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Can biostimulants help young maize use recycled phosphorus?
University of Hohenheim researchers found that humic substances and chitosan raised young maize growth by 21 to 34% and phosphorus uptake by 23 to 35% when paired with struvite or a soluble phosphate, but gave no detectable gain with a treated sewage sludge ash within six weeks.
Phosphorus recovered from wastewater could cut reliance on mined phosphate rock, but the authors note that recycled phosphorus fertilisers often perform less well than mined mineral ones. A team from the Institute of Crop Science at the University of Hohenheim in Germany, with co-authors from Southwest University and China Agricultural University, asked whether three biostimulants could close that gap, and whether the answer depends on which fertiliser they are paired with (Yousefi et al., 2026). This was a six-week greenhouse experiment with young plants, plus a plant-free soil incubation, so it says nothing yet about grain yield.
What did the team do?
The researchers grew maize (cultivar Ronaldinio) for 42 days in rhizoboxes, narrow boxes with a transparent window, filled with a low-phosphorus silty clay loam mixed with quartz sand. Each box got one of four phosphorus treatments:
- no phosphorus fertiliser;
- monocalcium phosphate, a water-soluble phosphate used as the reference;
- struvite (magnesium ammonium phosphate) recovered from municipal wastewater treatment;
- a sewage sludge ash treated by the ASH-DEC process (Na-SSA).
Each fertiliser was combined with one of three non-microbial biostimulants (humic substances, a seaweed extract from Ascophyllum nodosum, or chitosan) or with none. Biostimulants, as the paper defines them, are substances that enhance nutrient acquisition, growth or stress tolerance independently of their direct nutrient content. They were mixed into the substrate as powders at sowing, then given twice more as liquids on days 14 and 28.
At harvest the team measured plant dry matter, phosphorus uptake, rhizosphere pH, root length by diameter class and the nutrients taken up into the shoots. Alongside this, they ran a four-month incubation of the same fertiliser and biostimulant combinations in soil with no plants, to track how extractable phosphorus changed over time.
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What did they find?
- The fertiliser decided the response. With the soluble phosphate and with struvite, humic substances and chitosan increased plant biomass by 21 to 34% and total phosphorus uptake by 23 to 35%. With the sewage sludge ash, none of the biostimulants produced a detectable gain within the six weeks.
- Humic substances gave the biggest lift. Total dry matter rose from 4.60 to 6.04 g per plant with the soluble phosphate (31%) and from 4.58 to 6.13 g per plant with struvite (34%).
- Struvite kept up with the soluble phosphate. Without biostimulants, phosphorus partial factor productivity reached 10.2 g of dry matter per gram of phosphorus applied with both fertilisers.
- More fine roots. Humic substances increased fine-root length (roots under 0.2 mm) by 48% with the soluble phosphate and by 42% with struvite. Chitosan increased it by 24% and 51%.
- Two different routes. Humic substances were the only biostimulant that lowered rhizosphere pH (by 0.12 to 0.40 pH units). Chitosan grew fine roots without detectable acidification, which the authors read as two separable responses.
- A shift in other nutrients. Humic substances raised shoot potassium uptake by 163 to 182% with the soluble phosphate and struvite, while magnesium uptake fell by 46 to 51%, so the shoot potassium to magnesium ratio rose about fivefold. The authors say the magnesium drop warrants consideration alongside the phosphorus benefits. With the soluble phosphate and struvite, chitosan increased potassium, magnesium, calcium, zinc and manganese uptake together.
- The ash may just be slow. In the plant-free incubation, all three biostimulants raised extractable phosphorus with the sewage sludge ash by 12 to 18% at day 117, with statistically robust effects emerging between days 90 and 117. The authors suggest the six-week result reflects slow phosphorus release rather than an ash that cannot respond, while noting that microbial processes may have contributed.
What could it mean for growers?
The authors argue that biostimulants should not be treated as a broadly beneficial add-on across different fertilisers, and that recycled phosphorus sources should be matched to compatible biostimulant strategies according to how they dissolve. They suggest pairing struvite with humic substances may be a practical way to increase phosphorus-use efficiency in maize under circular fertilisation, while ash-based systems may need longer or different management.
The authors frame the work as relevant to circular fertiliser recommendation systems in maize. The results come from young maize in one soil under controlled conditions, and the authors say multi-season field trials across contrasting soils are needed to confirm it.
What were the limitations?
- Both experiments were run under controlled conditions with a single phosphorus-deficient soil and one maize cultivar; responses may differ in other soils and in the field.
- The rhizobox experiment covered only the first 42 days of growth, so whether the gains persist to grain yield, or whether the late phosphorus from the ash becomes available over a full season, is unknown.
- Each biostimulant was tested at a single rate and application mode, so dose-response relationships were not established.
- The proposed mechanisms were inferred from rhizosphere pH, root traits and shoot nutrient data rather than measured directly.
