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Mycorrhiza and soil phosphorus levels
Curtis E. Swift, Ph.D.
Area Extension Agent (Horticulture)
Colorado State University Cooperative Extension
Tri River Area
Introduction:
Mycorrhizae are an integral part of most plants in nature (Giazninazzi et al., 1982)
and occur on 83% of dicotyledonous and 79% of monocotyledonous plant
investigated (Wilcox - 1996). All gymnosperms are reported as being mycorrhizal
(Newman & Reddell, 1987). Infection of the root system of the plant by these fungi
creates a symbiotic (beneficial) relationship between the plant and fungus.
Upon root infection and colonization, mycorrhizal fungi develop an external
mycelium which is a bridge connecting the root with the surrounding soil (Toro et al.
1997). One of the most dramatic effects of infection by mycorrhizal fungi on the host
plant is the increase in phosphorus (P) uptake (Koide, 1991) mainly due to the
capacity of the mycorrhizal fungi to absorb phosphate from soil and transfer it to the
host roots (Asimi, et al. 1980). In addition, mycorrhizal infection results in an
increase in the uptake of copper (Lambert, Baker & Cole, 1979; Gildon & Tinker,
1983), zinc (Lambert, Baker & Cole, 1979), nickel (Killham & Firestone, 1983), and
chloride and sulphate (Buwalda, Stribley & Tinker, 1983). Mycorrhizae also are
known to reduce problems with pathogens which attack the roots of plants
(Gianinazzi-Pearson & Gianinazzi, 1983).
Influence of Phosphorus on Mycorrhizae:
The benefits listed above are greatest in P-deficient soils and decrease as soil
phosphate levels increase (Schubert & Hayman, 1986).
Very high and very low phosphorus levels may reduce mycorrhizal
infection/colonization (Koide, 1991). It is well established that:
�� infection by mycorrhizal fungi is significantly reduced at high soil phosphorus
levels (Amijee et al., 1989; Koide & Li, 1990)
�� the addition of phosphate fertilization results in a delay in infection as well as a
decrease in the percentage of infection of roots by mycorrhizae (deMiranda,
Harris & Wild, 1989; Asimi et al., 1989)
�� an increase in the level of soil phosphate results in a reduction in
chlamydospore production by the fungus (Menge, et al. 197
. These spores
are involved in root infection and spread of the fungus through the soil profile.
Research by Abbott and Robson (1979) concluded that levels of soil phosphorus
greater than that required for plant growth eliminated the development of the
arbuscles of vesicular-arbuscular (VA) types of mycorrhizae. Arbuscles are
structures produced within the host plant cells by the VA mycorrhizae. These
structures are responsible for the transfer of absorbed nutrients from the fungus to
the plant. The arbuscles resemble miniature shrub-like trees (arbuscular = shrub in
Latin). Mosse (1973) reports adding phosphate results in no arbuscles forming.
What levels of P are critical?
When the soil level of bicarbonate-soluble phosphorus exceeded 140 mg kg -1 (140
parts per million) the rate of infection was found to decrease (Amijee et al. 1989).
Abbott and Robson (1977 & 197 found the mycorrhizae Glomus fasciculatum
ceased to be effective when the soil level of phosphorus reached 133 mg kg -1 [133
parts per million (ppm)]. Schubert and Hayman (1986) found mycorrhizae was no
longer effective when 100 mg or more of P was added per kilogram of soil (100
ppm). Mycorrhizal infection virtually disappeared with the addition of 1.5 grams or
more of mono calcium phosphate per kilogram of soil (Mosse 1973). With small
additions of phosphorus fertilizer, entry points and fungal growth on the root surface
remained normal but arbuscles were small and fewer in number reducing the
effectiveness of the fungus/plant relationship. Other researchers have reported
mycorrhizal infections tend to die out in soils containing or given much phosphorus
(Baylis, 1967; Mosse, 1967). The development of mycorrhizal relationships were
found to be the greatest when soil phosphorus levels were at 50 mg kg -1 (50 ppm)
(Schubert & Hayman, 1986).
Summary and recommendations:
The benefits of mycorrhizae are greatest when soil phosphorus levels are at or
below 50 ppm (50 mg kg -1). Mycorrhizal infection of roots declines above this level
with little if any infection occurring above 100 ppm P even when soil is inoculated
with a mycorrhizae mix.
Prior to inoculating soil with mycorrhizae, a soil test should be conducted. If
phosphorus levels are greater than 50 ppm the addition of mycorrhizae will likely be
Mycorrhiza and soil phosphorus levels Colorado State University Cooperative Extension
ineffective.
The level of phosphorus in the plant also has been shown to influence the
establishment of VA mycorrhizae with high levels inhibiting colonization by
mycorrhizae (Menge, et al. 197. Foliar applications of phosphorus therefore
should be avoided when inoculating soil with mycorrhizae.
