Soil Quality Indicators

Available Water Capacity

Available water capacity is the maximum amount of plant

available water a soil can provide. It is an indicator of a

soil’s ability to retain water and make it sufficiently

available for plant use.

Available water capacityis the water held in soil between

its field capacity and permanent wilting point. Field

capacity is the water remaining in a soil after it has been

thoroughly saturated and allowed to drain freely, usually

for one to two days. Permanent wilting point is the

moisture content of a soil at which plants wilt and fail to

recover when supplied with sufficient moisture. Water

capacity is usually expressed as a volume fraction or

percentage, or as a depth (in or cm).

Factors Affecting

Inherent - Available water capacity is affected by soil

texture, presence and abundance of rock fragments, and

soil depth and layers.

Available water capacity increases with increasingly fine

textured soil, from sands to loams and silt loams. Coarse

textured soils have lower field capacity since they are high

in large pores subject to free drainage. Fine textured soils

have a greater occurrence of small pores that hold water

against free drainage, resulting in a comparatively higher

field capacity. However, in comparison to well-aggregated

loam and silt loam soils, the available water capacity of

predominantly clay soils tends to be lower since these soils

have an increased permanent wilting point (see Figure 1).

Rock fragments reduce available water capacity of soil

proportionate to their volume, unless the rocks are porous.

Soil depth and root restricting layers affect total available

water capacity since they can limit the volume of soil

available for root growth. (Restrictive layers may be

naturally occurring or a result of management activities.)

Plant rooting characteristics must be considered for a

practical understanding of the effects of soil depth and

restrictive layers on water available for plant growth. A

restrictive layer at 20 inches might have little consequence

on the water requirements of a shallow-rooted crop.

However, this layer might severely limit the volume of soil

a deep-rooted crop can explore for moisture.

Dynamic - Available water capacity is affected by

organic matter, compaction, and salt concentration of the

soil.

Organic matter increases a soil’s ability to hold water, both

directly and indirectly. When a soil is at field capacity,

organic matter has a higher water holding capacity than a

similar volume of mineral soil. While the water held by

organic matter at the permanent wilting point is also

higher, overall, an increase in organic matter increases a

soil’s ability to store water available for plant use.

Indirectly, organic matter improves soil structure and

aggregate stability, resulting in increased pore size and

volume. These soil quality improvements result in

increased infiltration, movement of water through the soil,

and available water capacity (see Figure 2).

Compaction reduces available water capacity through its

adverse affects on both field capacity and permanent

wilting point. Compaction reduces total pore volume,

consequently reducing water storage when the soil is at

field capacity. Compaction also crushes large soil pores

into much smaller micropores. Since micropores hold

water more tightly than larger pores, more water is held in

soil at its permanent wilting point.

Salts in soil water result from fertilizer application or

naturally occurring compounds. Salt concentration

increases as soil water decreases. For soils high in soluble

salts, moisture stress results when plants cannot uptake

USDA Natural Resources Conservation Service

0

2

3

4

Inches of water / ft soil depth

Sandy

loam

Loam Silt

oam

Sand Loamy

sand

Fine

sandy

loam

Silty

clay

oam

Clay

oam

Silty

clay

Clay

Field Capacity

Permanent W t ng Po nt

Ava ab e Water

Fineness of texture 

Unavailable Water

Figure 1. General relationship between soil moisture and texture.

Ohio Agronomy Guide, 14th edition, Bulletin 472-05

Indicator

P

Test

L/O

Function

D

Helping People Help the Land...
water across an unfavorable salt concentration gradient.

Soils with high salt concentration tend to have reduced

available water capacity because more water is retained at

the permanent wilting point than if water was held by

physical factors alone. These effects are most pronounced

in soils in dry regions where salts accumulate because of

irrigation or natural processes.

Relationship to Soil Function

Soil is a major storage reservoir for water. In areas where

rain falls daily and supplies the soil with as much or more

water than is removed by plants, available water capacity

may be of little importance. However, in areas where

plants remove more water than is supplied by precipitation,

the amount of water held by the soil may be critical. Water

held in the soil may be necessary to sustain plants between

rainfall or irrigation events. By holding water for future

use, soil buffers the plant – root environment against

periods of water deficit.

Available water capacity is used to develop water budgets,

predict droughtiness, design and operate irrigation systems,

design drainage systems, protect water resources, and

predict yields.

Problems with Poor Function

Lack of available water reduces root and plant growth, and

it can lead to plant death if sufficient moisture is not

provided before a plant permanently wilts. A soil’s ability

to function for water storage also influences runoff and

nutrient leaching.

Agricultural land management practices that lead to poor

available water capacity include those that prevent

accumulation of soil organic matter and/or result in soil

compaction and reduced pore volume and size:

• Conventional tillage operations,

• Low residue crop rotations, and burning, burying,

harvesting, or otherwise removing plant residues,

• Heavy equipment traffic on wet soils, and

• Grazing systems that allow development of livestock

loafing areas and livestock trails.

As natural areas are permanently converted to homes,

roads, and parking areas, the overall amount of water that

can be stored in the soil is reduced. This leads to higher

total runoff, increased pressure on storm water drainage

systems, a higher likelihood of flooding, and generally

poorer water quality in streams and lakes.

Improving Available Water Capacity

Farmers can grow high residue crops, perennial sod and

cover crops, reduce soil disturbing activities, and manage

residue to protect and increase soil organic matter to make

improvements in a soil’s available water capacity. When

feasible, tillage, harvest, and other farming operations

requiring heavy equipment can be avoided when the soil is

wet to minimize compaction; and compacted layers can be

ripped to break them and expand the depth of the soil

available for root growth.

For soil high in soluble salts, management activities that

maintain salts below the root zone can be used. These

include irrigation to leach salts below the root zone and

practices that promote infiltration, reduce evaporation,

minimize disturbance, manage residue, and prevent mixing

of salt-laden lower soil layers with surface layers.

Conservation practices resulting in available water capacity

favorable to soil function include:

• Conservation Crop Rotation

• Cover Crop

• Prescribed Grazing

• Residue and Tillage Management

• Salinity and Sodic Soil Management

Developers can incorporate the use of permeable parking

areas, green roofs, and other practices that minimize the

impact of development on soil water storage.

Measuring Available Water Capacity

Reference: U.S. Department of Agriculture, Natural

Resources Conservation Service, 2005. National Soil

Survey Handbook, title 430-VI. Soil Properties and

Qualities (Part 618), Available Water Capacity (618.05).

Online at: http://soils.usda.gov/technical/handbook/

Specialized equipment, shortcuts, tips:

Determination of permanent wilting point moisture content

requires a pressure membrane apparatus.

Time needed: One to two days is required for free

drainage and to allow soil to reach field capacity.

23 4 5 67

0.10

0.20

0.30

0.40

0.50

soil water f 3

t / f 3

t soil

% soil organic matter by weight

F e d Capac ty

Available Water

Pe manen w ng pe cen age

Figure 2. Effect of increasing organic matter on available water

capacity of silt loam soils. Adapted from Hudson, SWCS, 1994.

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