Timing Silicon Before Phosphorus Unlocks Locked-Up Nutrients in Tropical Soils

In the red, deeply weathered soils that underpin much of Brazil’s agriculture, phosphorus is a notoriously reluctant traveler. Farmers spread phosphate fertilizer year after year, yet crops often capture only a fraction of what is applied, because iron and aluminum oxides in tropical soils grab phosphate ions and lock them into forms roots cannot reach. Now a greenhouse study from São Paulo State University (UNESP), published in the journal Plant and Soil, has tested a deceptively simple idea: what if the timing of silicon application, relative to phosphorus fertilization, could keep more of that phosphorus available to maize plants? The answer, the researchers report, is a qualified but intriguing yes, with important caveats about what soil chemistry can and cannot deliver to the crop itself.

The research team, led by José Eduardo Petrin and corresponding authors Ana Paula Rodrigues da Silva and Dirceu Maximino Fernandes, set out to fill a genuine gap in the literature. Scientists have known for more than a century, with evidence dating back to 1905, that silicon can improve phosphorus availability, but almost no studies had systematically compared when to apply silicon relative to phosphate fertilizer. The team hypothesized that applying silicon before phosphorus would allow silicate anions to occupy the adsorption sites on soil minerals first, so that when phosphate arrived, fewer binding sites would remain to trap it. To test this, they grew maize in a clayey, unmanaged Ustox soil with low natural phosphorus, using a randomized block design with four replicates and six treatments: no phosphorus or silicon, phosphorus alone at sowing, silicon alone at sowing, phosphorus thirty days before sowing plus silicon at sowing, silicon thirty days before sowing plus phosphorus at sowing, and phosphorus and silicon applied together at sowing.

The doses were deliberately substantial: 100 milligrams of phosphorus per kilogram of soil, supplied as triple superphosphate, and 480 milligrams of silicon per kilogram, supplied as hydrophilic fumed silica with a vast specific surface area of 270 to 330 square meters per gram. All pots received dolomitic limestone to raise base saturation to 70 percent, along with balanced nitrogen, potassium, and zinc fertilization. After sixty days of maize cultivation, the researchers measured everything from leaf gas exchange and shoot biomass to tissue phosphorus and silicon concentrations, and, crucially, performed a sequential phosphorus fractionation of the soil using the Hedley method with modifications, separating labile phosphorus, inorganic phosphorus bound to iron and aluminum, calcium-bound phosphorus, organic phosphorus, and occluded phosphorus that is essentially unavailable to plants.

The headline result concerns the labile pool. When silicon was applied thirty days before sowing and phosphorus at sowing, plant-available phosphorus extracted by Mehlich-3 solution rose by 60.49 percent compared with applying phosphorus alone. Applying phosphorus and silicon together at sowing produced a similar gain of 54.78 percent. By contrast, the treatment in which phosphorus was applied first and silicon followed showed no such advantage, suggesting that the order of operations matters: silicon must be in place before, or simultaneously with, the phosphate to shield adsorption sites. The resin extraction method, which mimics the scavenging action of plant roots, told the same story, and the phosphorus saturation index, calculated from Mehlich-3 extractable phosphorus relative to iron and aluminum, was also highest in the combined treatments. Notably, total soil phosphorus did not change across treatments, confirming that silicon was not adding phosphorus but rather redistributing it from locked pools into accessible ones.

The mechanistic story behind these numbers is where the study becomes particularly compelling. The classic explanation holds that monosilicic acid competes with phosphate for the same adsorption sites on iron and aluminum oxides, and that this competition intensifies at alkaline pH, above roughly 9 to 10, where silicon adsorption is enhanced. But the soil in this experiment remained acidic throughout, with pH values between 4.5 and 4.9 across treatments. Under the conventional view, silicate-phosphate competition should have been weak. Yet the phosphorus saturation data showed clear evidence of phosphorus desorption even at these acidic pH values. The authors point to recent work showing that silicon can compete with phosphate on goethite and hematite, the dominant oxides in highly weathered tropical soils, even at pH 5.0, and they propose an additional mechanism: the formation of aluminum-silicon complexes under acidic conditions, which would deactivate adsorption sites that would otherwise bind phosphate.