Curtis E. Swift, Ph.D.
Area Extension Agent (Horticulture)
Colorado State University Cooperative Extension
Tri River Area
Introduction:
Mycorrhizae are an integral part of most plants in nature (Giazninazzi et al., 1982)
and occur on 83% of dicotyledonous and 79% of monocotyledonous plant
investigated (Wilcox - 1996). All gymnosperms are reported as being mycorrhizal
(Newman & Reddell, 1987). Infection of the root system of the plant by these fungi
creates a symbiotic (beneficial) relationship between the plant and fungus.
Upon root infection and colonization, mycorrhizal fungi develop an external
mycelium which is a bridge connecting the root with the surrounding soil (Toro et al.
1997). One of the most dramatic effects of infection by mycorrhizal fungi on the host
plant is the increase in phosphorus (P) uptake (Koide, 1991) mainly due to the
capacity of the mycorrhizal fungi to absorb phosphate from soil and transfer it to the
host roots (Asimi, et al. 1980). In addition, mycorrhizal infection results in an
increase in the uptake of copper (Lambert, Baker & Cole, 1979; Gildon & Tinker,
1983), zinc (Lambert, Baker & Cole, 1979), nickel (Killham & Firestone, 1983), and
chloride and sulphate (Buwalda, Stribley & Tinker, 1983). Mycorrhizae also are
known to reduce problems with pathogens which attack the roots of plants
(Gianinazzi-Pearson & Gianinazzi, 1983).
Influence of Phosphorus on Mycorrhizae:
The benefits listed above are greatest in P-deficient soils and decrease as soil
phosphate levels increase (Schubert & Hayman, 1986).
Very high and very low phosphorus levels may reduce mycorrhizal
infection/colonization (Koide, 1991). It is well established that:
�� infection by mycorrhizal fungi is significantly reduced at high soil phosphorus
levels (Amijee et al., 1989; Koide & Li, 1990)
�� the addition of phosphate fertilization results in a delay in infection as well as a
decrease in the percentage of infection of roots by mycorrhizae (deMiranda,
Harris & Wild, 1989; Asimi et al., 1989)
�� an increase in the level of soil phosphate results in a reduction in
chlamydospore production by the fungus (Menge, et al. 197
. These sporesare involved in root infection and spread of the fungus through the soil profile.
Research by Abbott and Robson (1979) concluded that levels of soil phosphorus
greater than that required for plant growth eliminated the development of the
arbuscles of vesicular-arbuscular (VA) types of mycorrhizae. Arbuscles are
structures produced within the host plant cells by the VA mycorrhizae. These
structures are responsible for the transfer of absorbed nutrients from the fungus to
the plant. The arbuscles resemble miniature shrub-like trees (arbuscular = shrub in
Latin). Mosse (1973) reports adding phosphate results in no arbuscles forming.
What levels of P are critical?
When the soil level of bicarbonate-soluble phosphorus exceeded 140 mg kg -1 (140
parts per million) the rate of infection was found to decrease (Amijee et al. 1989).
Abbott and Robson (1977 & 197
found the mycorrhizae Glomus fasciculatumceased to be effective when the soil level of phosphorus reached 133 mg kg -1 [133
parts per million (ppm)]. Schubert and Hayman (1986) found mycorrhizae was no
longer effective when 100 mg or more of P was added per kilogram of soil (100
ppm). Mycorrhizal infection virtually disappeared with the addition of 1.5 grams or
more of mono calcium phosphate per kilogram of soil (Mosse 1973). With small
additions of phosphorus fertilizer, entry points and fungal growth on the root surface
remained normal but arbuscles were small and fewer in number reducing the
effectiveness of the fungus/plant relationship. Other researchers have reported
mycorrhizal infections tend to die out in soils containing or given much phosphorus
(Baylis, 1967; Mosse, 1967). The development of mycorrhizal relationships were
found to be the greatest when soil phosphorus levels were at 50 mg kg -1 (50 ppm)
(Schubert & Hayman, 1986).
Summary and recommendations:
The benefits of mycorrhizae are greatest when soil phosphorus levels are at or
below 50 ppm (50 mg kg -1). Mycorrhizal infection of roots declines above this level
with little if any infection occurring above 100 ppm P even when soil is inoculated
with a mycorrhizae mix.
Prior to inoculating soil with mycorrhizae, a soil test should be conducted. If
phosphorus levels are greater than 50 ppm the addition of mycorrhizae will likely be
Mycorrhiza and soil phosphorus levels Colorado State University Cooperative Extension
ineffective.
The level of phosphorus in the plant also has been shown to influence the
establishment of VA mycorrhizae with high levels inhibiting colonization by
mycorrhizae (Menge, et al. 197
. Foliar applications of phosphorus thereforeshould be avoided when inoculating soil with mycorrhizae.
