Soil acidity is one of the most persistent challenges facing Australian farmers. National soil assessments have long shown that many agricultural soils sit at a surface pH of 5.5 or below—a level considered sub-optimal for healthy crop and pasture growth.
State agencies such as Agriculture Victoria highlight why this matters: adjusting soil pH directly influences nutrient availability, microbial activity, and even the risk of aluminium toxicity in acidic conditions.
When pH drifts too far in either direction, productivity suffers.
Because of this, understanding how assessing and adjusting soil pH has become essential knowledge for farmers working to improve soil performance and long-term resilience.
This article is divided into two practical parts;
Part 1 explores traditional pH management—how to test soil accurately, when to apply lime or sulfur for adjusting soil pH, and the common mistakes that can undermine results.
Part 2 explores the role of soil biology, beneficial microorganisms and biofertilisers in supporting nutrient cycling and soil fertility alongside traditional pH management.
Whether you’re managing broadacre cropping, pasture systems, or horticulture, this guide will help you understand both the chemical and biological levers behind adjusting soil pH.
Part 1: Traditional Soil pH Management Using Amendments
Adjusting soil pH has long been a standard part of Australian farming.
Before biological solutions became popular, most farmers relied on amendments like lime or sulfur to correct soil acidity or alkalinity. These tools are still highly effective when used correctly—and knowing how and when to apply them can prevent wasted effort and poor crop performance.
Why Adjusting Soil pH Matters
Soil pH measures how acidic or alkaline the soil is.
- A pH below 7 is acidic
- A pH above 7 is alkaline
- Many crops and pastures grow best in slightly acidic to neutral soils, although the ideal pH varies between species. Soil pH results should also specify whether they were measured in water or calcium chloride (CaCl₂), as these methods give different readings.
When soils sit outside this range, several issues occur:
- Nutrients (like phosphorus, calcium, zinc, and manganese) become harder for roots to absorb.
- Soil microbes slow down or die off in overly acidic conditions.
- In acidic soils, aluminium becomes soluble, which can damage young roots and reduce yield.
How to Test Soil pH Properly
Before adjusting soil pH, testing is essential. pH can vary across a paddock and even within small areas, so relying on guesswork is risky.
Common Testing Methods
- pH test kits (field-based): Quick and useful for spot checks, but less accurate.
- Laboratory soil tests: Some laboratory soil tests include measurements of buffering capacity or lime requirement. These help determine how much amendment may be needed to change soil pH.
- Digital pH probes: Convenient but must be calibrated regularly.
Why Buffer pH Matters
Lab reports often include buffer pH. This indicates how much amendment is required to change soil pH.
- Low buffer capacity: Soil pH changes easily.
- High buffer capacity (common in clay soils): Requires more input to shift pH.
Understanding these numbers prevents under- or over-correcting.
Using Lime to Raise Soil pH
Why Lime Works
Agricultural lime (calcium carbonate) is the most common amendment used to neutralise acidic soils. When added to the soil, lime reacts with hydrogen ions—one of the main contributors to acidity—and reduces their concentration.
The result: pH gradually increases toward a healthier, more productive range.
Types of Lime
1. Agricultural Lime (Ag Lime / Calcium Carbonate)
Origin: Crushed natural limestone (calcium carbonate).
Behaviour:
- Slow-acting, as it requires moisture and reacts gradually with soil acidity. Finer particles generally react faster.
- Long-lasting—effects can persist several years.
- Gentle on soil biology; ideal for regen systems.
Best for:
- Most broadacre scenarios
- Long-term pH correction
- Soils already adequate in magnesium
- Farms favouring biological stability
Regenerative note: Aglime integrates well with biological amendments. It buffers pH gradually, allowing microbes and fungi to adapt without shock.
2. Dolomitic Lime (Calcium + Magnesium Carbonate)
Origin: Natural dolomite rock containing both calcium carbonate and magnesium carbonate.
Behaviour:
- Similar speed and longevity to aglime
- Adds magnesium, which is helpful only if your soil is deficient
Best for:
- Sandy soils that leach magnesium
- Soils showing low Mg on the soil test
- Farms needing both pH correction and Mg replacement
Caution: Dolomitic lime supplies both calcium and magnesium. Where magnesium levels are already adequate, agricultural lime may be more appropriate. Base the choice on soil test results and the quality of the available lime.
Overuse may interfere with soil aggregation, especially in heavy clays.
3. Hydrated Lime (Calcium Hydroxide)
Origin: Limestone that has been heat-treated (calcined) to create quicklime, then reacted with water to make hydrated lime.
Behaviour:
- Very fast-acting, almost immediate pH rise
- Highly caustic—can harm soil biology and roots if misused
- Not typically used in broadacre or regenerative systems
Hydrated lime is highly reactive and caustic, making it unsuitable for routine broadacre soil pH management. Agricultural lime is generally the preferred option for correcting soil acidity.
Caution: High risk of ‘burning’ biological life. Not aligned with long-term regen principles unless used sparingly and for a specific purpose
Key Factors Affecting Lime Effectiveness
- Soil texture: Clays require more lime than sandy soils.
- Moisture: Lime reacts better in moist conditions.
- Particle size: Finer lime dissolves faster.
- Incorporation: Lime works more effectively when mixed into the topsoil, though surface applications can still help over time.
Using Sulfur to Lower Soil pH
When Sulfur Is Needed
While many Australian soils trend acidic, some regions—especially irrigated areas or calcareous soils—can become alkaline (pH > 7.5). In these cases, sulfur can help.
How Sulfur Works
Elemental sulfur is converted by soil microbes into sulfuric acid.
