicates a grid point and the Olsen P around that grid point. As you can see, the Olsen P measurements ranged from 8 to 40 mg/kg.

Figure 8.1   Variation of Olsen P (mg/kg) within one paddock.   Source: State Chemistry Laboratory (Macalister Research Farm)
Figure 8.1 Variation of Olsen P (mg/kg) within one paddock.
Source: State Chemistry Laboratory (Macalister Research Farm)

 

To minimise variation within paddocks and between times of testing, transect sampling is recommended.

Transect sampling means sampling the same path through the paddock each time you do a soil test. Recent experiments have shown that this technique can substantially reduce the variation in the soil test results. Permanently mark the fence posts opposite the end of each path so that future sampling can be carried out along the same line, or use a GPS unit. In irregularly shaped paddocks or sections, a permanent landmark, such as a tree, fence corner or dam, can be used to identify where sampling lines cross – see Figure 8.2.

Figure 8.2   Recommended sampling sites for transect sampling
Figure 8.2 Recommended sampling sites for transect sampling

In addition to transect sampling of representative paddocks over time, soil testing is also used to investigate specific problem areas. In these cases, take the sample from the problem area or poor section of the paddock. This is referred to as poor patch sampling. It is extremely useful to take a sample from a nearby area that is representative of good pasture so that results can be compared to the ‘poor patch’ sample. If the soil test results are similar for both the poor area and the good area, then the problem may be related to some other factor, such as a trace element deficiency (which can be tested via a plant tissue analysis; see Section 8.4), disease or insect pests, lack of suitable pasture species or inadequate drainage or pugging.

8.3.3 Soil sampling depth

Soil nutrient levels vary with depth and usually reduce in concentration as you go deeper. It is critical that soil cores be collected to a standard sampling depth, if results are to be interpreted reliably.

The standard surface soil sample depth for pasture, cereal, oilseed, grain & legume crops is 10 cm, except Tasmania where the standard depth for pasture is 7.5cm and 15cm for field crops.

Optimum soil test values vary with depth due to a concentration of some nutrients such as phosphorus and potassium near the soil surface. Subsurface samples (taken from a depth of 10 to 60 cm below the soil surface, in 10 cm increments) may need to be taken for such problems as:

  • Salinity.
  • Poor structure.
  • Sulphur deficiency.
  • Subsoil pH.
  • Aluminium toxicity.

Subsurface soils are usually sampled using an auger or hydraulic soil coring machine to remove soil at depth. For phosphorus in particular, it is essential to minimise contamination by the top 10 cm of soil, whilst inserting the auger to collect samples at the deeper profile depths. This is because phosphorus is concentrated in the surface soil where it is applied as fertiliser and dung. After the top 10 cm has been collected, a hole wider than the auger circumference can be dug before collecting the 10-20 cm profile depth with the auger, thereby minimising surface soil contamination. If required, this process can be repeated for subsequent depths.

Correct soil sampling equipment such as hand augers, foot probes, and hydraulic cores, or soil testing services are available from fertiliser companies, private consultants and dedicated soil testing services. Using other sampling tools, such as spades or galvanised pipe is not preferred due to possible contamination or inaccurate sampling methods. In the case of galvanised pipe, the galvanising can affect the soil test results.

8.3.4 Taking core samples

Take cores from at least 30 sites for surface tests and from at least 15 sites for subsoil tests.

The samples should be evenly spaced along one or more straight-line or zig-zag paths that are representative of the sampling area. Cores should be taken from spots of average or below-average growth. 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 waterlogged or pugged soil, obvious dung or urine patches, stock camps, stock tracks, fertiliser dump sites, recently grazed strips, and silage or hay storage or feedout areas. It is best not to sample within 20 metres of fencelines, gates, troughs or trees. Remember, if 3 or 4 cores in a 30-core sample are from urine patches, it can cause the potassium soil test result to be substantially higher than it should be. Take samples from sacrifice paddocks and those that are soon to be grazed before they are grazed, so you can avoid the urine and dung patches more easily as you can see the extra growth – See Figure 8.3.

Figure 8.3   Urine and dung patches should be avoided when sampling soils. The prevalence of these is a common indicator of nutrient deficiencies in other areas of the pasture.   Source: http://www.caes.uga.edu/Publications/pubDetail.cfm?pk_id=7780
Figure 8.3 Urine and dung patches should be avoided when sampling soils. The prevalence of these is a common indicator of nutrient deficiencies in other areas of the pasture.
Source: University of Georgia, 2011

 

When taking cores, avoid growing plant material by inserting the sampler tip between leaves and stems. Remove each core carefully from the sampler, using a clean tool or fingers, and place the core in a clean container such as a plastic bucket. Discard any partial cores and resample near that site.

If the 30 cores weigh more than about 1 kg, thoroughly mix the cores in clean containers and then take a subsample of about 0.5 kg for mailing to the laboratory.

Transfer the cores or the subsample to a clean sample bag and label the bag with the paddock (or area) name and the number of cores taken. Fill in as much detail as possible when completing the paddock information form to ensure that the best possible recommendation can be made. This information is crucial for the person who will interpret your soil test results and formulate a fertiliser strategy for a paddock or area. Labels should be written on paper or cardboard labels with an indelible pen and attached to the outside of the sample bag. Use a second bag to protect the label from being rubbed during transport to the lab. Never put a label in with the sample because it will quickly deteriorate and become unreadable. Your samples are now ready for posting to the laboratory.

Exercises 4, 5, and 6 will help you understand how to choose representative areas to sample and how to take transect samples. Download Exercises 4 to 6.