Across Australia’s vast farmlands, fertilisers are a lifeline, transforming conventionally farmed soils into productive ground for crops and pastures. High volumes of nitrogen (N), phosphorus (P), and potassium (K) re applied to keep yields up—often over 200 kg/ha of nitrogen for wheat in high-rainfall zones or 150 kg/ha of phosphorus for grazing land (Fertiliser Australia, 2023). Yet, a stubborn problem lingers: much of this fertiliser ends up "locked up" in the soil, unavailable for plant up-take. This leaves farmers facing rising costs with less bang for their buck—a frustrating cycle that’s been decades in the making. So, what’s behind this? It’s a mix of soil chemistry, the vital role of microbes, and unbalanced ratios.
The Fertiliser Overload
Australia’s soils are typically low in nitrogen, phosphorus, and organic carbon—often under 1% organic matter compared to 5-10% in fertile regions globally (CSIRO, 2021). Farmers counter this with hefty fertiliser doses, yet only 30-50% of nitrogen and 10-25% of phosphorus reach the plants (Food and Agriculture Organization, 2019). The rest is trapped in a nutrient bank or leaches away.
As nutrients stay locked, more fertiliser goes on to keep yields steady. The Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES, 2023) notes fertiliser costs in some regions climbed 15% yearly from 2015 to 2020, while yield boosts tapered off.
The Microbial Middlemen
Soil isn’t just a medium—it’s a living system driven by microbes. Bacteria and fungi are the unsung heroes of nutrient cycling, turning nutrients into forms plants can absorb. Nitrogen-fixing bacteria like Rhizobium (found in legume root nodules) convert atmospheric nitrogen into ammonium, while mycorrhizal fungi unlock phosphorus from mineral complexes, delivering it straight to plant roots.
But when fertiliser applications go overboard, this microbial workforce gets overwhelmed and nutrient ratios are thrown out. Excessive nitrogen, often from urea or ammonium-based products, throw out the C:N ratio. In cases where no carbon soured is added alongside the nitrogen source, a carbon loss will be seen and humus stores depleted. This shift disrupts the soil food web, reducing the fungi that help plants access phosphorus and water. These NPK ratios, which are often unbalanced with the excessive application of potentially just a single nutrient, will lock the other nutrients up in organic forms that the plants can't use.
Imbalanced Ratios: A Legacy Off-Balance
Standard practice has long favoured nitrogen-heavy fertilisers, a holdover from the need to spark growth on nutrient-starved soils. But when N:P:K ratios tilt—say, 20:1:1 instead of a 10:1:4 for wheat, Those N:P:K ratios (rough benchmarks from crop response data (Grains Research and Development Corporation, 2022)),—the soil ecosystem stumbles:
- Nitrogen Surge: Excess ammonium or nitrate fuels nitrifying bacteria like Nitrosomonas, outpacing mycorrhizal fungi. Phosphorus availability drops—sometimes 50%—even when soil tests flag high total levels. (Dig deeper: tests measure total phosphorus; separating soluble PO₄³⁻ from insoluble fractions shows what’s bioavailable versus locked.)
- Phosphorus Bind: Unabsorbed phosphate precipitates with cations, blocking micronutrients like zinc (Zn²⁺) and iron (Fe²⁺), quietly stunting growth.
- Carbon Deficit: Microbes need organic carbon to metabolise fertilisers. With soils scraping below 1% organic matter, heterotrophic bacteria slow down, leaving nutrients stranded.
A Way Forward
This isn’t about pointing fingers—farmers are doing what’s always been done to feed the nation. Many of our enquiries come from farmers tired of putting more on and getting less out, seeking alternative methods. Understanding and utilising microbes offers an important tool but is not a silver bullet. Adding organic matter, tweaking fertiliser ratios to soil tests, or using slow-release blends can ease the lockup, but it’s ultimately the intersection of all these strategies that works. Trials in Queensland with phosphorus-solubilising bacteria have lifted uptake by 20% (Grains Research and Development Corporation, 2022), showing what’s possible. Fertilisers will likely always have an important role to play—we’re looking to balance these strategies for better yields and a better bottom line for farmers.