Soil water is not equally available in all soils. Differences in particle size, pore space, and organic matter control how much water is stored and how this storage supports nutrient cycling, plant growth, and overall land productivity.

Water holding capacity (WHC): what it is and why it varies

WHC matters because it influences how long soil stays moist after precipitation or irrigation and how buffered plants are against short-term drought.

Soil texture and pore space

Soil texture (relative amounts of sand, silt, and clay) strongly controls WHC by changing pore sizes.

• Sandy soils

  • Large particles and macropores drain quickly

  • Lower WHC; water moves through rapidly, increasing drought stress risk

• Clay-rich soils

  • Very small particles and many micropores hold water tightly

  • Higher WHC overall, but some water is held too strongly for roots to access

• Silt loams and loams

  • Balanced pore sizes often provide high plant-available water

  • Commonly productive for agriculture when well-managed

Soils with a mix of pore sizes can both store water and supply oxygen to roots, supporting higher land productivity.

Organic matter and aggregation

Soil organic matter (including humus) increases WHC by acting like a sponge and improving aggregation.

• More stable aggregates create a mix of pore sizes

• Improved structure increases infiltration while also increasing storage

• Organic matter can reduce rapid evaporation by improving surface condition and plant cover support

Compaction reduces WHC that is useful to plants by collapsing macropores, which can increase runoff and limit root growth even if the soil still contains water.

Plant-available water: a useful way to think about WHC

Some stored soil water drains quickly, some is available to plants, and some is held too tightly to be extracted by roots.

This diagram shows how water occupies pore spaces at three key soil-moisture states: saturation, field capacity, and permanent wilting point. It visually separates gravitational water that drains from larger pores from the plant-available water stored after drainage, and the tightly bound water that remains unavailable. The dashed boundaries reinforce that plant-available water is a bounded storage pool rather than all water present in the soil. Source

Two common reference points are field capacity (water remaining after drainage) and the permanent wilting point (water content at which plants cannot recover).

Soil water retention curves compare how volumetric water content changes with water potential for different soil textures. The vertical reference lines show where soils are typically considered at field capacity (left) and permanent wilting point (right), illustrating why plant-available water differs even when two soils have similar total water content. The figure highlights that finer-textured soils generally hold more water at a given tension, but not all of it remains accessible to roots. Source

PAW tends to be greatest in well-aggregated loams and lower in very sandy or very clayey soils for different physical reasons.

This figure summarizes how available water varies across soil textures, emphasizing that the gap between field capacity and permanent wilting point is not maximized at the clay end-member. It helps explain why many loams (and clay loams) can provide more plant-available water than sands (low storage) or heavy clays (more water held too tightly). The visual reinforces that texture controls both total water held and how much of that water is actually extractable by roots. Source

How WHC influences soil fertility and land productivity

Nutrient retention and loss

Water movement affects whether nutrients remain in the root zone.

• Low-WHC soils drain quickly, which can increase leaching of soluble nutrients (especially nitrate, NO$_3^-$)

• Higher-WHC soils slow percolation, increasing the time nutrients remain available for uptake

• Greater moisture stability supports microbial processes that transform nutrients into plant-available forms

WHC also connects to cation exchange capacity (CEC) because clay minerals and organic matter both hold water and provide negatively charged surfaces that retain nutrient cations (for example, K$^+$, Ca$^{2+}$, Mg$^{2+}$), reducing nutrient loss.

Water–air balance and root function

Productive soils must hold water without becoming oxygen-poor.

• Excessively wet, fine-textured soils can become waterlogged, limiting oxygen and reducing root respiration

• Drier, coarse-textured soils can limit nutrient uptake because many ions move to roots dissolved in soil water

• Consistent moisture supports root growth, soil biota, and decomposition that replenishes nutrients

Implications for land productivity

Across soil types, WHC influences:

• Irrigation frequency and drought resilience

• Crop yield stability (reduced stress between watering or rainfall)

• Fertiliser efficiency (less loss when water remains in the root zone)

• Vegetation patterns and carrying capacity on rangelands