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Soil Sampling

Essential Nutrients | Toxicities and Deficiencies | Soil pH | Nitrogen Cycle

Collecting soil samples for testing is more complex than one may think. It is important that natural variability across the paddock and temporal trends are considered, plus the accuracy required of the test being completed. The number of samples you need depends on the test being conducted, the area the test result will represent, depth of sampling, cost of analyses, and most importantly, why you are sampling and testing. If sampling is not completed correctly and without contamination, the laboratory results may provide data unsuitable for making farming decisions.

Samples must not be taken on areas such as stock camps, fence lines, headlands, table drains, waterlogged areas, etc. It is also advised that samples are not taken with three months of liming, or two months of applying fertiliser.
Soil sampling can be done for one-off measurements to optimise inputs (e.g. fertiliser), or monitoring of changes in your soil over time. Whether you are sampling for a once-off, or to monitor, greatly alters the strategy you should have to sampling your soil.

When sampling for monitoring you need to record:

  • Detail description of the location/s of sampling point/s (accurate GPS)
  • Pattern of sub-sampling around the monitoring location (Intensity considerations: appropriate to the knowledge of spatial variability of key parameter under investigation)
  • Sampling depth/s
  • Sub-sampling method
  • Time of year/ rotation, time after rainfall, time after break of season, soil moisture, soil temperature, etc
  • Management information - pasture composition, stocking rate, fertilisers, amendments
  • Description of post sampling storage and handling procedure
When monitoring, the more strict standard sampling protocols for input optimisation need not be followed unless the laboratory data is to be used for this purpose also. When monitoring, the method consistency between times of sampling are paramount. The selection of time, depth and other factors is dependent solely on the parameter or process to be monitored. For example, soil organic carbon is generally measured from 0-100 mm, but if you wanted to measure in the 0-50 mm layer in zero-till system then it is essential that the same depth is measured in subsequent sampling events.

Monitoring is a self-calibrating system with each new sampling date adding to the body of information against which subsequent samplings are measured. Graphical representation of the data is frequently very useful in the interpretation process.

Sample hygiene, storage and transport to the laboratory are also very important things to consider. It is important that samples are not contaminated during sampling (e.g. fertiliser dust in sampling bags, sunscreen on the samplers’ hands, samples for organic carbon put into paper bags, etc), and that samples are stored in a cool environment and transported to the laboratory within 48 hours of collection.

For step-by-step instructions on how to take a soil sample for crops and pastures.

For step-by-step instructions on how to take a soil sample for horticulture.
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