Manitoba Agriculture, Food and Rural Initiatives
Mycorrhizae – More than the basics.
All agronomists and many farmers should now be aware of the benefits that mycorrhizal fungi offer in meeting phosphorus (P) nutrition needs of plants. This concept has been a lesson at several Crop Diagnostic Schools (2000, 2003-5) and presented at the Manitoba Agronomist Conference (2006) and Manitoba Soil Science Society meeting (2009). As a refresher on this subject, I would refer you to the excellent summary presented at the 2006 Manitoba Agronomist’s Conference (Turmel, 2006).
Unfortunately some confusion still exists regarding the need for mycorrhizae in Manitoba cropping systems.
- There is a difference between host crops and their dependence on mycorrhizae.
Even though plants may be hosts to mycorrhizae they may differ in their dependence on them for uptake of phosphorus and some micronutrients (Table 1).
Table 1. Mycorrhizal dependent and non-supportive crops
| Host |
Host plants |
Non-host plants |
|
| Highly dependent upon mycorrhizae | Less dependent upon mycorrhizae | Not dependent | |
| Crop plants | Corn, flax, pulse crops, legumes, potatoes | Wheat, oat, barley | Canola, mustard, buckwheat |
| Weeds | Green Foxtail, wild oats, Canada thistle, chickweed, cleavers, dandelions | Wild buckwheat, lamb’s-quarters, stinkweed, redroot pigweed, kochia | |
(Turmel, 2006).
- The form of overwintering mycorrhizae affects its success.
Mycorrhizae may overwinter as hyphal strands following production of a host plant. When this hyphal strand network is left intact (through minimal tillage) it should be able to colonize the roots and access soil P quite early in the season. If hyphal strands are severed through tillage, the establishment of the P-absorbing network is delayed. Similarly, hyphal strands are very much reduced, if existent at all, following fallow or non-host crops.
Following fallow or non-host crops, recolonization must occur from the spore or resting state form of the mycorrhizae, and colonization could be delayed some 2 months before matching the level of colonization of a crop following a host crop (Turmel, 2006). This misses the important early season period where P uptake by annual crops is so important.
For an indication of typical spore levels in Prairie soils, refer to Table 2.
Table 2. Mycorrhiza spore population in typical Saskatchewan soils and cropping systems.
| Soils | Previous crop |
Spores/100g soil
|
| Canora silt loam | Wheat | 302 |
| Barley | 326 | |
| 1 yr summerfallow | 189 | |
| Hoey silty clay loam | Barley | 332 |
| 1 yr summerfallow | 127 | |
| 2 years summerfallow | 55 | |
| Rapeseed | 104 |
(Kucey and Paul, 1983)
It is apparent that spore numbers are less following rapeseed (a non-mycorrhizal crop) and fallow than following other crops. In fact after a year of canola the levels are similar to that of a year of summerfallow (Kucey and Paul, 1983). Nevertheless spores are still present to colonize dependent crops when next seeded. Even after 2 years of summer fallow there are spores present. Multiply the above values by 9 million to get an estimate of spore numbers per acre.
- Should one add more mycorrhizae to the soil?
Mycorrhizae have been found beneficial to many plants and are registered for use on some plants under certain soil conditions. These generally involve high value plants and soil conditions where mycorrhizal populations are low to non-existent. Examples of soils low in mycorrhizae may be sterilized potting soil, non-agricultural soil like subsoil in new housing developments or mine reclamation sites. Often the inoculants are most beneficial and useful for perennial plants, where they are inoculated once and remain for the life of the plant (trees, lawns).
Under the Canadian Food Inspection Agency, soil organisms like mycorrhizae, must be tested to ensure they are safe (non-injurious to plants and the environment) and efficacious (they are beneficial and do what they are supposed to). Only after testing and regulatory approval are they eligible for use and sale in Canada.
Mycorrhizal inoculants are currently registered for:
- Some greenhouse or some vegetable crops. Some may be grown on soils that have been sterilized.
- Reclamation of soils, such as mine sites, where soils may have little presence of mycorrhizae.
- Some perennials like trees, shrubs and turf grass.
A current brand of mycorrhiza inoculant registered for turf grasses provides 3.75 propagules (reproductive structures) per g and is applied at 20 g/sq m. That would equal 300,000 reproductive structures per acre or 1/1650th of what is naturally present as spores even in soil fallowed for 2 straight years.
In controlled studies comparing mycorrhizae-free soil and inoculation, plant growth advantages are often reported. However in agricultural soils, where many million mycorrhizal spores reside, there appears to be mixed and unpredictable results for field crops. Following are the observations from two Manitoba studies.
A) In a single year study of mycorrhizal inoculant on 6 flax cultivars, yield was depressed between 15-33%, possibly due to reduced stands from a formulation problem (WADO, 2008).
B) In studies with wheat in western Manitoba and Alberta there was no advantage to mycorrhizal inoculation, and inoculation could not substitute for P fertilizer to produce optimum yield (Table 3)
Table 3. Hard red spring wheat yield from treatments without and with mycorrhizal inoculation. (Grant et al, 2006).
| Site | Control | Plus Inoculant |
|
Wheat yield bu/ac |
||
| Brandon 1 | 47.7 | 47.7 |
| Brandon 2 | 22.4 | 20,.0 |
| Lacombe AB | 72.4 | 74.0 |
Mycorrhizal colonization of wheat in control treatments were between 74 and 89% of the inoculated treatments. The effect of the inoculant when applied with P fertilizer varied, causing increases at some sites and decrease or no effect at others. If there is no advantage to the plant from the mycorrhizal colonization, the fungus may depress yield potential by using the photosynthate of the crop (Grant et al, 2006).
In summary, until mycorrhizal inoculants are registered for field crops in Canada, they cannot be legally sold or used for this purpose. Companies looking to develop such a market must first conduct proper research showing the product to be safe and efficacious on these crops. The indiscriminate misuse of such inoculants in field crops may not only be ineffective but costly to growers.
References
Grant, C., G. Clayton, M. Monreal, N. Lupwayi, K. Turkington, and D. McLaren. 2006. Improving phosphorus nutrition in wheat – 2006 Update. Research progress report.
Kucey, R.M. and E.A. Paul. 1983. Vesicular arbuscular mycoohizal spore populations in various Saskatchewan soils and the effect of inoculation with Glomus mosseae on faba bean growth in greenhouse and field trials. Can. J. Soil Sci. 63: 87-95.
Turmel, M. S. 2006. Exposing the Mycorrhizae in agriculture. Manitoba Agronomists Conference.http://www.umanitoba.ca/afs/agronomists_conf/proceedings/2006/turmel_exposing_the_Mycorrhizas.pdf
Westman Agricultural Diversification Organization (WADO). 2008. Annual report. Variety variation in flax and responses from mycorrhizal inoculation. Pp. 94-96.
For further information, contact your GO Representative.