This gradual acidification can lower soil pH, although the extent of the change depends on application rate, soil type and conditions.
Important Notes
- The process requires microbial activity, so warm, moist soils work best.
- It takes time—weeks to months depending on soil conditions.
- Overuse can lead to excessive acidity, so testing and monitoring are essential.
Common Mistakes in pH Adjustment
1. Applying amendments without testing
Guessing leads to wasted inputs or overcorrection. Always test first.
2. Expecting instant results
Both lime and sulfur take time. Lime may take months to fully react, and sulfur requires microbial activation.
3. Ignoring soil texture
Clay soils resist change. Sandy soils shift quickly. Knowing this helps set realistic expectations.
4. Only treating the topsoil
Acidity often exists below the surface, especially in no-till systems where acidity can “stack up” at depth.
5. Forgetting the biology
Traditional amendments adjust pH chemically, but soil biology also plays a major role—something we explore in Part 2.
Part 2: How Soil Biology Supports pH Management
While lime and sulphur remain important tools for correcting soil pH, soil biology also plays a valuable role in maintaining soil fertility.
Soil microorganisms interact with minerals, organic matter and plant roots, influencing nutrient availability and cycling. These biological processes can affect acidity in localised areas of the soil, although they don’t necessarily correct or stabilise overall soil pH.
Supporting healthy soil biology is therefore an important part of soil management, working alongside appropriate soil testing and pH amendments.
How Do Soil Microbes Influence pH?
Soil microbes continuously interact with minerals, organic matter, and plant roots. These interactions create natural buffering processes that help pH stabilise. Here are the key ways they do it:
1. Microbes Produce Organic Acids
Beneficial microorganisms, including lactic acid bacteria, produce organic acids and other compounds as part of their metabolism.
These compounds can influence nutrient availability and mineral interactions within the soil, particularly around plant roots.
Their effects depend on soil conditions, microbial activity and the compounds produced. While these processes contribute to nutrient cycling, they don’t necessarily prevent changes in overall soil pH.
2. Microbes Break Down Organic Matter Into Stable Compounds
As microorganisms break down plant residues, compost and other organic materials, some of that carbon becomes incorporated into more stable forms of soil organic matter. These can contribute to soil structure, nutrient retention and buffering capacity.
Humus does several things:
- Improves soil structure
- Supports nutrient cycling
- Increases soil’s cation exchange capacity (CEC)
- Helps soils resist sudden shifts in pH
CEC is particularly important—soils with higher CEC can hold onto nutrients and hydrogen ions better, which keeps pH more stable over time.
3. Microbes Support a Functional Microbial Community
A diverse microbial population is more resilient to stress and can adapt to fluctuations in moisture, temperature, and nutrient availability. When microbial ecosystems are stable:
- Organic matter decomposition and nutrient cycling can become more efficient.
- Different microbial groups contribute to nutrient transformations and interactions with plant roots.
- Diverse soil communities may be better able to respond to changing environmental conditions.
How Biofertilisers Support Soil Biology
Biological fertilisers can provide nutrients, organic compounds and beneficial microorganisms that interact with existing soil biology.
Fermented fish and seaweed biofertilisers contain compounds formed during fermentation that may contribute to microbial activity and nutrient cycling.
Their effects will vary depending on soil conditions, application rates and existing biological activity.
Why Biological Fertilisers Work Best With Organic Amendments
Soil microorganisms require suitable carbon sources, nutrients and environmental conditions to grow and function. Organic inputs such as fish, seaweed, compost and molasses can provide different forms of nutrition for soil microorganisms.
The benefits will depend on the material used, application rates and existing soil conditions. These inputs may support microbial activity and nutrient cycling, but they shouldn’t be relied upon solely to correct soil pH.
Organic Amendments that Support Soil Biology
1. Fish
Fish fertilisers, such as liquid fish hydrolysate or fish extract, provide a balanced carbon source and amino acids that microbes quickly utilise. SONIC Liquid Fish Hydrolysate combines ocean fish with beneficial soil microbes making it a mineral-rich power house for soil fertility and crops.
2. Seaweed and Kelp
Seaweed contains a full spectrum of minerals and trace elements as well as naturally occurring plant growth-regulating compounds that may influence plant growth and root development. For a premium product that combines ocean kelp and beneficial microbes, see SONIC Liquid Seaweed Biofertiliser.
3. Compost
Good compost is rich in humic substances. It increases CEC (cation exchange capacity), feeds microbes, and can contribute to soil organic matter, nutrient retention and buffering capacity over time.
4. Mixed Cover Crops
A variety of roots exude a variety of organic compounds that feed microbes and help regulate nutrient availability, indirectly influencing pH.
When to Focus on Supporting Soil Biology
Biological pH management is especially useful when:
- pH is close to the desired range, and you want to maintain stability
- You’ve applied lime or sulfur and want to extend the benefits
- You’re working in a regenerative or low-input system
- Soil organic matter is low and needs rebuilding
- You want to reduce long-term reliance on chemical amendments
Conclusion
Managing soil pH is an important part of maintaining productive Australian soils. Lime and sulphur can be effective tools for correcting acidity or alkalinity when used appropriately, while healthy soil biology supports nutrient cycling, organic matter breakdown and soil fertility.
Biological inputs such as compost, fermented fish and seaweed biofertilisers can complement these practices by providing nutrients and organic compounds that may support soil microbial activity.
The best approach is to consider soil chemistry and biology together, using soil testing to guide pH corrections while building the conditions for a healthy, productive soil ecosystem.
Disclaimer:
The information in this article is for general guidance and not professional advice—always consider your individual circumstances or consult with a professional before making decisions. For more details, please review our full Disclaimer.
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