Soil Testing

Why do I need soil testing?

At Nutrilab we understand how important the accuracy of results is but also how easy it is to obtain. We work closely with our customers to ensure we offer appropriate services as well as an easy-to-use system. Many other laboratories have complicated processes for sample receipt and reporting. At Nutrilab we accept digital order forms as well as have them available at our sample reception. We are always able to help you choose the testing that suits your needs and talk you through the process.

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When to collect soil samples for testing

The main areas to consider when sampling is the timing, placement, depth, number of cores, handling and storage. The timing of soil sampling is vital to ensure successful assessment of the soil nutrient status and chemical parameters. There is high temporal variation in several analytes illustrating marked changes in some nutrient levels when tested at different time frames throughout the year. For this reason testing is more effective if performed at the same time each year.


How to take the Sample

Before commencing soil sampling ensure all equipment is clean and free of contamination. You should use a soil probe either a hydrolic or a hand held device. For most soil types if you are after a deep soil analysis you will need the use of a hydrolic probe.

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Placement

When considering areas to sample there are several aspects to consider, most importantly time and cost. To achieve a detailed analysis of the soil profile throughout the farming area, you would need to sample every production unit and soil type; however, cost and time make this often unsuitable. To ensure the results received are a correct and accurate guide, the samples need to be an adequate representation of the conditions in the field under consideration. This means the sample must be typical to the section of the paddock being tested, adequately represent the soil type and be at a depth that is able to be used for the plant’s needs. As soils are not static and are inherently variable there is a large area for difference within a cropping region. Within one farm there will be paddock, horizon, vertical and lateral variation. A decision needs to be made on how the results from testing will be used and from this a sampling plan needs to be determined.


There are a number of different designs or patterns that are commonly used for soil sampling. The type of pattern used is dependent on why the soil is being tested and the environmental conditions of the field. The standard patterns can be seen below, these are zig-zag, transect (with permanent markers at either end), random and systematic.

Number of Cores

To obtain a representative soil sample a number of soil cores must be taken from within a uniform area.  For surface soils the number of cores depends on the parameters being tested.  10 to 20 cores have been shown to be enough when testing for pH, electrical conductivity, and exchangeable cations but 20 to 30 is needed for extractable phosphorus and Potassium.  For accurate representation it is recommended to collect a large number of cores (30-40), mix thoroughly together and take a random sub-sample to send for analysis. 

Depths

To provide accurate information in the critical root zone, the recommended sampling depths have been calibrated for different crops.  The surface or topsoil analysis is often used for all nutrients and determination of starter fertiliser where as the subsoil gives an indication into sodicity, salinity, nitrate and chloride.


Pasture, cereal, oilseed, grain legumes and rain-fed cotton are sampled at 0–10 cm for surface and 10–100 cm for subsoil testing.  Irrigated cotton is sampled at 0-30 cm for surface and 30-100 cm for subsoil testing.  Subsoil sampling commonly only reaches 90cm in depth and is normally divided into multiple stages for more accurate interpretation.

Sampling Do’s and Don’ts

  • DO divide the field into homogenous groups based on soil type, crop growth, topography and management history.
  • DO clean equipment and hands before starting collection.
  • DO clean equipment between subsequent sampling.
  • DO place sample in a clean plastic bag.  These are available from Nutrilab FREE of charge.
  • DO collect sufficient volume for analysis (approx. 500g).
  • DO store samples at a cool temperature, preferably in the refrigerator or cool room.  An esky with cool bricks or ice is suitable if in transit.
  • DO document each sample site; ensuring the soil samples are stored in a plastic bag labelled with the site location and date.
  • DO avoid potential contamination.  Be aware of everything that comes into contact with the sample.  For example don’t use the same bucket for fertiliser as for soils or drop cigarette ash on samples.
  • DON’T sample unusual areas (unless for comparison purposes) including stock camps, corners of paddocks which have been cultivated, poorly drained areas, near irrigations channels, near tail drains, near old stumps or trees, excessively we areas as well as within 10 – 20 meters of current    and  old fence lines.
  • DON’T sample areas of extremely poor growth or excessively good growth (unless for comparison purposes).
  • DON’T overuse lubricant when collecting – this can interfere with extractions of certain soil chemical parameters.
  • DON’T handle samples in hot weather with bare hands.
  • DON’T sample within 2 months of fertiliser application or 3 months of liming.

Soil is needs to be dried at a low temperature to ensure the nutrient concentrations do not change.  Excessive heat can alter the solubility of certain nutrients, affecting the measurement of nutrient availability.  Nutrilab conducts this process during the weekend.  The soils are placed in the oven on a Friday (sample submission closed at 12.00 Friday), crushed on a Monday and analysed during the week with reports sent out on the following Friday.  There may be slight variations to this timetable and we understand the need for quick results.  Some analytes such as Colwell P take a 3 day period for extraction and analysis, so while we try to ensure the quickest possible turn-around time we do have procedures to follow.  We are happy to provide earlier N results if requested and we are able to do so (a small surcharge applies if a separate extraction is needed).

Sample Handling and Transport

Once a soil sample is collected it needs to be sent to the laboratory as soon as possible.  If it will be stuck in transport over the weekend it is best to refrigerate or freeze the sample and send the following week.  Once collected, given the correct conditions biologically-induced changes may occur to the chemical properties.  To avoid this as soon as samples are collected you should keep in either an esky or cool box with cooler bricks or in a dark space in an air conditioned car. Samples are best kept in plastic bags, but if there is sufficient heat and moisture biological activity will continue to occur.  You do not need to post or courier samples in an esky but please try ensure they are not left in the sun to the best of your ability.  When first posting samples to Nutrilab it is advised to email the office so we can be on the look out for your samples. 

Turn Around Time

From sample receipt until receival of results is between 5-10 working days.  Soils need to be dried at a low temperature (maximum of 40°C) to minimise chemical reactions and biological transformations that may lead to changes in components of soil chemical values.  Dependent on the soil type and moisture content this can take up to 48 hours.  Due to the longer drying time we start the process on a Friday with results out the following Friday.  Under certain conditions this may take slightly longer, for example if we need to re-run an analyte or double check some information before reporting it, instrument servicing or unusually large sample numbers.  Any soil samples we receive will be refrigerated until they are able to be dried to ensure the nutrient levels do not change and are an accurate indication to what is present in the field.  The soils need to be prepared before drying, the cut-off of is 12:00 on Friday (or Thursday if Friday is a public holiday).  If there are any problems with paperwork or missing samples and we are unable to get in contact with you, the samples may be stored until we can confirm the information.  Any soils received after 12 will be stored until the next run.