When water refuses to soak into the ground, leading to runoff and dry plants, you are witnessing a failure of water infiltration. This common problem is caused by the soil’s inability to accept and transport water deeper into the profile. The issue often presents as water pooling immediately on the surface or running off quickly, even when the soil beneath is dry. This failure is typically due to a physical or chemical barrier near the surface, blocking the pathways water needs to travel.
Soil Compaction and Density
One frequent cause of poor water absorption is soil compaction, a physical problem caused by external pressure. Compaction occurs when heavy forces, such as foot traffic, machinery, or the impact of raindrops, press soil particles closer together. This pressure reduces the total pore space within the soil volume, altering the soil structure.
The most affected spaces are macropores, the large air-filled channels that facilitate the rapid movement of water and air. When soil is compacted, these macropores collapse, transforming into smaller micropores or being eliminated. The loss of these larger pathways restricts the soil’s hydraulic conductivity, meaning water cannot move past the surface layer efficiently.
An increase in bulk density (the weight of the soil per unit volume) is a direct result of compaction and a clear indicator of this structural degradation. This dense layer acts like a physical barrier, preventing water from infiltrating more than a few inches before pooling or running off. The soil may appear hard and difficult to dig.
Hydrophobicity: The Water-Repellent Layer
Another barrier to water absorption is soil hydrophobicity, or water repellency, which is a chemical phenomenon. This condition occurs when organic compounds coat individual soil particles, preventing water molecules from adhering. These compounds are typically waxy, long-chained substances, such as lipids or hydrocarbons. They are derived from the decomposition of plant materials, fungal activity, or intense heat from fires.
The soil particle surface, normally hydrophilic (water-attracting), becomes shielded by the hydrophobic “tail” of the organic coating. When water hits this coated soil, it beads up and resists soaking in, much like water on a waxed car. This layer can be very thin, sometimes only a few centimeters below the surface, but it is enough to cause significant runoff and erosion.
Hydrophobicity is common in sandy soils because the large particles provide less surface area for organic matter to bind. This allows a thin waxy coating to be more effective at repelling water. When the soil becomes extremely dry, the water-repellent effect is exacerbated. Water then bypasses the affected areas and flows through existing cracks or channels in a process called preferential flow.
Surface Crusting and Sealing
A localized cause of infiltration failure is the formation of a surface crust or seal, a thin, dense layer at the very top of the soil. This process begins with surface sealing, where the mechanical impact of raindrops or high-pressure hose spray breaks down soil aggregates. The impact energy disintegrates the soil clumps into individual sand, silt, and clay particles.
These fine, dispersed particles, especially silt and clay, are carried by surface water and settle into the soil’s pores, clogging them. The rapid drop in infiltration rate is due to this seal, which can have a permeability several orders of magnitude lower than the soil beneath it.
Once this muddy, sealed layer dries out, it hardens into a structural crust that can be just a few millimeters thick. This thin, hard barrier is sufficient to block nearly all water infiltration and can impede seedling emergence. Soils with low organic matter or a high percentage of silt are susceptible to this type of surface degradation.
The Role of Texture and Organic Content
The inherent makeup of your soil, defined by its texture and organic content, determines its vulnerability to absorption problems. Soil texture refers to the proportion of sand, silt, and clay particles. Clay particles are the smallest, while sand particles are the largest. This ratio dictates the size and distribution of pores.
Soils high in sand have large pores, allowing for rapid initial infiltration. However, these large pores hold less water, and sandy soils are prone to developing water repellency due to their coarse nature. In contrast, fine-textured clay soils have numerous small pores, leading to slower infiltration rates but a higher capacity to hold water once it has soaked in.
Organic matter is the glue that creates stable soil aggregates, which form the macropores necessary for healthy infiltration. Low organic matter content reduces aggregation, making the soil structure weak and susceptible to both compaction and surface crusting. Increasing soil organic matter by just one percent can help the soil hold thousands of gallons more water per acre.