Soil sampling guide

To achieve accurate simulations, there are two important things that Yield Prophet needs to know about the soil in your Yield Prophet paddocks;

  1. How much water can your soil hold? (plant available water capacity, determined by soil characterisation)
  2. How much water and nitrate is currently in your soil? (determined through an annual soil test)
Soil characterisation

An appropriately measured soil characterisation is an essential input for Yield Prophet to accurately simulate crop growth and yield.

The plant available water capacity (PAWC) and bulk density of a specific soil type are used to determine how much of the measured water and nitrogen is available to the crop for growth during the season. PAWC is determined by a soil’s ‘drained upper limit’ (DUL, or field capacity) and its ‘crop lower limit’ (CLL, similar to permanent wilting point).

Yield Prophet has a catalogue of approximately 500 soil characterisations for many of the major cropping regions throughout Australia. The locations of these characterisations are available for download in google maps here (new window). However, many subscribers have soil types for which there is no available measured characterisation data. In these circumstances, a soil characterisation is estimated based on soil type and previous rainfall and crop yields, and any information available from existing soil surveys. An estimated characterisation is less likely to produce accurate results in comparison to a measured characterisation, and it is recommended that potential subscribers to Yield Prophet consider characterising their soil if no appropriate data exists.

If you would be interested in having your soils characterised or would like more information please feel free to contact Tim McClelland for further information. Alternatively, if you would like to understand more about the soil characterisation process, view a 36-page guide to the methodology of estimating plant available water content (PDF 2.1MB, new window).

Soil sample

There are three basic steps to obtaining initial conditions for Yield Prophet:

  1. Sampling – taking soil cores from the paddock;
  2. Sample preparation – preparing and packaging soil samples for accurate analysis; and
  3. Sample analysis – measuring the amount of water, nitrate etc in the soil.

If your Yield Prophet paddocks are located in north-west Victoria, BCG can sample, prepare and analyse your soil for Yield Prophet initial conditions. For further details contact the BCG office on 03 5492 2787 or email.

If you are elsewhere in Australia, you or a contractor will have to take and prepare your own samples and you will need to get a soil lab to analyse your samples. Prices and instructions in sending soil samples to CSBP and Incitec Pivot.

Discuss Yield Prophet soil sampling with your agricultural consultant – you may be able to combine it with regular deep N, phosphorus and moisture testing.

What soil analysis do I need?

If your paddock has not been in Yield Prophet before you will need to obtain the following information through soil sampling:

  1. Gravimetric or volumetric soil water content
  2. Soil nitrate (NO3) and ammonium (NH4) concentration
  3. Soil EC (electrical conductivity)
  4. Soil chloride (Cl-) concentration
  5. Soil pH (CaCl2)
  6. Soil organic carbon content (top soil only)

If you paddock has previously been included in Yield Prophet and measurements have already been taken for EC, chloride, pH and organic carbon, you will only be required to get measurements for soil water, nitrate and ammonium.

When should I be sampling?

Before sowing! March-April is ideal, so finish reading this document, get anything you don't understand clarified and go and begin sampling! The earlier it’s done, the earlier you will get your results back and the earlier you can log on to Yield Prophet to maximise its use and value.

At what depths should I be sampling?

Yield Prophet models the growth of your crop's roots during the growing season, so knowing where the resources (water and nitrogen) are located in the soil profile is essential for accurate simulation of your crop. Therefore, you need to measure initial conditions in at least four different layers down the soil profile eg 0-10, 10-40, 40-70 and 70-100cm depth. These are the depth intervals used by BCG. It is not recommended that you use less than four layers, and you can have more layers if you like (eg. 0-10, 10-25, 25-40, 40-55, 55-70, 70-85, 85-100 cm). This will increase model accuracy, but is more expensive to sample and analyse. You should sample down to the maximum potential rooting depth of your crop (which is probably at least 1m) because Yield Prophet needs to know the total amount of water and nitrogen potentially available to your crop for an accurate simulation.

How do I take my soil samples?

Sampling for Yield Prophet initial conditions involves taking a number of soil cores from across your Yield Prophet paddock, splitting them into the depth intervals described above, bulking the separate intervals and preparing and packaging them for analysis.

Soil sampling can be difficult and time-consuming work if you do not have the correct equipment, and accuracy of analysis can be jeopardised by inappropriate sampling methods. Soil sampling contractors (eg. fertiliser companies, resellers, agronomists, farming groups, Departments of Ag) operate in most districts and are an easy, cost-effective way of getting your paddocks sampled. However, it is important that you tell contractors exactly what you are sampling for, as they will often not be used to sampling for soil moisture or at as many depth intervals as required by Yield Prophet. Give them this document to read and if you or they have any doubts or queries about what is required, or need anything explained, please contact Tim McClelland.

Where in my paddock should I take cores?

You need to make sure that your soil sample is representative of your Yield Prophet paddock. Ideally, Yield Prophet paddocks should be of a consistent soil type, but all paddocks have variation to a certain degree, so you must ensure that your soil cores come from a region of the paddock that is consistent in soil type and covers the most area in that paddock. (If you have a paddock with significant variation in soil type, you should consider each soil type as a separate 'paddock' in Yield Prophet and sample them individually). ‘Averaging’ soil types does not work!

How many cores should I take?

The more cores you take, the more accurate the measurements will be. You need to take at least six cores across a paddock to obtain accurate results. Once cores have been taken, they are usually combined (bulked) according to the depth interval they are taken from and lumps broken up and soil thoroughly mixed before samples (few hundred grams) are taken and bagged ready for analysis.

It is a good practice to record where you took your cores from within your paddock, a rough map or description will do - or record the actual points with a GPS if you have one. This will allow you take soil samples from the same places in future creating sample consistency from year to year.

How do I prepare my sample?

It is very important that you label your samples CLEARLY with:

A permanent (waterproof) texta on zip-lock bags works well. It may also be advantageous to mark your sample bags prior to going to the paddock. When handling samples, always keep in mind what you are trying to measure in the soil and how that will change as you take it out of the paddock and handle it. Water will start evaporating as soon as a core is taken from the ground, so work quickly and get the samples into sealed plastic bags (zip-lock bags available from supermarkets are ideal) as soon as possible. Minimise further evaporation and changes in nitrate levels by keeping samples cool - take an esky out in the field, and store them in the fridge when you get them back home.

If you are posting the samples for analysis (particularly soil water analysis) make sure they are very well sealed in durable plastic bags (again, zip-lock bags are ideal, use a couple). Post them by Express Post to minimise the amount of time in which they can dry out, and post them early in the week so they are not sitting around in a post office over the weekend.

Sample analysis

Soil sampling contractors can often organise soil chemical analysis for you, but make sure you specify the measurements you need (water, nitrate, ammonium, EC, pH, chloride, and organic carbon in the 0-10cm layer) and that they need to be done for all depth intervals (except organic carbon).

For further information...

Please contact:

Tim McClelland
Yield Prophet Coordinator
Phone: (03) 9882 2670
Fax: (03) 9882 0996
Mobile: (0400) 090 659
Email: yieldprophet@bcg.org.au