Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
subsoils become the growing environment and, unless mitigating measures are taken, the
results are disastrous for plant establishment.
Soil acidity or alkalinity, as measured by pH, affects mineral nutrient availability, mineral toxicity
(Palmer 1990), and nitrogen fixation (Thorup 1984). In acid soils, the ability of plants to utilize
many nutrients decreases, especially for calcium and magnesium. As soil pH becomes more
acid (less than 4.5), aluminum becomes more soluble and more toxic to plant growth. Low
pH soils also hinder the establishment of nitrogen-fixing plants, such as legumes (Bloomfield
and others 1982). Significant loss of rhizobia viability has been documented at pH levels less
than 6 (Brown and others 1982).
Basic soils with pH values of 8.0 or greater indicate the presence of calcium carbonate (Thomas
1967). Calcium and magnesium are at such high levels that they can interfere with the uptake
of other nutrients, notably phosphorus, iron, boron, copper, and zinc (Campbell and others
1980). High pH soils typically have high salt levels, which can also restrict the growth of many
plants. For example, as soil pH approaches 9, sodium concentrations become toxic to plants
(Tisdale and Nelson 1975).
Soil salinity is the measure of the total amount of soluble salts in a soil. The term soluble
salts refers to the inorganic soil constituents, or ions, that are dissolved in the soil water. The
principal soluble salts in a soil contain the cations sodium, calcium, and magnesium, and the
anions chloride, sulfate, and bicarbonate (Landis and Steinfeld 1990).
Almost all plants are susceptible to salt injury under certain conditions, with germinants and
young seedlings being particularly susceptible to high salt levels (Figure 5-37). Soluble salts
can injure plants in several ways:
◾◾
Reduced soil moisture – Salts can lower the free energy of water molecules, causing
an osmotic effect and thereby reducing the moisture availability to plants.
◾◾
Reduced soil permeability – High salt concentrations (specifically sodium salts) can
change the soil structure by reducing the attraction of soil particles, causing them
to disperse. Pore space is lost and air and water movement within the soil profile are
restricted.
◾◾
Direct toxicity – High levels of certain ions, including sodium, chloride, and boron,
can injure plant tissue directly.
◾◾
Altering nutrient availability – Certain nutrients as salts can change the availability
and utilization of other plant nutrients (Landis 1981; Landis and Steinfeld 1990).
Soluble Salts
mS/cm or mmhos/cm
3,500
3,000
2,500
2,000
1,500
1,000
500
High Salt
Hazard -
Damage
to Plants
Moderate
Salt Hazard -
Injury to
Germinants
Low Salt
Hazard
Figure 5-37 | Soluble salt
affects on plants
Soluble salts will injure germinants
and, at high concentrations, damage
established plants. (Values are based
on the saturated media extract method
of conductivity measurement.) Soil
laboratories often report conductivity
in dS/m units. To convert to mS/cm or
mmhos/cm, multiply by 100.
Figure 5-38 | Soils with high salts
Soils with high salt content typically occur in
regions of the United States with low annual
precipitation (http://bonap.org/2008_Soil/
saline.png).
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
98