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Beyond SAR: evaluating CROSS for predicting soil structural stability under recycled water irrigation

This study demonstrates that the Cation Ratio of Soil Structural Stability (CROSS) is a superior indicator to the traditional Sodium Adsorption Ratio (SAR) for predicting soil structural degradation and permeability hazards under recycled water irrigation by more accurately accounting for the dispersive effects of potassium and the flocculating roles of calcium and magnesium.

Original authors: Usama Aldughaishi, Stephen R. Grattan, Srinivasa Peddinti, Francisco Pedrero Salcedo, Isaya Kisekka

Published 2026-08-12

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Usama Aldughaishi, Stephen R. Grattan, Srinivasa Peddinti, Francisco Pedrero Salcedo, Isaya Kisekka

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Great Soil Party: Why Water's "Personality" Matters More Than Its Saltiness

Imagine the soil beneath our feet not as a static pile of dirt, but as a bustling, microscopic city. In this city, tiny clay particles are the buildings, and the water flowing through them is the traffic. For this city to function—allowing rain to soak in and roots to breathe—the buildings need to stick together in sturdy neighborhoods called "aggregates." But sometimes, the water flowing through the city brings in a guest list that causes chaos.

For decades, farmers and scientists have worried about one specific troublemaker: Sodium. Think of Sodium as a rowdy party guest who loves to push people apart. When too much Sodium gets into the soil water, it forces the clay buildings to separate, turning the sturdy neighborhoods into a muddy, disorganized mess. This process is called "dispersion," and it clogs the city's streets, stopping water from draining and causing the ground to turn into a puddle. To measure this risk, scientists have long used a scorecard called the Sodium Adsorption Ratio (SAR). It's like a bouncer checking the ID of the Sodium guest; if the number is too high, the water is considered dangerous for the soil.

However, there's a new twist in the story. As the world runs low on fresh water, farmers are increasingly turning to "recycled water"—treated wastewater from our homes and cities. This water is a lifesaver for crops, but it often has a different chemical personality than natural rain or river water. It might have less Sodium, but it could be packed with other guests, like Potassium. For a long time, we assumed that if the Sodium bouncer said "all clear," the soil was safe. But what if Potassium is also a troublemaker, just a quieter one? This is the question that a team of researchers set out to answer, asking whether our old scorecard is missing the full picture of who is crashing the soil party.

The Experiment: A Greenhouse Drama with Strawberries

To find out if our old rules still apply, the researchers set up a dramatic experiment in a greenhouse at the University of California, Davis. They didn't just look at dirt; they grew strawberry plants in pots, treating them like actors in a play designed to test how different types of water affect the soil's structural stability.

They created 11 different "synthetic recycled waters." Imagine these as 11 different cocktails, all with the exact same amount of total saltiness (salinity), but with very different mixes of ingredients. Some were heavy on Sodium, some on Potassium, and some were dominated by the "good guys"—Calcium and Magnesium, which act like the glue that holds the soil buildings together. They even included a mix with only Potassium and no Sodium at all, just to see if Potassium could cause trouble on its own.

Over a 40-week growing season, they watered their strawberry pots with these different cocktails. They watched closely for signs of soil distress: Did water pool on the surface and refuse to drain (standing water)? Did the water struggle to sink into the soil (reduced infiltration)? And if they took a sample of the soil and shook it in water, did the clay fall apart into a cloudy soup (clay dispersion)?

The Results: The Old Scorecard Missed the Plot

The findings were a wake-up call for the old way of thinking. The team discovered that the traditional SAR scorecard was failing to predict the soil's behavior accurately, especially when Potassium was involved.

Here is the plot twist: The researchers found that water with zero Sodium (meaning a perfect SAR score of 0) could still cause the soil to clog and drain poorly if it was high in Potassium. In the experiment, the pots watered with these Potassium-heavy, Sodium-free solutions showed significantly more standing water and clay dispersion than the pots watered with Calcium-rich water. The old rule said, "No Sodium? No problem!" The experiment said, "Not so fast. Potassium is also causing the clay to fall apart."

The study showed that the soil's health wasn't just about how much Sodium was present; it was about the balance between all the players. The "disruptive" cations (Sodium and Potassium) were fighting against the "stabilizing" cations (Calcium and Magnesium). When the disruptors won, the soil structure collapsed, leading to drainage failures.

The New Hero: CROSSf

To solve this puzzle, the researchers turned to a newer, more sophisticated scorecard called CROSS (Cation Ratio of Soil Structural Stability), specifically a version they called CROSSf.

Think of SAR as a bouncer who only checks the ID of the Sodium guest. CROSSf, on the other hand, is a bouncer who checks the IDs of everyone in the room. It weighs the disruptive power of both Sodium and Potassium against the stabilizing power of Calcium and Magnesium, giving each a specific "weight" based on how strong they are at causing trouble or holding things together.

The results were clear: CROSSf was a much better predictor of what was happening in the soil pots.

When the researchers plotted the data, CROSSf showed a very strong, clear line connecting the water's chemistry to the soil's problems. As the CROSSf score went up, the amount of standing water increased, the infiltration rate dropped, and the clay dispersion skyrocketed.
In contrast, the old SAR score was a mess. It couldn't explain why the Potassium-only water was causing such bad drainage, nor could it distinguish between the different effects of Calcium and Magnesium.

The study found that clay dispersion was the main culprit behind the drainage issues. When the water chemistry was "wrong" (high CROSSf), the clay particles broke loose, floated around, and clogged the tiny pores in the soil, turning a sponge into a brick.

