Oak Hill Case

Soil Physical Problems

Poor Drainage

Surface Drainage• Reflects the ease with

which water can move downslope.

• Reflects access to catch basins through which surface water can be removed from a site.

Internal Drainage• Reflects the ease with

which water can move through the soil matrix.

• Reflects the presence or absence of obstacles (e.g., pans, layers) to internal soil water movement.

Surface Drainage

To the extent that water falls at a rate in excess of a turf’s infiltration capacity, the excess will flow downslope and accumulate in depressions.

Catch basins situated in depressions can remove surface water and conduct it to drain lines or elsewhere.

Internal Drainage

Water moves through the pores permeating the soil matrix.

The larger the pores, the faster the movement of water through the soil.

Soil Aeration

As water drains from the macropores, O2 is drawn in and CO2 and other gases are liberated from the soil.

A favorable relationship between O2 and CO2 in the turf rootzone is thus maintained.

Satu

rate

d s

oil

Decreasing Soil Moisture

Gravitational Water

Dry

soil

Capillary (available & unavailable water)

Unavailable water

Soil Water MovementTherefore, the rate at which

water moves through the soil reflects its porosity and pore-size distribution.

Soils with a high proportion of macropores (i.e., coarse textured soils) conduct water more rapidly than finer textured soils.

As the surface dries from ET, water moves up from lower regions of the soil.

Water Potential (w)

W is a measure of the energy status of water; as free standing water has no energy, its W = 0.

Soil water potential is symbolized by SW

The components of soil water potential are: – matric potential (M)

– osmotic potential (O)

– pressure potential (P)

SW = M + O + P

SW is measured in units of pressure, including bars and Pascals;

1 bar = 100 kP or 1 cb = 1 kP.

pure water

W

= 0

W

= > 0 (due to P)

W

= < 0 (due to M)

Low SW

High SW

Water potentialgradient

Water potentialgradient

Matric Potential M

lower

higher

Lower W

Higher W

lower

higher

Matric Potential (M)

This reflects the amount of water retained by the soil matrix.

As this amount declines, the water films surrounding soil particles become thinner and are held more tightly, and W decreases correspondingly.

• At saturation, M is near 0.

• At field capacity, M = -0.1 to -0.33 bar (-10 to -33 kPa).

• At the permanent wilting point, M = -15 bar (-1500 kPa).

Osmotic Potential O

pure water

salty water

lower

higher

Osmotic Potential (O)

This reflects the concentration of solutes in the soil water.

As this concentration increases, O decreases.

In pure water (containing no solutes), O = 0.

In saline soils, the combination of O and M can reduce SW dramatically, especially as the soil dries (e.g., where O = -216 kP and M = -200 kP, SW = -416 kP, which indicates

a major reduction in soil water availability).

O

H H

-√

105°

+√ +√

OH

HO

H

HO

H

HO

H

H

O

H H

O

H H

O

H H

O

H H

O

H H

OH

H

O

H H

OH H

Ca2+

Pressure Potential (P)

This reflects the positive pressure to which water may be subjected in some environments.

In a glass of water, the water at the top of the glass would have a P of 0; however, the P of the water at the bottom would have a positive number.

Where a perched water table exists above the base of a soil or sand layer, the O may be positive as well; however, O = 0 in most soils.

Components of SW

SW = ψM + ψO

soil-waterpotential

matricpotential

osmoticpotential

SW

Units of Measurement

bar cb kPa MPa

1 100 100 0.1

M

+ O

= SW

- 0.3 - 0.1 - 0.4 bar

- 0.3 - 2.1 - 2.4 bar

- 2.0 - 0.1 - 2.1 bar

- 2.0 - 2.1 - 4.1 bar

Textural LayersTextural layers within the soil

profile can seriously disrupt water movement.

Where a fine textured layer occurs above a coarse textured layer, a perched water table can form.

Conversely, where a coarse textured layer occurs above a fine textured layer, a temporary water table can form.

Black Layer

SOIL

THATCH

ET

Soil Structure

As a soil becomes more compacted:

• bulk density increases

• porosity (especially macroporosity) decreases

• water movement through the soil is restricted