Physical Soil Health

The physical properties of soil play a critical role in determining how well soils support crops, manage water, and resist degradation. This fact sheet is for Ohio farmers, advisors, and conservation professionals who want to better understand how soil texture, structure, compaction, and water movement affect soil health and crop productivity. By focusing on the key drivers of physical soil health and highlighting practical management practices, producers and land managers can connect the science behind soil health with everyday decision-making to more effectively address Ohio soil health challenges.

Soil Physical Properties

Soil is composed of three general components (Figure 1):

1. Minerals.
2. Organic matter.
3. Pore space.

The mineral section (sand, silt, and clay) forms the basic framework of soil and changes very little over human timescales. Organic matter, while more dynamic, changes slowly and typically requires years of management to build or deplete. In contrast, pore space is the most dynamic component of soils. It shifts in response to management, weather, and biological activity. Farmers can directly influence pore space—how soil stores water and air—through management.

The physical characteristics of soil have a large influence on how soil interacts with water, air, roots, nutrients, and biology. At the most general level, the physical properties of soil are shaped by five soil-forming factors outlined by the father of soil science, Hans Jenny:

1. Climate.
2. Organisms.
3. Topography.
4. Parent material.
5. Time.

On the farm, these factors are often recognized in more practical terms—soil texture and parent material (sand, silt, clay content), the relative flatness of the land, plants grown, and the patterns of rainfall and temperature that influence drainage and crop growth. Each of these factors affects the amount of water that can be stored in soil, the rate water moves through the soil, the soil’s susceptibility to compaction, and the soil’s potential for aggregation. Therefore, understanding the processes that affect the physical properties of soil can be informative in interpreting testing data on the physical health of soil.

Physical Soil Health

In Ohio, poor drainage and, to a certain extent, degraded soil structure and increased compaction are often observed as physical health issues in soil. Combating these issues requires an understanding of the soil’s physical properties (i.e., physical health). The optimal level of the soil’s physical health is limited by the soil’s composition and texture. For example, sandy, coarser-textured soils cannot aggregate as well as clayey, finer-textured soils; the presence of more organic matter increases soil-aggregation potential. In general, physically healthy soils have relatively strong aggregation (i.e., resistance to wind and water erosion), the ability to hold and infiltrate water (i.e., resilience to drought and extreme rain events), and limited compaction (i.e., adequate pore space for water, air, and roots).

Key strategies for improving physical soil health:

1. Adding or increasing the soil’s organic matter content.
2.  Minimizing compaction.
3. Reducing physical disturbance.

Organic matter can be added with the use of cover crops, adding manure or compost, and retaining in-field crop residue. By adding organic matter, more water can be held and infiltrated, aggregation is promoted, and plant nutrient availability is improved.

Minimizing compaction starts with managing agronomic operations because approximately 80% of soil compaction occurs during the first pass of machinery. Using machinery on wet fields destroys aggregates as a result of crushing the soil which reduces pore space. Lowering the number of passes in a field, following the same tracks, optimizing contact pressure, and operating in fields during proper moisture conditions are the main strategies for reducing soil compaction.

Lastly, reducing physical disturbance is one of the four pillars of soil health (i.e., minimizing disturbance, maximizing living roots, maximizing soil cover, and maximizing biodiversity). It is critical for improving aggregation and soil structure. Physical disturbance from tillage degrades the physical health of soil. Options for reducing the level of tillage include implementing no-till, strip till, or conservation tillage.

Key Physical Properties of Soil

Soil Texture

• dependent on proportions of sand, silt, and clay (Figure 2)
• affects water retention, drainage, aggregation, and nutrient availability

Soil Structure

• dependent on how soil particles are arranged into aggregates (Figure 3)
• good structure improves aeration, water movement, and root penetration
• poor structure can lead to surface crusting and decreased infiltration

Bulk Density

• mass of dry soil per unit volume
• higher bulk density means compaction which can restrict root growth

Porosity

• the percentage of soil volume occupied by pore space
• healthy soils have a balance of macropores for air, and micropores for water retention

Aggregate Stability

• the ability of soil aggregates to resist erosion when exposed to water or wind
• stable aggregates reduce erosion and improve water infiltration (Figure 4)

Soil Hydraulic Properties

• water infiltration rate, available water capacity, hydraulic conductivity, and sorptivity
• healthy soils allow optimum movement of water in the soil profile and minimize water logging or quick dry-out scenarios

Conclusions

In Ohio, maintaining good physical soil health is essential for productive cropping systems, particularly due to the state’s diverse soil types and weather patterns. By focusing on the key factors reviewed in this fact sheet, farms can improve water infiltration, reduce compaction, and build aggregates. Implementing practices such as reduced tillage, cover cropping, and adding organic matter can mitigate common challenges such as erosion and surface crusting. Improving or maintaining the soil’s physical health is crucial for improving farm profits, mitigating environmental issues caused by agriculture, and building resilience towards extreme weather events.

Additional Resources

• Baseline Assessment of Soil Health in Ohio
  (soilhealth.osu.edu/sites/soilhealth/files/imce/WhitePapers/Baseline%20Ohio%20Soil%20Health.pdf)
• Biological Soil Health
  (ohioline.osu.edu/factsheet/anr-0205)
• Chemical Soil Health
  (ohioline.osu.edu/factsheet/anr-0204)
• Hans Jenny and the Art of Soil or Soil in Art
  ucanr.edu/blog/topics-subtropics/article/hans-jenny-and-art-soil-or-soil-art
• Soil Health
  (ohioline.osu.edu/factsheet/anr-0202)
• Soil Health Testing
  (ohioline.osu.edu/factsheet/anr-0206)
• Soils and Soil Health
  (ohioline.osu.edu/factsheet/anr-0136)