How Carbohydrates Help Soil Hold Water: Molecular Insights with Big Agricultural Implications

Redacción HC
11/09/2025

Water retention in soils is one of the most pressing challenges in agriculture, especially in arid and semi-arid regions where water scarcity limits crop productivity and ecosystem health. Farmers have long known that adding organic matter improves soil’s ability to hold moisture, but the molecular mechanisms behind this effect remained unclear.

A groundbreaking study published in PNAS Nexus (Kelch et al., 2025) now sheds light on how carbohydrates—key components of soil organic matter—interact with clay minerals to retain water even under dry conditions. This research not only advances soil science but also carries profound implications for sustainable farming, climate resilience, and land restoration strategies.

Why Soil Moisture Retention Matters

Soil’s ability to retain water depends on both its mineralogy (for example, the presence of clays with high surface area) and its organic matter content (including plant-derived carbohydrates). In degraded soils or drought-prone landscapes, the interaction between these two components is a critical determinant of how much water remains available to plants and microbes.

Until now, however, the precise molecular interactions between carbohydrates and clay surfaces were not fully understood. The central research question addressed by Kelch and colleagues was: How do carbohydrates at the molecular and structural level trap and retain water in clay-rich soils, even under drought-like conditions?

Innovative Laboratory Approach

To answer this question, the research team designed controlled model systems in the laboratory. Different carbohydrates—ranging from simple sugars to complex polysaccharides—were deposited on clay surfaces such as montmorillonite, a common soil clay.

They then used advanced spectroscopic and physical techniques, including:

• Solid-state nuclear magnetic resonance (NMR) and infrared spectroscopy, to track water–carbohydrate–clay interactions at the molecular scale.
• Adsorption isotherms and wetting experiments, to measure how much water was retained at varying humidity levels.
• Microscopy and structural analysis, to visualize how polysaccharides altered the clay’s surface architecture.

These methods allowed the researchers to distinguish between strongly bound water (immobile), structured water (semi-mobile), and free water (easily lost). While the experiments simplified natural soils into laboratory models, they provided critical clarity into how carbohydrates modify the water-retention properties of clay.

Dual Mechanisms: Chemistry Meets Architecture

The study uncovered two complementary mechanisms through which carbohydrates enhance soil water retention:

1. Hydrophilic bridges – Hydroxyl groups in carbohydrates formed hydrogen bonds with water molecules, stabilizing thin layers of water bound tightly to the clay surface.
2. Nanostructural porosity – When polysaccharides coated the clay, they created a micro- and nano-textured architecture that acted as “microreservoirs,” trapping water in pores and preventing rapid evaporation.

Importantly, not all carbohydrates were equally effective. The researchers found that complex, branched polysaccharides retained more water than simple sugars, both by increasing hydrogen bonding potential and by generating more robust structural porosity.

As the authors note, organic matter effectively acts as a “glue-sponge”: chemically attracting water while physically storing it in microstructures. This dual role goes beyond traditional models of soil moisture, which often emphasize only texture or simple adsorption processes.

Practical Relevance for Agriculture and Soil Management

The practical implications of these findings are significant. Even small additions of organic matter—through compost, crop residues, or biochar enriched with polysaccharides—could substantially boost soils’ ability to store water and buffer crops against drought.

For policymakers, this research reinforces the importance of soil organic matter programs aimed at food security and climate adaptation. For farmers, it provides molecular evidence supporting practices they may already use, such as adding compost or cover crops, but also encourages new approaches, like designing organic amendments specifically rich in polysaccharides.

The authors emphasize the need for follow-up research in natural soil systems to confirm durability and scalability of these effects. They also highlight the potential for improving hydrological models by incorporating these molecular mechanisms, which could inform irrigation planning and watershed management.

Opportunities for Latin America and Beyond

This research holds particular promise for regions such as the Andes and the Gran Chaco, where soil water management is critical for agriculture. Practices like incorporating cover crops that naturally release polysaccharides through root exudates, or applying organic amendments rich in complex carbohydrates, could help build soil resilience.

By creating these “hidden microreservoirs” of water in soils, farmers could reduce vulnerability to increasingly frequent droughts while improving productivity.

Conclusion: Small Molecules, Big Impact

The study by Kelch and colleagues provides molecular-level evidence for how soil organic matter helps retain water, revealing a dual mechanism of chemical bonding and structural porosity. For agriculture, the message is clear: managing organic matter isn’t just about fertility—it’s also about building water resilience from the ground up.

Future work will focus on scaling these insights from controlled lab models to diverse field soils worldwide. If successful, such knowledge could transform how we design soil management strategies in the face of global climate change.

Call to action: Farmers, soil managers, and policymakers should look closely at the role of organic matter not just as fertilizer but as a key ally in water conservation.

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