Are You Applying Lime or Just Expensive Sand?
or: When it comes to Lime, size matters!
by Roger Martyn
After evaluating my home garden, I concluded that lime application would benefit the soil. I purchased a 20 kg bag of lime from a local farm supply store, of which the product packaging advertised as being for agricultural and gardening use. However, upon applying it, I noticed the product resembled limestone sand rather than high-quality lime. Approximately 20% of the material contained particles around 1.5mm. That portion of my purchase at least would have little to no impact on adjusting soil pH or delivering any long-term benefits even to my children's grandchildren!. If I were a farmer receiving a truckload of lime like this, I would be justifiably upset.
Why Apply Lime?
Lime is a crucial input for improving soil fertility and pasture production. It supplies calcium, an essential nutrient that enhances soil structure, retains moisture, and supports soil microbes, plants, and soil-dwelling organisms. Lime also raises soil pH to optimal levels, ensuring key nutrients—nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, copper, zinc, molybdenum, and boron—are chemically in their most available form.
In short, lime acts as a fertilizer in its own right.
The Importance of Agricultural Lime Quality
To be effective, agricultural lime must be finely ground. Lime is primarily composed of calcium carbonate (CaCO₃), obtained from grinding natural limestone deposits. While this material is generally affordable and widely available, its effectiveness depends on its particle size, regardless of purity (% calcium carbonate) or chemical reactivity test result (the latter of which is very much a function of particle size).
Agricultural lime quality can be determined by assessing its Effective Neutralising Value (ENV), a laboratory analysis that evaluates purity, solubility, and critically, particle size. Coarse lime, even if chemically pure, will have poor solubility and chemical reactivity without sufficient particle surface area to react effectively in the soil.
Why Particle Size Matters
The smaller the lime particles, the greater is the surface area (per kg of lime), which determines how well they react in the soil. For example, a given weight of limestone reducing particle size from 1mm to 0.5mm doubles its total surface area thereby significantly improving its neutralising value effectiveness.
Quality lime should contain:
At least 50% of particles smaller than 0.3mm.
Very few particles larger than 0.85mm.
A Persistent Problem
The production of substandard lime is not a new problem for farming. Unfortunately though, it appears that each new generation of farmers have to learn the hard way that it will be a problem for them - unless they demand better of their suppliers. Producing well ground lime necessitates slower product throughput and more frequent maintenance of grinding machinery to finer tolerances compared to that required for producer coarser grades. These are strong incentives for some to cut corners. It is galling to find that these include even large multinational companies who with their slick marketing claim product credibility in producing suitable agricultural lime products but choose to prioritize profit over quality, despite knowing better. Farmers have faced this issue for decades, and new generations must continually be made aware of it.
In short, don't let them get away with it!
Without proper quality product, spreading substandard lime is no better than spreading sand.
Testing Lime Quality
When purchasing lime for your farm:
Request Product Specifications
Ask for details on the lime’s Effective Neutralizing Value (ENV) and sieve test results.Conduct Independent Testing
If in doubt, collect a sample and send it to a reputable soil testing lab. This standard test is affordable compared to the cost of applying ineffective lime.
Further Reading
For more background on agricultural lime and quality considerations, refer to the following resource:
GRDC on Lime and Liming - Managing Soil Health