What This Means for the Future

The paper concludes that while the old SAR method is useful for simple, natural waters, it is insufficient for the complex world of recycled water irrigation. Recycled water often has high Potassium levels that the old SAR ignores. By relying solely on SAR, farmers might think their water is safe when it is actually slowly destroying their soil's ability to drain.

The researchers suggest that incorporating CROSSf into how we judge irrigation water could be a game-changer. It offers a more accurate way to predict if a specific batch of recycled water will turn a field into a swamp or keep it healthy for crops. While this study was done in a controlled greenhouse with potted strawberries, the results strongly suggest that the same rules apply in the real world. It's a reminder that in the complex city of soil, we need to know the whole guest list, not just the most famous troublemaker, to keep the party from turning into a disaster.

Technical Summary: Beyond SAR – Evaluating CROSS for Predicting Soil Structural Stability Under Recycled Water Irrigation

Problem Statement
The increasing reliance on recycled water for irrigation necessitates reliable indicators to assess potential impacts on soil physical properties. Traditional water quality assessments predominantly utilize the Sodium Adsorption Ratio (SAR) to evaluate sodicity hazards and soil permeability. However, SAR has significant limitations in the context of recycled water: it assumes sodium is the sole dispersive cation, ignoring the potential dispersive effects of potassium (K⁺), which is often elevated in treated wastewater. Furthermore, SAR assumes calcium (Ca²⁺) and magnesium (Mg²⁺) exert equal flocculating effects, a simplification that may not reflect actual soil system behavior. While previous laboratory column experiments suggested that the Cation Ratio of Soil Structural Stability (CROSS) offers improved predictions over SAR, the applicability of these findings under actual cropping conditions—where factors like root activity and soil heterogeneity exist—remains insufficiently explored.

Methodology
To address this gap, the authors conducted a controlled greenhouse experiment using strawberry plants cultivated in clay loam soil. The study employed a randomized complete block design with 11 treatments and four replications per treatment (176 pots total).

Synthetic Irrigation Waters: Researchers prepared synthetic recycled waters using chloride salts (NaCl, KCl, CaCl₂, MgCl₂) and one calcium sulfate treatment. All solutions maintained a constant electrical conductivity (1.5 dS/m) but varied in cation ratios to create distinct SAR and CROSS values.
Experimental Conditions: Plants were irrigated daily for 40 weeks using a drip system. Irrigation pulses were adjusted based on soil moisture (tensiometer readings) and a leaching factor of 30% to maintain soil EC below strawberry thresholds.
Measured Variables:
- Drainage Impairment: Monitored via the occurrence of standing water in pots prior to irrigation events during the cropping period (weeks 26–38) and a 39-hour drainage test post-irrigation termination.
- Relative Infiltration Rate: Measured using a Wooding infiltrometer on selected pots at the experiment's conclusion, normalized against the calcium-dominated control.
- Clay Dispersion: Quantified via spontaneous dispersion methods on soil samples collected from 5–15 cm depths.
- Volumetric Soil Water Content: Measured 7 days after irrigation cessation.
Indices Evaluated: The study compared the predictive power of traditional SAR against two CROSS formulations: CROSSf (incorporating specific flocculating factors for K⁺ and Mg²⁺) and CROSSopt (optimized factors).

Key Results

Drainage and Infiltration: Irrigation with waters dominated by dispersive cations (Na⁺ and K⁺) significantly increased standing water occurrence and reduced relative infiltration rates compared to calcium-dominated treatments. Notably, treatments containing potassium but no sodium (SAR = 0) still exhibited drainage impairment greater than the control, demonstrating that permeability risks exist even when SAR suggests safety.
Clay Dispersion: Clay dispersion was strongly correlated with the cation composition of the irrigation water. Treatments with higher proportions of dispersive cations showed significantly higher clay dispersion, which the authors identify as the primary mechanism driving soil structural degradation, aggregate breakdown, and pore clogging.
Predictive Performance:
- CROSSf Superiority: Among the evaluated indices, CROSSf demonstrated the strongest relationships with all measured soil physical indicators. Specifically, it showed the highest coefficient of determination ( $R^2$ ) for standing water occurrence at 6 hours ( $R^2 = 0.84$ ), relative infiltration rate ( $R^2 = 0.73$ ), and dispersed clay ( $R^2 = 0.93$ ).
- SAR Limitations: SAR showed weaker correlations ( $R^2$ values ranging from 0.20 to 0.63) and failed to capture the negative impacts of potassium or the differential roles of calcium and magnesium.
- Temporal Dynamics: CROSSf consistently outperformed SAR and CROSSopt during the early stages of the drainage experiment (3–6 hours post-irrigation), indicating its sensitivity to immediate structural responses.

Significance and Claims
The paper claims that the results confirm CROSSf provides improved predictions of soil permeability hazards under cropping conditions compared to the traditional SAR. The study validates that clay dispersion is the key mechanism linking irrigation water cation composition to soil structural degradation.

The authors assert that incorporating indices such as CROSS into irrigation water quality assessment frameworks is necessary for managing agricultural systems increasingly reliant on recycled water. They argue that relying exclusively on SAR may underestimate soil structural risks when recycled waters contain significant concentrations of potassium or variable divalent cation compositions. However, the paper maintains a modest stance, noting that CROSSf should serve as a complementary screening tool rather than a standalone replacement for field monitoring, and that further field studies are required to establish practical threshold values across diverse soil textures and management strategies.