The appearance of water sitting on top of soil, known as ponding or runoff, is a common issue that signifies a failure of the ground to absorb moisture quickly. The water is unable to soak in because the soil’s infiltration rate—the speed at which water enters the soil surface—is too low to keep up with the rate of precipitation or irrigation. When the supply of water exceeds the soil’s maximum absorption capacity, the excess water either accumulates on the surface or flows away. This low infiltration is not a single problem but a symptom of underlying physical, textural, or chemical conditions within the soil structure.
The Role of Soil Compaction
Soil compaction is a physical barrier that dramatically slows water absorption. It occurs when soil particles are pressed tightly together, which reduces the total volume and connectivity of the pore spaces between them. The large pores, or macro-pores, are particularly important for rapid water movement and air exchange, and these are the first to be crushed under pressure. Once these pathways are destroyed, water movement is severely restricted, leading to surface ponding and runoff.
Common causes of this structural damage include the pressure from heavy equipment, repeated foot traffic, or the impact of intense rainfall on bare soil. This compression results in a denser soil structure, which causes a significant decrease in the water infiltration rate. Compacted layers, often called hardpans, can form just beneath the surface, preventing both water and plant roots from penetrating deeper into the soil profile.
How Soil Texture Affects Drainage
Soil texture refers to the fundamental composition of the soil, specifically the ratio of sand, silt, and clay particles. Unlike compaction, which is a structural problem, texture is an inherent property that affects drainage by determining the size of the pore spaces. Sandy soils are composed of large particles, creating large pore spaces that allow water to drain very quickly, sometimes at a rate of 10 inches per hour.
Clay soils, conversely, consist of extremely fine particles that are tightly packed together. These small particles create tiny micro-pores that hold water tenaciously, resulting in very slow drainage and water movement. Their slow infiltration rate (less than 0.05 inches per hour when saturated) means they are prone to surface ponding and waterlogging. Loam soils, a balanced mixture of the three particle types, generally offer the best compromise, providing sufficient water retention without sacrificing good drainage.
When Soil Becomes Water Repellent
A less obvious cause of poor absorption is soil water repellency, also known as hydrophobicity. This condition is often chemical in nature, where organic compounds coat the soil particles, preventing water from being absorbed. The waxy, non-polar substances responsible come from the decomposition of organic matter, such as decaying plant material or fungal hyphae.
When soil dries out, especially after a prolonged drought, these hydrophobic compounds become exposed and physically repel water molecules. The compounds attach to soil particles, leaving their water-repelling tail facing outward. This coating limits the capillary action necessary for water to penetrate the soil, causing water to bead up and run off. This phenomenon is most noticeable in coarse-textured soils, like sandy soils, or in areas with thick, dry mulch layers.
Practical Steps to Improve Water Absorption
To counteract compaction, mechanical aeration is an effective method that physically breaks up the dense soil layers and restores the larger pore spaces for water and air. Using cover crops or reducing tillage also helps to limit the surface pressure that causes this structural degradation.
Soil texture limitations can be improved by incorporating organic amendments, such as compost or well-rotted manure. Organic matter increases the soil’s ability to store water, particularly in sandy soils, by creating absorptive surfaces and improving aggregation. For hydrophobic soils, applying a wetting agent (soil surfactant) can temporarily break the surface tension, allowing water to bypass the waxy coating and penetrate the soil. Long-term improvement is achieved by maintaining consistent soil moisture and increasing overall organic matter content.