Notes for Chapter 5: Soil Particles, Water and Air

Soil Particles, Water and Air

  • The physical condition of soil greatly influences crop production because roots must be anchored and water must be stored and made available to plants and soil organisms. Capillarity (capillary action) allows soil to hold and slowly release water, store precipitation, and gradually transmit water to groundwater or streams.
  • Mineral weathering and slow dissolution of minerals in water supply plant nutrients over long times, enriching soil organic matter pools used for agricultural production.
  • Degraded soils typically show reduced water infiltration and percolation (drainage to deeper layers), poorer aeration, limited root growth, and diminished nutrient supply. They also struggle to maintain diverse soil organisms and to neutralize hazardous compounds.
  • Small changes in a soil’s physical state can have large effects on water movement, aeration, nutrient supply, and biological activity. Building a good physical soil environment is a core part of healthy soil management.

Texture, a basic soil property

  • A typical mineral soil consists of about 50% solid particles by volume, with the remaining 50% as pores (air and water). A photo (Figure 5.1) illustrates the solids–pores distribution.
  • Soil texture is defined by the relative amounts of sand, silt, and clay, which determine the soil’s textural class (e.g., clay, clay loam, loam, sandy loam, sand).
  • Texture changes little over time, regardless of management practices.
  • Textural class is a fundamental inherent characteristic because it affects physical, biological, and chemical processes in the soil.
  • Particles larger than 2 mm are rock fragments (pebbles, cobbles, stones, boulders) and are not counted in the textural class because they are relatively inert.
  • Soil particles form the soil skeleton; however, the pores between particles and aggregates are equally important because most physical and biological processes occur there. Pore quantity and size distribution (large, medium, small, very small) govern water and air movement and host soil organisms and roots.
  • In a typical soil:
    • Clay soils have mostly small pores (generally < 0.002 mm).
    • Sandy soils have mostly large pores (though generally still smaller than 2 mm).
  • Aggregation and structure influence pore sizes: beach sands have large particles with little aggregation and few binding agents, while good loams or clays form aggregates (crumbs) with larger pores between aggregates and small pores within.
  • Texture itself remains relatively stable, but total pore space and the distribution of pore sizes are strongly affected by management practices (tillage, rotations, cover crops).
  • Management can destroy or improve aggregation and structure depending on tillage intensity, crop rotations, cover crops, and organic matter inputs.

Water and Aeration

  • Soil pore spaces are filled with water, air, and biota; their proportions change with wetting and drying.
  • Wet extreme: when all pores are filled with water, gas exchange with the atmosphere slows, creating potential anaerobic conditions (low or no oxygen).
  • Dry extreme: very little water can impair plant/organism water supply but can improve gas exchange; poor moisture reduces biological activity.
  • Water in soil is controlled by two opposing forces:
    • Gravity pulls water downward (drainage).
    • Capillarity (adhesion to solid surfaces and cohesion among water molecules) holds water in pores, especially in small pores.
  • Soils behave like sponges: after saturation, larger pores drain quickly due to gravity, while smaller pores retain water longer.
  • Field capacity is reached after about two days of free drainage following saturation; it represents the amount of water that remains in the soil when drainage has slowed and is available for plant use.
  • Consequences for different textures:
    • Coarse sands: many large pores drain quickly, low field capacity, limited water available to plants; good aeration but higher risk of drought stress.
    • Fine-textured dense clays/compacted soils: many small pores retain water, high field capacity, but risk prolonged saturation and anaerobic conditions (e.g., denitrification discussed in Chapter 19).
  • Ideal soils exhibit a balance: moderate texture with well-aggregated crumbs that provide both good drainage and sufficient water-holding capacity.
  • Figure references: water storage varies among soils (Figures 5.4 and 5.5 illustrate pore structures and waterholding characteristics).

