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).

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. 1978). 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.

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 & 1978) 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).

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 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. 1978). Foliar applications of phosphorus therefore should be avoided when inoculating soil with mycorrhizae.

This pdf file explains the benefits of mycorrhizae and then delves into the reasons why inoculants of these products do not work. If soils are completely devoid of mycorrhizal-forming fungi, i.e. if the soil has been pasteurized to kill everything, inoculants of this type may be beneficial. Research documenting the points in this presentation are provided. This talk was presented at the 2008 ProGreen Expo in Denver in January 2008.

References:

Abbott, L.K. & Robson, A.D. 1977. Growth stimulation of subterranean clover with vesicular-arbuscular mycorrhizas. Australian Journal of Agricultural Research 28:639-649.
Abbott, L.K. & Robson, A.D. 1978. Growth of subterranean clover in relation to the formation of endomycorrhizas by introduced and indigenous fungi in a field soil. New Phytologist 81:575-585.
Abbott, L.K. & Robson, A.D. 1979. A quantitative study on the spores and anatomy of mycorrhizas formed by a species of Glomus, with special reference to its taxonomy. Australian Journal of Botany 27:363-375.
Amijee, F., Tinker, P.B. & Stribley, D.P. 1989. The development of endomycorrhizal root systems. VII. A detailed study of effects of soil phosphorus on colonization. New Phytologist 111: 435-446.
Asimi, S. Gianinazzi-Pearson, V. & Gianinazzi, S. 1980. Influence of increasing soil phosphorus levels on interactions between vesicular-arbuscular mycorrhizae and Rhizobium in soybeans. Canadian Journal of Botany 58:2200-2205.
Baylis, G.T.S. 1967. Experiments on the ecological significance of phycomycetous mycorrhizas. New Phytologist 66:231.
Buwalda, J.G., Stribley, D.P. & Tinker, P.B. 1983. Increase uptake of anions by plants with vesicular-arbuscular mycorrhizas. Plant and Soil 71:463-467.
De Miranda, J.C.C., Harris, P.J. & Wild, A. 1989. Effects of soil and plant phosphorus concentrations on vesicular-arbuscular mycorrhizae in sorghum plants. New Phytologist 112:405-410. Gianinazzi-Pearson, S., Gianzinazzi-Pearson, V. and Trouvelot, A. (editors) 1982. Mycorrhizae, an integral part of plants: biology and perspectives for their use. INRA-Presse, Paris, France.
Gianinazzi-Pearson, V. & Gianinazzi, S. 1983. The physiology of vesicular-arbuscular mycorrhizal roots. Plant and Soil 71:197-209.
Gildon, A. & Tinker, P.B. 1983. Interactions of vesicular-arbuscular mycorrhizal infections and heavy metals in plants. II. The effects of infection on uptake of copper. New Phytologist 95:263-268.
Guillemin, J.P., Orozco, M.O., Gianinazzi-Pearson, V. & Gianinazzi, S. 1995. Influence of phosphate fertilization on fungal alkaline phosphotase and succinate dehydrogenase activities in arbuscular mycorrhizae of soybean and pineapple. Agriculture, Ecosystems and Environment 53:63-69.
Kilham, K. & Firestone, M.K. 1983. Vesicular arbuscular mycorrhizal mediation of grass response to acidic and heavy metal depositions. Plant and Soil 72:39-48.
Koide, R.T. & Li, M. 1990. On host regulation of the vesicular-arbuscular mycorrhizal symbiosis. New Phytologist 114:59-65.
Koide, R.T. 1991. Nutrient supply, nutrient demand and plant response to mycorrhizal infection. New Phytologist 117:365-386
Lambert, , D.H., Baker, D.E. & Cole, H. 1979. The role of mycorrhizae in the interactions of phosphorus with zinc, copper and other elements. Soil Science Society of America Journal. 43:976-980.
Menge, J.A., Steirle, D., Bagy Araj, D.J., Johnson, E.L.V., & Leonard, R.T. 1978. Phosphorus concentrations in plants responsible for inhibition of mycorrhizal infection. New Phytologist 80:575-578.
Mosse, B. 1967. Effects of host nutrient status on mycorrhizal infection . Annual Report of the Rothamsted Experiment Station, p. 79.
Mosse, B. 1973. Plant growth responses to vesicular-arbuscular mycorrhizae. IV. In soil given additional phosphate. New Phytologist 72:127-136.
Newman, E.I. and Rydell, P. 1987. The distribution of mycorrhizas among families of vascular plants. New Phytologist 106:745-751.
Schuybert, A.& Hayman, D.S. 1978. Plant growth responses to vesicular-arbuscular mycorrhizae. XVI. Effectiveness of different endophytes at different levels of soil phosphate. New Phytologist 103:79-80.
Toro, M., Azcon, R. & Barea, J. 1997. Improvement of arbuscular mycorrhizae development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (32P) and nutrient cycling. Applied and Environmental Microbiology. Nov, 1997. pages 4408-4412.
Wilcox, H.E. 1996. Mycorrhizae. In: Plant Roots: the hidden half - second edition. Waisel, Y. Eshel, A & Kafkafi, U. (eds.) Marcel Decker, Inc.