Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu
For soil to store water, it must first infiltrate — or enter — the soil profile. Infiltration begins at the surface, and whether water soaks in or runs off depends largely on the porosity of the top layer and how it interacts with rainfall intensity.
Under slow, steady rainfall, many soil types can absorb water effectively. But during intense rainfall events, infiltration depends on the presence of macropores — large, open channels created by root systems, soil fauna, or stable aggregates. Without these, heavy rainfall overwhelms the soil surface, leading to runoff and erosion.
Soil porosity can be broken down into two main categories, textural and structural:
Textural porosity relates to the soil’s sand, silt, and clay proportions — properties that are inherently tied to the soil’s mineral makeup.
- Sandy soils have larger pores, but also less pore volume. So, they are good for drainage, but poor at holding water for plant growth.
- Clay-rich soils, with their fine particles, contain a greater total pore volume, which enhances water holding capacity. However, these pores are small and slow to drain, which means infiltration rates are typically lower in finer-textured soils.
A good example of textural water holding comes from the Virginia Coastal Plain, where the Bojac soil series, common to the region, has a sandy subsurface. This limits the soil’s water holding capacity, which, combined with poor structure, can reduce plant-available water and limit crop growth. The Wickham has more clay in the subsurface, holding more water for crop growth in rainfed fields.
Figure 1. Left: An aerial image of grain crops in Virginia, with the soil map on the right. The sandier Bojac2A clearly reduces crop growth.
Structural porosity, on the other hand, is created by biological activity, plant roots, and aggregation, and it can be managed and improved by minimizing disturbance. Stable soil structure creates larger pores that enhance water movement into and through the soil profile. Infiltration and conductivity of water in the soil is improved by structural porosity (i.e. macropores), able to handle greater flow associated with heavier rainfall.
Figure 2. Aggregated soil particles have created macropores which allow for water storage, but also drainage and conductivity due to their larger size. Tilling a soil will breakup those macropores, although some may find it necessary to reduce weed pressure or create a conducive seedbed.
How to Promote Soil Aggregates and Improve Infiltration
To promote larger aggregates, you must limit their disturbance and promote their strength. This includes:
- Reducing compaction and tillage – Both practices can destroy aggregates at the soil surface and reduce infiltration. Tillage breaks apart existing aggregates, while compaction collapses pore space, especially under wet conditions (Figure 3)
- Maintaining or building organic matter – Organic matter is the strongest glue we have for holding soil particles together. It feeds microbes and supports the biological processes that form and stabilize aggregates.
- Promoting plant growth – This includes grain crops like corn or soybean, but also cover crops, which help improve structure in multiple ways. Cover crops provide macropores through their roots, structural integrity through root exudates, and surface protection through their residues. Maintaining residue cover is important, as it helps reduce the impact of rainfall on soil aggregates.
- Encouraging soil macrofauna – While harder to do in agricultural soils, earthworms and other burrowing insects can create macropores. Earthworms, in particular, are also known to help form stronger aggregates through their movement and casting activity.
- Not calcium – The promotion of calcium for improving soil structure is mostly relevant in arid climates where sodium may dominate soil CEC or in soils with specific types of clay minerals, such as smectites. Neither of those conditions is common here. It is unlikely that adding calcium will significantly change your soil structure in most Mid-Atlantic soils. I have observed very few soils in our region with abnormally low calcium.
Figure 3. Left: Irrigation water ponds on the surface of a recently tilled field (foreground), while a no-till field with cover crops in the background shows no standing water, illustrating better infiltration. Right: Close-up of the cover crop residue that helped protect soil structure and promote water entry.