MaxSil Farming Guides
Soil pH: Why Acid Soils Cost Australian Farms Billions, and What Silicon Does About It
Low pH releases aluminium that prunes roots. Lime fixes the pH. Silicon helps the crop cope while it works.
The scale of soil acidity in Australia
Soil acidity is one of the biggest hidden costs in Australian agriculture. The National Land and Water Resources Audit (2001) found soil acidity affects about 50 million hectares of Australian surface soils and 23 million hectares of subsoils. About half of Australia's intensively used agricultural land is acidic.
The lost production is estimated at $1,585 million a year. For comparison, dryland salinity, which gets far more attention, is estimated at $187 million a year. Acid soil is the quiet billion dollar problem.
Globally, an estimated 30 to 40% of the world's arable soils are acid. This is not a niche issue for a few unlucky paddocks. It is the norm across huge areas of Australian cropping and grazing country.
What low pH actually does to your crop
The pH number itself is not what hurts the plant. As soil pH falls below about 4.8 (measured in CaCl2), aluminium locked in clay minerals is released into the soil solution in a form that is toxic to roots. Aluminium toxicity prunes the root system: roots stop elongating, thicken and stub off, and the crop loses its ability to reach water and nutrients deeper in the profile.
A crop with a pruned root system looks like a crop short of water or nitrogen, because functionally it is. You can pour on fertiliser and the plant simply cannot access it. That is why acid soils make every other input less efficient.
The ideal soil pH (CaCl2) for most crops is 5.5 to 8.0. Below 4.8 is classed as acidic in the root zone, and that is where aluminium starts doing damage.
| pH (CaCl2) range | What happens | What to do |
|---|---|---|
| Below 4.8 | Acidic in the root zone. Aluminium toxicity prunes roots and shuts down nutrient uptake. | Lime is the priority. Silicon helps plants cope with aluminium while the lime works. |
| 4.8 to 5.5 | Marginal. Aluminium risk rising, phosphorus tie-up increasing, silicon availability falling. | Plan a liming program. Test plant available silicon. |
| 5.5 to 8.0 | Ideal range for most crops. Nutrients available, aluminium locked away. | Maintain pH. Silicon still supports stress tolerance. |
| Above 8.0 | Alkaline. Trace element tie-up (zinc, iron, manganese) becomes the risk. | Manage trace elements. Avoid over-liming. |
The acid soil and silicon connection
Here is the part most farmers have never been told: acid soils are often low in plant available silicon as well. A South African sugar industry study found that extractable soil silicon falls as soil pH falls. The same acidity that releases toxic aluminium also strips out the silicon that would help the plant cope with it.
Research shows silicon can reduce aluminium toxicity in maize, wheat, sorghum, cotton and white clover. Silicon does not neutralise the aluminium in the soil the way lime does. It helps the plant tolerate the aluminium it encounters, protecting root function while your liming program works through the profile.
That distinction matters because lime is slow. It takes months to years to move through the profile and fix subsoil acidity. Silicon is the bridge that helps the crop perform in the meantime.
Where silicon fits in a liming program
Lime is the fix for pH. Nothing else raises pH the way lime does, and on genuinely acid country, liming is not optional. But lime does not supply silicon, and acid soils are frequently silicon depleted, so the two work on different parts of the same problem.
MaxSil Powder supplies 40,000 ppm soluble silicon at a typical starting rate of 50 kg/ha, confirmed by a MaxSil agronomist after a soil test. It can be spread with your liming program or with your fertiliser, through standard gear.
Be honest with yourself about where silicon earns its keep: under stress. Acid soil with aluminium pressure is exactly the stress condition where silicon does its best work. On a perfect soil at perfect pH, the response is smaller.
How to raise soil pH, step by step
Test first. Sample to at least 10 cm, and deeper if you suspect subsoil acidity, because surface lime does not fix a sour subsoil quickly. Check pH in CaCl2, not just water, and ask for plant available silicon while you are at it, because most standard tests never report it.
Apply lime at the rate your test and target pH require, and give it time. Fine lime reacts faster than coarse. Incorporation speeds it up where tillage fits your system. Re-test every few years because acidity creeps back, driven by nitrate leaching and product removal.
MaxSil offers a free soil test analysis. Send your results and an agronomist will interpret them, flag whether silicon is likely limiting on your acid country, and recommend a rate if it is.
The proof behind the claims
- Soil acidity affects about 50 million hectares of Australian surface soils and 23 million hectares of subsoils (National Land and Water Resources Audit, 2001).
- Lost production from soil acidity is estimated at $1,585 million a year, compared with $187 million for dryland salinity.
- Silicon has been shown to reduce aluminium toxicity in maize, wheat, sorghum, cotton and white clover.
- Professor Zed Rengel (University of Western Australia) found MaxSil supplies 13 to 25 times more plant available silicon than diatomite.
Trial and grower results are reported outcomes, not guaranteed results. Your soils, seasons and management are your own.
What you get with MaxSil
Frequently asked questions
What is the ideal soil pH for pasture?+
For most pastures and crops, the ideal soil pH measured in CaCl2 is 5.5 to 8.0. Below 4.8 is classed as acidic in the root zone, which is where aluminium toxicity starts pruning roots. Lucerne and barley are more sensitive to low pH; pastures like phalaris tolerate a bit more acidity.
How do I raise soil pH?+
Lime is the standard fix. Test first (in CaCl2, and check the subsoil too), apply lime at the rate your test requires, and give it months to react. Fine lime works faster than coarse. Re-test every few years because acidity returns through nitrate leaching and product removal.
Is lime enough on acid soils?+
Lime fixes pH, but acid soils are often low in plant available silicon too, because extractable silicon falls as pH falls. Lime does not replace that silicon. Adding MaxSil alongside a liming program supplies the silicon the soil has lost and helps plants cope with aluminium while the lime works.
What is aluminium toxicity?+
When soil pH (CaCl2) falls below about 4.8, aluminium is released from clay minerals into the soil solution in a form toxic to roots. It stops root elongation, so roots become stubby and shallow. The crop then cannot reach water or nutrients deeper in the profile, no matter how much fertiliser you apply.
Does silicon change soil pH?+
No. MaxSil is not a liming product and does not meaningfully change soil pH. Silicon helps plants tolerate the aluminium and stress that acid soils create, while lime does the actual pH correction. They work on different parts of the same problem.
Can I spread MaxSil and lime together?+
Yes. They do different jobs and can go out in the same program through standard spreading equipment. Lime corrects the pH over months, while MaxSil supplies plant available silicon that helps the crop cope in the meantime.
Find out if your soil is short on plant available silicon
Free soil test analysis from a MaxSil agronomist.
Get a free soil test analysisSources
- National Land and Water Resources Audit (2001): Australian agriculture assessment
- Silicon and aluminium toxicity research (PMC)
- Frontiers in Plant Science: Silicon and plant stress tolerance review
- South African sugar industry study: soil pH and extractable silicon
Part of the MaxSil Farming Guides series.