Water Stable Aggregate Calculator

Sand and coarse mineral fragments aren't true soil aggregates — they're inert particles that survive wet-sieving regardless of how well-structured your soil actually is, so leaving them in would overstate stability. The calculator's formula, WSA% = (stable weight − sand weight) / (initial weight − sand weight) × 100, removes that inert mass from both the aggregates that survived and the total sample, isolating the fraction of true water-stable structure.

The calculator predicts an expected WSA% from your organic matter reading using a rough correlation of about 8 times your OM percentage plus 10, then subtracts that from your actual measured WSA%. A positive vsExpectedPct means your soil structure is holding together better than its organic matter content alone would predict — often a sign that good management practices like reduced tillage are paying off structurally — while a negative value suggests structure is lagging behind what your organic matter level should support.

The calculator scales infiltration rate linearly with your WSA% between a floor of 0.1 inches per hour (severely unstable, crusted soil) and a ceiling around 6 inches per hour (fully stable, well-aggregated soil), using the formula infiltrationRate = 0.1 + (WSA% / 100) × 5.9. It's a simplified proxy relationship rather than a measured infiltrometer test — real infiltration also depends on texture, compaction, and antecedent moisture — so use it as a directional estimate alongside your WSA% result.

When aggregates aren't water-stable, raindrop impact and wetting cause them to disperse into individual mineral particles at the soil surface, which then dry into a dense, low-porosity crust that blocks water infiltration and can prevent seedling emergence. The calculator flags this as "Very High" crust risk below 15% WSA, stepping down through "High" and "Moderate" bands as stability improves, since more stable aggregates physically resist that dispersion process.