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By Rafael De Toledo
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Modern agriculture faces an overlooked efficiency problem: crops often struggle to access phosphorus already present in agricultural soils, leading farmers to apply additional fertilizer to maintain productivity.
Much of the phosphorus applied over time becomes immobilised through interactions with soil minerals, creating accumulated reserves that plants cannot readily use.
The next major advance in agricultural sustainability may therefore depend less on increasing fertilizer application and more on improving the soil’s ability to release the nutrients it already contains.
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For decades, agricultural productivity has been closely associated with fertilizer use. Phosphorus, in particular, is essential for plant development, supporting energy transfer, root growth, and crop reproduction.
When plants cannot access sufficient phosphorus, yields can suffer. The conventional response has been straightforward: apply more fertilizer.
But phosphorus does not behave as simply as that approach suggests. Once applied, much of it can bind with minerals in the soil, making it less accessible to plant roots.
Although the nutrient remains present, its availability to crops becomes limited. Over successive growing seasons, these accumulated reserves contribute to what scientists describe as legacy phosphorus.
The Challenge
Research published in Soil and Tillage Research examines the accumulation and management of residual phosphorus in agricultural soils.
A field can contain substantial phosphorus reserves while still requiring additional fertilizer to maintain productivity. This creates an inefficient cycle, particularly when farmers face rising input costs and increasing pressure to improve the environmental sustainability of food production.
The challenge, therefore, is not simply to replenish nutrients. It is to understand the biological and chemical processes that determine whether those nutrients become available to plants.
Soil is not an inert growing medium. It is a complex ecosystem in which microorganisms, plant roots, organic matter, and minerals interact.
Microorganisms play an important role in nutrient cycling, including the processes that release phosphorus from compounds that plants cannot readily absorb.
Certain soil bacteria produce enzymes that help break down organic phosphorus compounds. One example is phytase, which can release phosphate from phytate, a form of phosphorus found in organic matter.
These biological processes offer a potential route to improving nutrient availability without relying exclusively on additional fertilizer inputs.
Microorganism and Nutrient Availability
My interest in this area developed through early work in vermicomposting and a growing focus on soil biology. This led me to explore how microorganisms influence nutrient availability, particularly the processes that can make phosphorus more accessible to plant roots.
Working alongside university researchers and studying a collection of more than 10,000 microbial strains provided a broader understanding of the potential role of soil bacteria in improving nutrient cycling.
Among the findings was a soil-derived bacterial isolate exhibiting phytase-like activity, with relatively low sequence homology to phytases previously described in the literature.
This presents an interesting research direction, although its practical value ultimately depends on independent validation, reproducibility, and performance under real agricultural conditions.
An organism that demonstrates useful enzyme activity under controlled conditions does not automatically become an effective agricultural treatment.
Soil composition, temperature, moisture, pH, and interactions with other microorganisms can all influence performance.
Cadmium Contamination
Agricultural technologies must also be affordable, scalable and straightforward for farmers to incorporate into existing practices. Scientific potential must ultimately translate into measurable agronomic and economic value.
There is another important reason to reconsider how phosphate fertilizers are used: their potential contribution to cadmium contamination.
Certain phosphate rocks used in fertilizer production naturally contain cadmium, a toxic heavy metal that can accumulate in agricultural soils over time.
According to the United Nations Environment Programme, phosphate fertilizers derived from phosphate rock are a significant source of cadmium releases into the environment.
The metal can be absorbed by crops and enter the food chain, including through animals consuming contaminated feed.
A Public Health Concern
This is a significant public-health concern. The World Health Organization identifies cadmium as toxic to the kidneys and skeletal system and classifies it as a human carcinogen.
Its persistence in the environment and accumulation through prolonged exposure make limiting unnecessary contamination of agricultural soils particularly important.
Regulators have also recognised the issue. Under European Union Regulation 2019/1009, phosphate fertilisers carrying the CE marking are subject to a maximum cadmium concentration of 60 mg per kilogram of phosphorus pentoxide (P₂O₅).
France has introduced further measures to progressively tighten national limits on cadmium in fertilisers, with thresholds of 60 mg/kg P₂O₅ from January 2027, 40 mg/kg from January 2028 and 20 mg/kg from January 2030. These measures are outlined by the French Government.
Improving phosphorus-use efficiency and developing biological approaches to nutrient management could form part of a broader effort to reduce unnecessary fertilizer inputs.
The Importance of Understanding Soil Biology
Such approaches will not eliminate the need for conventional fertilizers, particularly in soils where phosphorus is genuinely deficient.
However, making better use of existing nutrient reserves could help farmers reduce avoidable applications while supporting healthier, more productive soils.
Agriculture needs to move beyond the assumption that maintaining productivity always requires more inputs. Researchers, policymakers, and the agricultural industry should place greater emphasis on understanding soil biology, improving nutrient availability, and developing technologies that deliver consistent results under practical farming conditions.
Farmers should not have to keep paying for nutrients that remain inaccessible to their crops, nor should agricultural productivity come at the cost of introducing avoidable contaminants into the soil and food chain.
The next generation of agricultural progress will depend on making better use of the resources already present in the ground, while ensuring that the inputs we do apply are both effective and safe.
Rafael De Toledo is the founder of HeartsBio and Supersol, where his work focuses on the development of enzyme and microbial approaches to agricultural soil health and phosphorus availability. All views are his own.
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