Soil Test: The Number Most Australian Soil Tests Don't Report (Silicon)

What a standard soil test covers, and what it misses

A standard Australian soil test reports pH, phosphorus, potassium, sulfur, organic carbon, cations and maybe some trace elements. It is a useful snapshot, and if you are not testing regularly, start. But there is a number that almost never appears on the report: plant available silicon.

That is a strange omission when you think about it. Silicon is the second most abundant element in the earth's crust, and plants take it up in quantities comparable to the major nutrients. The reason it is missing is historical: standard labs were never set up to measure it, so nobody asked for it, so the labs never added it.

The result is that most Australian farmers have no idea whether their soil is short of silicon. And the data we do have suggests many are.

How silicon is measured, and what the numbers mean

The standard test for plant available silicon uses 0.01 M calcium chloride (CaCl2) extraction. Australian sugarcane research by Berthelsen classed the results: very low is 0 to 5 mg/kg, low is 5 to 10, limited is 10 to 20, and sufficient is 20 to over 50 mg/kg.

A 2025 study of Western Australian farm soils through Murdoch University found 74.9% of samples were below 20 mg/kg CaCl2 silicon, and 88.2% were below 40 mg/kg. In other words, three quarters of the WA farm soils tested were below the sufficiency threshold used in the sugar industry.

Tissue testing tells the other half of the story. In Australian sugarcane research, leaf silicon around 0.53% matched 95% relative yield. Below that, the crop was leaving yield on the table.

How to take a soil sample properly

A soil test is only as good as the sample. Here is how to take one that means something:

• Sample at two depths: 0 to 10 cm and 10 to 20 cm. Surface and subsurface tell different stories.
• Take 20 to 30 cores per paddock zone. One core tells you about one spot, not the paddock.
• Avoid gateways, stock camps, fence lines and old burn piles. They skew the results.
• Sample in a consistent pattern across the zone, and keep different soil types in separate bags.
• Bag and label each sample clearly with paddock name, depth and date.
• Test at the same time of year each time so results are comparable year to year.

What to do with your results

If your test comes back below 20 mg/kg CaCl2 silicon, your soil is likely short of plant available silicon by the sugar industry classification. That is the point where a silicon fertiliser is most likely to pay.

MaxSil offers a free soil test analysis. Send your existing test results, even if silicon is not on them, and a MaxSil agronomist will interpret what you have, tell you whether silicon is likely to be limiting, and recommend a rate if it is. MaxSil Powder typically starts at 50 kg/ha, with the rate confirmed for each paddock after the soil test.

Once you have applied, MaxSil recommends a tissue test about 2 weeks before harvest to measure the response. That is how you turn a hunch into a number you can bank on for next season.

Why silicon pays off most under stress

One honest caveat before you test: silicon is a beneficial nutrient, not a magic bullet. The research consistently shows it pays off most under stress: drought, heat, salinity, acidity, pest and disease pressure. If your soil is short of silicon and your seasons are kind, the response will be modest. If your soil is short and the season turns nasty, silicon is the difference between a crop that holds and a crop that folds.

That is exactly why the soil test matters. You want to know your silicon status before the stress arrives, not after.