Soil Analysis Preparation

Lachlan

Lachlan

— 2 min read
Picture of a soil sampling rig on the back of a ute taking a core from a field of harvested wheat at sunset.
Taking soil from the field is just the first step.

Following on from my previous article on Soil Sampling, the next step is preparing the sample for testing. This requires a good mixing of the sample to avoid skewed results and grinding the sample down to a fine mixture of less than 2mm particle size to make the testing as consistent and accurate as possible.

Before grinding samples they are recommended by the lab we use to oven dry at 50 degrees Celsius. This is to ensure the removal of all moisture for cleaner grinding and sieving but also nitrogen in the sample can get lost at high temperatures affecting the results. Seeing the resulting sample to be sent, it may not seem like shipping would be an issue but sending more than 400 samples to the other side of the country can also get expensive on top of the analysis costs. Drying the sample removes that water weight and can reduce costs on both ends as the lab will likely charge for this preparation.

Below is a series of five photos showing the progression of soil preparation and grinding to be sent away for testing.

Soil in aluminium tray after drying.

Photo 1 is an example of the soil straight out of the bag it was placed in from coring. Notice the larger sized peds and soil pieces that will have to be broken down and the residual plant material including leaves and roots that need to be removed. This was a sample from two 0-10cm cores that were combined or 'bulked'.

Soil in aluminium tray after being put through grinding machine.

Photo 2 is the result of that soil simply being placed in the grinding machine set to approximately 2mm. Notice the plant material is still there and the larger portions have been broken up into a course mix.

2mm sieve with soil and plant material remaining after a sample was sieved.

Photo 3 shows how the sieve removes any remaining particles larger than 2mm and any large pieces of plant material.

Soil in aluminium tray after sample was ground and sieved.

Photo 4 shows that final sieved sample and the goal of how fine it ideally should be for sending to a testing lab.

Prepared soil in small plastic ziplock bag ready for labelling and shipping.

Photo 5 shows an approximate 70g sample placed in a small ziplock bag ready to be labelled and sent away to a lab. The amount you will need to send depends on the lab and what tests you require.

Note as well this is soil preparation for scientific research purposes to ensure consistent results. When looking at a testing labs soil preparation guidelines they will have a minimum weight for dried or moist soil.

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