- Phosphorus availability was assessed with a single extraction (CAL), and phosphorus speciation was not determined.
- Microbial processes were not measured, and repeated mixing before subsampling in the incubation may have stimulated microbial activity.
Trial details
- Where: Germany. Greenhouse at the University of Hohenheim, Stuttgart; soil from the topsoil (0 to 30 cm) of the university’s Oberer Lindenhof experimental research station (Baden-Württemberg), collected in August 2024. Incubation in a temperature-controlled chamber.
- Crop: maize (Zea mays L.), cultivar Ronaldinio; three seeds sown per rhizobox, thinned to one seedling after emergence; harvested 42 days after sowing.
- System and substrate: rhizoboxes 50 × 20 × 5 cm holding 3.0 kg substrate (dry matter), kept at approximately 45° inclination. Substrate: silty clay loam (14% sand, 59% silt, 27% clay; pH (CaCl2) 5.9; CAL-P 15.3 mg P per kg, VDLUFA class A, very low) mixed with washed quartz sand (0.6 to 1.2 mm) at 70:30 by mass. Water-holding capacity 0.24 g g⁻¹; moisture held at approximately 70% of water-holding capacity by weighing three times a week.
- Climate and lighting: 28/18 °C day/night, 70% relative humidity, 12 hour photoperiod. Rhizoboxes repositioned weekly within blocks.
- Nutrition: phosphorus fertilisers at 150 mg P per kg dry matter, mixed in as fine powders: monocalcium phosphate (Ca(H2PO4)2·H2O), struvite (11.1% total P), and ASH-DEC sewage sludge ash (7.97% total P). Basal supply to all treatments: Ca(NO3)2 (70 mg N per kg), K2SO4 (70 mg K per kg) and MgSO4 (50 mg Mg per kg). Potassium supplied with the liquid seaweed product was balanced in all other treatments with K2SO4.
- Biostimulants: humic substances (powder humic acid; liquid micronised leonardite suspension with 28% total humic acids and 4 to 5% fulvic acids), Ascophyllum nodosum seaweed extract (powder; liquid with 4% K2O), and chitosan (powder; liquid chitosan acetate). At sowing, 1 g per kg dry matter for humic substances and seaweed extract and 0.1 g per kg for chitosan; liquid follow-ups of 1 mL per kg (0.1 mL per kg for chitosan) on days 14 and 28 (days 15 and 42 in the incubation).
- Design: 4 phosphorus levels × 4 biostimulant levels, fully factorial, in four randomised complete blocks (n = 4; 64 rhizoboxes). Incubation: 800 g substrate in 1 L jars, same factorial design (64 jars), in the dark at 20 ± 1 °C, sampled at days 0, 14, 27, 63, 90 and 117.
- Statistics: linear mixed-effects models (fertiliser and biostimulant fixed, block random) with Type II F-tests and Fisher’s LSD at α = 0.05, in R 4.3.1. Fertiliser × biostimulant interaction for biomass and phosphorus uptake F = 6.51, p < 0.001.
- Results: total dry matter with humic substances 6.04 g per plant (soluble phosphate) and 6.13 g per plant (struvite), against 4.60 and 4.58 without. Phosphorus uptake with humic substances 6.63 and 6.92 mg per plant, against 5.41 and 5.11. Agronomic phosphorus-use efficiency raised by 63 to 77% with humic substances and chitosan under those two fertilisers; at or close to zero with the ash. Fine-root length with humic substances 5908 mm (from 3996) and 5562 mm (from 3905). Extractable phosphorus at day 117 of the incubation 12 to 18% higher with biostimulants on the ash and 18 to 22% higher on struvite.
- Funding: DFG (German Research Foundation) grant 328017493/GRK 2366 (Sino-German International Research Training Group AMAIZE-P); publishing fees supported by the University of Hohenheim’s Open Access Publishing funding programme.
- Conflicts of interest: the authors declare none. They also state that generative AI was used to improve the clarity of the writing.
- Publication: Frontiers in Agronomy, volume 8, article 1971558, published 2 November 2026 (peer reviewed), https://doi.org/10.3389/fagro.2026.1971558. Licence: CC BY.
- Not reported in the paper: greenhouse light source, PPFD, DLI and spectrum; CO2; greenhouse dates; and grain yield (the plants were harvested at 42 days).
Links
- The study: Fertilizer source governs biostimulant efficacy for phosphorus acquisition in maize: comparison of recycled phosphorus fertilizers and soluble phosphate
- The research programme: AMAIZE-P, Sino-German International Research Training Group, University of Hohenheim
References
- Yousefi, A., Müller, T., Chen, X., Zhang, W., Kondra, R., & Malakshahi Kurdestani, A. (2026). Fertilizer source governs biostimulant efficacy for phosphorus acquisition in maize: comparison of recycled phosphorus fertilizers and soluble phosphate. Frontiers in Agronomy, 8, 1971558. https://doi.org/10.3389/fagro.2026.1971558