A biodegradable hydrogel produced from starch and carboxymethyl cellulose could improve seed germination and early plant development by retaining water around seeds, according to research published in Scientific Reports.
The researchers developed hydrogel formulations containing approximately 90% bio-based materials and tested their potential as coatings for sugar beet seeds.
The study was conducted by Raikhan Rakhmetullayeva, Botakoz Khavilkhairat, Assel Toktabayeva, Nurzhan Mukhamadiyev, Elmira Nurgaziyeva and Munziya Abutalip.
The findings suggest that natural polymer-based hydrogels could support more efficient water use in agriculture while reducing reliance on conventional petroleum-derived superabsorbent polymers.
Addressing water shortages and plastic pollution
Superabsorbent hydrogels are three-dimensional polymer networks capable of absorbing and retaining large quantities of water.
In agriculture, these materials can help maintain soil moisture, reduce irrigation requirements and gradually release water and nutrients near plant roots. They may be especially valuable in arid and semi-arid regions where limited rainfall and drought can prevent successful seed germination.
However, many commercially available superabsorbents are produced from synthetic, petroleum-derived polymers. Their long-term persistence in soil may contribute to environmental pollution.
To develop a more sustainable alternative, the research team combined two renewable polysaccharides: starch and carboxymethyl cellulose.
Starch is abundant, biodegradable and relatively inexpensive, while carboxymethyl cellulose offers water-retention and film-forming properties. Glutaraldehyde was used as a chemical crosslinking agent to create a stable three-dimensional polymer network.
The researchers prepared hydrogels using several starch-to-carboxymethyl cellulose ratios and different concentrations of the crosslinking agent.
Hydrogel absorbs more than 17 times its dry weight
The hydrogel containing starch and carboxymethyl cellulose in an 80:20 ratio demonstrated the strongest overall performance.
It absorbed up to 17.5 grams of distilled water per gram of dry hydrogel. In river water, the same formulation absorbed approximately 12.5 grams per gram.
The reduction in river water was associated with the presence of dissolved ions, which can limit the expansion of the polymer network.
Both hydrogel formulations reached equilibrium swelling after approximately 4.2 hours.
Scanning electron microscopy revealed a porous internal structure. These pores can facilitate the movement and storage of water and nutrients, which is important when hydrogels are used as soil conditioners or seed-coating materials.
Chemical analysis using Fourier-transform infrared spectroscopy confirmed that the starch and carboxymethyl cellulose had formed a crosslinked hydrogel structure.
The 80:20 formulation also achieved the highest gel fraction, approximately 88.13%, indicating the formation of a comparatively stable crosslinked network.
Creating uniform coated seeds
The researchers used a dry pelleting process to apply the hydrogel to sugar beet seeds.
Seed pelleting changes irregularly shaped seeds into more uniform particles, which can simplify handling and sowing. A polyvinylpyrrolidone adhesive solution was used to bind the coating materials to the seeds.
The hydrogel was combined with a small amount of wood ash. Wood ash contains plant nutrients including potassium, calcium, magnesium and phosphorus, although excessive application can increase soil alkalinity or introduce undesirable elements.
Four coating structures were evaluated:
- polymer–ash;
- ash–polymer;
- ash–polymer–ash;
- polymer–ash–polymer.
The coated seeds were compared with untreated seeds and seeds treated with conventional protective and growth-stimulating formulations.
Coated seedlings grew approximately twice as long
The best results were observed with the ash–polymer–ash coating containing the 80:20 starch-to-carboxymethyl cellulose hydrogel.
Seedlings produced using this formulation reached approximately 6 ± 0.8 centimetres. The best 50:50 hydrogel formulation produced seedlings approximately 5.5 ± 0.3 centimetres long.
By comparison, uncoated seeds produced seedlings approximately 3 ± 0.3 centimetres long after 21 days in the reported experiment.
The researchers found that the 80:20 hydrogel formulations generally performed better than the 50:50 formulations. This was attributed to their higher gel fraction and greater water absorption capacity.
Seeds coated with the ash–polymer–ash structure also demonstrated the highest visible water absorption and germinated within seven days.
The hydrogel may create a moist microenvironment around the seed and release stored water gradually. Wood ash may additionally supply minerals that support early plant development.
The results indicate that the coating could be particularly useful when irrigation is limited or temporarily unavailable.
Approximately 67% degradation in soil
Biodegradability is an important requirement for agricultural hydrogels because persistent polymer residues could accumulate in soil.
Both principal formulations lost approximately 66.7–67.2% of their mass during the soil degradation study.
The researchers observed rapid degradation during the early stages, followed by a slower phase. The initial degradation was associated with water absorption, swelling and microbial access to the starch-rich regions of the material.
The remaining crosslinked and crystalline parts of the hydrogel were more resistant to enzymatic breakdown.
These degradation results suggest that starch and carboxymethyl cellulose hydrogels may offer greater environmental compatibility than many slowly degrading synthetic superabsorbents.
Further safety and field testing required
The researchers emphasise that the technology remains at the laboratory stage.
The current purification procedure required approximately 372 millilitres of water per gram of hydrogel over the full washing period. Industrial production would therefore require more efficient systems, such as water recycling, continuous purification or counter-current washing.
The study also did not quantitatively assess the possible presence or toxicity of residual glutaraldehyde after washing.
In addition, some seed treatments produced too few germinated seeds for robust statistical comparison. Larger experiments are required to confirm the effect of the coatings across different crops, soil types and environmental conditions.
Future research will evaluate residual crosslinking agents, investigate less toxic alternative crosslinkers and improve the hydrogel’s mechanical stability and swelling behaviour.
Field trials will also be needed to determine whether the technology can improve germination, irrigation efficiency and crop productivity under realistic agricultural conditions.
The researchers conclude that starch–carboxymethyl cellulose hydrogels provide a promising platform for biodegradable seed coatings, water-retaining soil conditioners and controlled-release agricultural systems.