Aside from monotropoid mycorrhizas, the juvenile stages of all orchids, and the whole lives of a few orchid species (which are achlorophyllous) there is always a 2-way transfer of nutrients.

Direct uptake of nutrients by plant roots can often result in zones of nutrient deficiency around the root system.  Subsequent absorption of nutrients is then limited entirely by the rate at which nutrients move through the soil.
Hyphae can grow past these depletion zones and therefore maintain the rate of absorption (Harley, 1991).  Maintaining and extending hyphae is a more economical way of sustaining contact with nutrients than by continually extending roots (Harley, 1989).

Nitrate (NO₃^-) is more mobile than ammonia, and is the main source of nitrogen in most soils.  The exception to this rule is in acidic soils, such as those of forests and heathlands, where ammonia is in grater abundance.

Most plants will preferentially uptake ammonia over nitrate (Marschner & Dell) and research with ECM fungi has shown that many may not be able to utilise nitrate (Harley, 1991).  This is advantageous to plants, as even if ammonia is in low concentration, the ECM fungi will absorb it more rapidly than nitrate (Smith & Read, 1997).

Ericoid mycorrhizas are particularly efficient at assimilating ammonia, since their partner plants are mostly heathland plants, which grow on the acidic soils which are high in that form of nitrogen.

In contrast, AM inoculation has little effect on N nutrition, and when nitrate is plentiful in the soil, AM plants have no advantage over non-AM plants (Harley, 1991).

In agreement with this, ECM and ericoid plants tend to dominate in N-limited soils, and AM dominate in P-limited soils.

Phosphorus (P) is the limiting nutrient in most soils and it can be said that, in general, large growth increases due to mycorrhizal infection are due to increases in P absorption (Bolan et al, 1987).  In AM plants, when root growth is restricted, up to 80% of P obtained by the plant can come via the hyphal network (Li et al, 1991).

Maintaining the acquisition of P may occur in 3 ways:
i)   forming polyphosphates in the hyphae and therefore maintaining a low internal phosphate concentration (low Pi) and allowing phosphate to passively diffuse in,
ii)  small hyphal diameter leading to larger volume of soil explored per unit surface area of hyphae, and
iii) production of extracellular acid phosphatases, which releases P from organic complexes in the soil.

AM fungi are actually unable to liberate P in this way (Bolan,1991) and as with N in organic complexes, it is ECM and ericoid mycorrhizas that this is particularly well developed in.

The cases of monotropoid and seedling orchid mycorrhizas show different movement of nutrients between plant and fungus.  The plants can be considered to be myco-heterotrophic, since they obtain their supplies of organic C from a fungus, but give nothing back in return.  Mineral nutrients may also be supplied to the plants by their fungi.

However, the fungi appear not to be disadvantaged by this relationship as in monotropoid mycorrhizas, the fungi sequesters its carbon from other photosynthetic plants.  Monotropoid mycorrhizas are often ECM with surrounding trees, and carbohydrates form that plant sustain both the fungus and the associated Monotropa sp.

The monotropoid relationship has been referred to as epiparasitic, but since the fungi is not damaged in this symbiosis, this term has been dropped in favour of myco-heterotrophic (Smith & Read, 1997).

The fungi almost certainly receives nothing in return from Monotropa sp. although whether fungi do in the adult stages of orchid growth remains to be seen.

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