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5.10 Soil Water Facts

Field Capacity

Field capacity is the amount of water a soil continues to hold a few days after rain


This will be higher for soils with greater clay content. But clay holds soil water so strongly that about half of the water cannot be extracted by plants.


The result is that most soils (sandy loams to clays) hold about the same amount of plant available water.

Plant Available Water

This is the amount of soil water that can be extracted by roots and used for growth


Most plant available water is held in pores which range in size between 0.02mm and 0.005mm. Sands don't have many pores of this size

Wilting Point

When all available soil water has been used, the plant's wilting point is reached


In clay soils where approximately 50 percent of soil water is unavailable, moisture may still be recorded even when the soil is at wilting point.

Soil Structure

The water holding, release and infiltration properties of soil are determined by its clay content and can be changed if soil structure can be improved


The best soil structure for storage of available water and root growth will have a high number of water stable aggregates 1 - 2mm in size.


Between these aggregates will be stable pore spaces which hold available water and oxygen


Stable aggregates are formed when the basic soil particles i.e. sand, silt and clay, are bound together by 'organic glues' and 'fungal ropes'.


  • More organic residues = more food for soil biota and more soil biota

  • More soil biota = more 'organic glue' and 'fungal ropes'

  • More 'glue' and 'ropes' = more stable aggregates

  • More stable aggregates = more pore spaces = more available water storage

  • More water = more plant growth

The effects of water on different soil particles.

5.10 Soil Water Facts

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