The fractionation data add further texture to this picture. Across all treatments, the overwhelming majority of soil phosphorus, between 85.61 and 89.53 percent, remained in the occluded, essentially inert fraction, a sobering reminder of how tightly tropical soils hoard this nutrient. But within the accessible pools, the silicon-and-phosphorus treatments shifted the balance. Inorganic phosphorus extracted by sodium hydroxide, which represents phosphorus weakly bound to iron and aluminum, increased in the combined treatments, while the organic phosphorus fraction was highest in the unfertilized control. The principal component analysis reinforced these patterns: available phosphorus and the iron- and aluminum-bound inorganic fraction clustered with the silicon-and-phosphorus treatments, while the control aligned with organic phosphorus. The authors suggest, cautiously, that silicon may stimulate microbial decomposition of organic matter, converting organic phosphorus into inorganic forms, though they did not measure microbial activity directly and flag this as a hypothesis requiring further testing.

Here, however, comes the twist that gives the study its most important practical lesson. Despite the substantial increase in plant-available soil phosphorus, the silicon treatments did not increase phosphorus concentration or total uptake in the maize shoots compared with phosphorus alone. Plant height and shoot dry mass followed the same pattern: they responded to phosphorus application, whether alone or combined with silicon, but silicon offered no additional growth benefit. The researchers offer two plausible explanations. First, the maize hybrid used, BRS 2107, is a low-input genotype recommended for low-cost production systems and may simply not have demanded more phosphorus than the conventional treatment supplied. Second, the sixty-day experimental window may have been too short for differences in soil phosphorus availability to propagate into measurable differences in tissue accumulation. Either way, the finding underscores a recurring theme in soil-plant research: a larger labile pool in the soil does not automatically translate into a larger plant.

The physiological measurements, however, revealed subtler effects. At the tasseling stage, plants receiving phosphorus and silicon together at sowing showed the highest photosynthetic rates and stomatal conductance, though these did not differ statistically from phosphorus alone. The authors suggest that silicon’s well-documented ability to mitigate aluminum toxicity may have played a role: silicon can form hydroxyaluminosilicate complexes in the rhizosphere and root apoplast, reducing aluminum’s phytotoxic effects on roots and potentially improving water uptake and stomatal opening. Silicon applied alone did increase height and dry mass relative to the untreated control, consistent with literature attributing such benefits to enhanced root systems and leaf area. The multivariate analysis also hinted at a coordinated response of growth and nutrient-uptake variables to the combined treatments, a trend the authors note was visible in the ordination even where individual means tests found no significance.

What should farmers and agronomists take away from this work? The practical message is that silicon fertilization, timed either thirty days before phosphate application or co-applied at sowing, can meaningfully increase the plant-available phosphorus pool in highly weathered tropical soils, potentially improving the efficiency of a finite and increasingly precious resource. Phosphate rock, the raw material for most phosphorus fertilizer, is a non-renewable resource whose reserves could be depleted within roughly two centuries, and only about 20 percent of first-year fertilizer phosphorus is typically taken up by crops. Any strategy that keeps more applied phosphorus out of the occluded pool has real value for both economics and sustainability. At the same time, the study is candid about its limits: the growth response did not follow the soil chemistry, the test genotype had modest phosphorus demands, and the short duration leaves yield-level questions unanswered.

The authors themselves chart the path forward. Future experiments should test lower phosphorus rates, where the gap between silicon-treated and untreated soils might widen enough to matter for the plant, and should use maize genotypes with higher phosphorus requirements to give the enhanced availability a chance to express itself in biomass and grain. They also call for direct measurement of soil microbial activity to test the organic-phosphorus hypothesis, and for longer growing seasons to track whether physiological gains compound into yield. For now, the study stands as a careful, technically rigorous demonstration that in the battle between phosphate and the iron and aluminum oxides of tropical soils, silicon can tip the balance, provided it arrives at the right time, and provided we remember that soil chemistry and plant nutrition, while connected, do not always move in lockstep.

Subject of Research: Effect of silicon application timing on phosphorus availability, soil phosphorus fractionation, and maize growth in tropical soil

Article Title: Silicon application timing on phosphorus availability and fractionation in tropical soil and its effects on maize growth

Article References: Petrin, J. E., da Silva, A. P. R., da Silva, L. J. R., Ferreira, T. C., Deus, A. C. F., Büll, L. T., & Fernandes, D. M. (2026). Silicon application timing on phosphorus availability and fractionation in tropical soil and its effects on maize growth. Plant and Soil. https://doi.org/10.1007/s11104-026-09165-4

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09165-4

Keywords: silicon, phosphorus, tropical soils, maize, soil chemistry, phosphorus fractionation, adsorption competition, iron and aluminum oxides, fertilizer efficiency, plant nutrition, greenhouse experiment, soil fertility