Available water and rooting

  • Access to water and nutrients depends not only on storage in pores but also on root penetration and growth in the soil volume.
  • Compaction reduces accessible rooting volume, limiting water and nutrient uptake.
  • Example: a compacted surface horizon may be penetrated by only a single corn root with few fine lateral roots, leaving much of the soil water/nutrient niche inaccessible.
  • By contrast, subsoiling can break hard layers (e.g., a plow pan) and allow roots to reach deeperwater and nutrients, improving plant access and drought resilience.
  • Consequences of compaction:
    • Reduced rooting depth limits water extraction and nutrient uptake, especially during dry periods.
    • Deep rooting (plus root hairs and mycorrhizal associations) enhances water/nutrient uptake.
  • Figure 5.6 shows two corn root scenarios: left in compacted soil with limited exploration; right after subsoiling with deep rooting and better access to water/nutrients.
  • Rooting depth can be constrained by a plow pan; subsoiling opens up soil for deeper rooting and better water use.
  • The depth and effectiveness of rooting depend on soil structure, compaction, and presence of facilitative biota (mycorrhizal fungi, etc.).

Infiltration versus runoff

  • Soils absorb rainfall at the land surface and either store water for plant use or transmit it slowly to groundwater, depending on infiltration and percolation.
  • Infiltration capacity is the maximum amount of rainwater entering a soil per unit time and is influenced by soil type (pore size distribution), soil structure, and initial moisture at the start of rain.
  • In gentle rain, infiltration typically remains within capacity and most precipitation enters the soil.
  • In intense storms, initial infiltration is rapid as the ground is dry, but as the soil wets and moisture increases, infiltration rates decline and some rainfall runs off.
  • Sandy and gravely soils, with more large pores, tend to sustain higher infiltration rates during storms than fine loams and clays; strong aggregation in finer-textured soils can also maintain high infiltration rates.
  • Poorly aggregated soils may lose infiltration capacity quickly, increasing runoff and erosion risk.
  • Runoff occurs when rainfall exceeds infiltration capacity; frozen ground can exacerbate runoff due to blocked pores by ice.
  • Runoff erodes soil and carries nutrients and agrochemicals with sediment; management practices aim to reduce runoff by maintaining strong aggregates, cover, and residue cover.
  • Surface coverage (mulch, crop residues, cover crops) reduces erosion by protecting soil from raindrop impact and moderating surface temperatures and moisture extremes; continuous organic matter inputs maintain soil aggregation and biological activity.

Crop water needs and management implications

  • Different crops require different water amounts. Examples:
    • Alfalfa: high water demand with long taproot reaching deep water sources.
    • Vineyards and wheat: lower water requirements.
    • Corn and potatoes: intermediate water needs.
  • Crop choices may be influenced by climate projections toward drier and warmer conditions, which affect irrigation availability.
  • Managing soils to improve infiltration and water storage helps meet crop water needs under climate change scenarios.

Aggregation, tilth, and surface protection

  • Aggregation and soil structure influence erosion resistance, infiltration, and water storage; strong aggregation reduces dispersion and erosion.
  • Good tilth means a well-structured seedbed after soil preparation.
  • Practices that improve aggregation include:
    • Mulching or leaving surface residues on the soil
    • Reducing or eliminating tillage
    • Supplying continuous organic matter via roots, organic residues, and mycorrhizal networks
  • Surface cover (crop residue, mulch, or sod) and continuous organic matter promote a healthy soil food web and forward a chain of organisms that support aggregation.
  • Soils high in sodium can pose challenges to aggregation and structure; this is discussed in Chapters 6 and 20.
  • Figure 5.11 shows a well-aggregated soil under organic management with rye cover crop, illustrating good surface structure and aggregation.
  • Water and moisture extremes contribute to aggregation dynamics: saturated soils are soft and prone to dispersal; moist soils promote cohesion; very dry soils reduce cohesion and can be wind-eroded if organic matter is low.
  • Pore water curvature and surface tension contribute to cohesion in moist soils, aiding aggregation; this cohesion is stronger when there is substantial organic matter and biological activity.
  • Figures 5.9–5.11 illustrate conditions from erosion susceptibility to well-aggregated soils and the effects of moisture on aggregation.

What comes from the sky: the lifeblood of ecosystems

  • Climate factors affecting soils and crop production include:
    • Annual precipitation amount (arid vs humid climates)
    • Seasonal distribution and alignment with the growing season (rainfall vs irrigation needs)
    • Rain intensity, duration, and frequency (regular gentle showers are preferable to infrequent, intense events that cause runoff and erosion)
  • Climate variability often leads to water shortages during the growing season, which remains a major yield-limiting factor globally.
  • Water excess can be problematic in humid or monsoonal regions due to impaired air exchange and reduced oxygen availability; subsurface drainage and raised beds can improve aeration, while irrigation can compensate for rainfall deficits.
  • Chapter 17 discusses irrigation and drainage in more detail.

Climate risk and resilience

  • Climate risk integrates the cost of adverse events with their likelihood; extreme weather events are increasing, elevating risk to farms and communities.
  • Risk is characterized by three aspects:
    • Exposure: weather-related challenges likely to be faced
    • Sensitivity: extent to which events threaten a given operation
    • Adaptive capacity: ability to minimize damage and exploit new opportunities
  • Strategies to build resilience include:
    • Reducing greenhouse gas emissions through better cropping systems and nutrient management
    • Building soil health to improve crop vigor and reduce runoff and drought stress
    • Diversifying crops and livestock to spread risk
    • Integrating climate risk management into farm planning
    • Installing physical infrastructure like irrigation and drainage
    • Building social networks and financial resilience (insurance, savings)
    • Developing adaptive nitrogen management tools and weather-informed practices
  • The goal is to create a resilient farm system that can recover quickly from weather-related disruptions and accommodate new crops or practices as needed.
  • Adaptations must be anticipated, such as growing alternative crops if the initial crop fails.
  • This section draws on Lengnick (2015) and related sources for resilience concepts.

Historical context: climate, erosion, and soil conservation

  • The Dust Bowl era of the 1930s in the U.S. Great Plains demonstrated how drought, wind erosion, and poor soil management can devastate agricultural systems, trigger crop failures, and trigger large-scale human migrations.
  • The soil conservation movement emerged in response to those crises and has achieved substantial gains, but erosion risk remains a concern, underscoring the need for sound soil management practices across regions.

References (SOURCES)

  • Brady, N.C. and R.R. Weil. 2008. The Nature and Properties of Soils, 14th ed. Prentice Hall: Upper Saddle River, NJ.
  • Hill, R.L. 1990. Long-term conventional and no-tillage effects on selected soil physical properties. Soil Science Society of America Journal 54: 161–166.
  • Karunatilake, U. and H.M. van Es. 2002. Temporal and spatial changes in soil structure from tillage and rainfall after alfalfa-corn conversion in a clay loam soil. Soil and Tillage Research 67: 135–146.
  • Kay, B.D. 1990. Rates of change of soil structure under cropping systems. Advances in Soil Science 12: 1–52.
  • Lengnick, L. 2015. Resilient Agriculture: Cultivating Food Systems for a Changing Climate, New Society Gabriola Island, Canada. Available summary at Sare.org link.
  • Nunes, M. et al. 2018. Dynamic changes in compressive properties and crop response after chisel tillage in a highly weathered soil. Soil & Tillage Res. 186: 183–190.
  • Nunes, M. et al. 2018. Soil Health and Maize Yield Analysis Detects Long-Term Tillage and Cropping Effects. Geoderma 328: 30–43.
  • Shepard, G. et al. Visual Soil Assessment, vol. 2: Soil Management Guidelines for Cropping and Pastoral Grazing on Flat to Rolling Country. Horizons.mw and Landcare Research: Palmerston North, New Zealand.
  • Whitman, H., ed. 2007. Healthy Soils for Sustainable Vegetable Farms: Ute Guide. Land and Water Australia, AUSVEG, Ltd.: Clayton North, Victoria.