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[SANET-MG] On-farm production of AM fungus inoculum
I just ran across an article about "On-farm production of AM fungus
inoculum..." that may be of interest to some of you...
Volume 97, Issue 6 , April 2006, Pages 809-818
On-farm production of AM fungus inoculum in mixtures of compost and
D.D. Douds, Jr.a, , , G. Nagahashia, P.E. Pfeffera, C. Reiderb and W.M.
On-farm production of arbuscular mycorrhizal (AM) fungus inoculum can reduce
the cost of the inoculum and increase utilization of this symbiosis in plant
production. Bahiagrass (Paspalum notatum Flugge) seedlings, colonized by AM
fungi, were transplanted into raised bed enclosures. Media within the
enclosures was vermiculite mixed with either field soil or yard clippings
compost in Experiment I and vermiculite mixed with yard clippings compost or
dairy manure/leaf compost in Experiment II. Compost and vermiculite mixtures
yielded more propagules of AM fungi than soil-based mixtures in Experiment
I. Growth of plants in a 1:4 (v/v) mixture of yard clippings compost and
vermiculite produced more inoculum (503 propagules cm−3) than growth
in 1:9 and 1:99 (v/v) mixtures (240 and 42 propagules cm−3,
respectively). Water, inorganic nutrient solution minus P, and fish protein
digest were added to inoculum production enclosures in Experiment II.
Results indicated that supplemental nutrient addition was unnecessary. This
method produces a concentrated inoculum of AM fungi in a form readily used
as an amendment to horticultural potting media for the production of
Keywords: Sustainable agriculture; Organic agriculture; Biofertilizer
Inoculum of AM fungi was successfully produced in compost diluted with
vermiculite. Overall averages of 530 and 505 propagules cm−3 were
produced in Experiments I and II, respectively, and up to 80 spores
cm−3, in 1:4 (v/v) yard clippings compost and vermiculite. This
greatly surpasses the target inoculum density of 80?100 propagules
cm−3 to qualify as mass production (Feldmann and Grotkass, 2002). The
lower average of 505 propagules cm−3 yields 37.57 × 106 propagules per
0.61 × 0.61 m enclosure section. A target inoculation of 100 propagules per
vegetable seedling to ensure colonization during growth in the greenhouse
prior to outplanting means one enclosure section can be used to inoculate
375,700 plants. This translates to 7.6 ha of tomatoes at a planting density
of 49,400 plants ha−1, or five times that with use of the entire
enclosure. Uniformly mixing such a small amount of inoculum into the potting
media is difficult, so a more generous inoculation may be more practical.
On-farm production of AM fungus inoculum via different techniques has been
described previously. Sieverding, 1987 and Sieverding, 1991 developed a
procedure in Colombia whereby Glomus manihotis was introduced into fumigated
soil. Direct comparison with the method reported here is difficult because
total propagules were not estimated. After 4 months of growth of host
plants, 42?96 spores cm−3 were produced (assuming an average bulk
density of soil of 1.2 (Thompson and Troeh, 1973)). Only 9 spores cm−3
were produced without fumigation. Dodd et al. (1990) used this method to
produce inocula containing three AM fungi, including approximately 208
spores cm−3 of G. manihotis.
Another method was developed at the Tata Energy Research Institute in India
(Douds et al., 2000 and Gaur, 1997). One of a number of AM fungi was
inoculated into furrows in raised beds of fumigated soil. Three crops, e.g.,
sorghum, corn, and carrot, were grown successively in the bed, each for 3
months. The soil and roots were chopped and remixed prior to planting the
next nurse host. This method produced an average of 22.7 infectious
propagules cm−3 after three 1-year cycles. A modification of this
method using one of three vegetable crops as a host plant produced
approximately 1 propagule cm−3 in soil amended 1:1 or 1:2 (v/v) with
compost (Gaur et al., 2000 and Gaur and Adholeya, 2002).
The method developed here is competitive with existing methods and, in
addition to not requiring fumigation, offers several advantages to farmers
in temperate climates. First, bahiagrass is a proven host plant that
supports abundant colonization and sporulation by most, if not all, AM
fungi. In addition, as a tropical C4 grass it is frost killed. This
minimizes its potential to become a weed pest when the inoculum is used in
the field. Also, bahiagrass is taxonomically unrelated to most crops and,
therefore, is unlikely to allow the proliferation of crop pathogens. Second,
all materials for the construction of the enclosures are readily available
to farmers. The precolonized host plants could be purchased or grown on-site
using commercially-available inocula. Another strength lies in the benefits
of compost. Compost provides all the nutrients necessary for the growth of
bahiagrass. The P concentration of compost, however, requires that it be
diluted with a relatively inert material, such as vermiculite, prior to use
because high levels of available P suppress the colonization of roots by AM
fungi (Smith and Read, 1997). Compost also provides a broad spectrum of
microbes to benefit the biological health of the soil and is known to
suppress plant diseases (Hoitink and Fahy, 1986 and Kim et al., 1997).
Lastly, a taxonomically diverse mixed inoculum can be produced when
different AM fungi are inoculated into separate enclosure sections. Recent
research indicates that there is functional diversity among AM fungi (Smith
et al., 2000 and Stampe and Daehler, 2003). Though some AM fungi are
beneficial in a wide variety of soil?plant combinations (Sylvia et al.,
1993), some data suggest that host benefits are greater with certain AM
fungi than others (Blaszkowksi, 1993 and Pope et al., 1983). A diverse
inocula increases the opportunity for an optimal match with the eventual
crop host, and it also allows for the inclusion of Gigaspora spp. to aide
soil aggregation (Miller and Jastrow, 1992 and Wright and Upadhyaya, 1996).
Gigaspora spp. are typically absent from commercial formulations.
This inoculum production system targets vegetable producers who grow their
own seedlings for transplant to the field. The most practical use for this
inoculum is mixing it into horticultural potting media in which the
seedlings are grown. Inocula produced by all currently available on-farm
methods are not amenable to large scale direct application to fields via
farm machinery. Direct application to the field is possible in labor
intensive farming systems in the tropics (Gaur et al., 2000 and Sieverding,
1987) or in small scale organic or community supported agriculture (CSA)
Though plant response to AM fungus inoculation may vary from positive to
negative in the presence of different AM fungi (Klironomos, 2003 and Marin
et al., 2003), there are management situations in which the response to AM
fungus inoculation can be more reliably predicted. Certain crops such as
Cruciferae, spinach, and sugar beet do not become colonized by these fungi
and will not respond to inoculation. Soil solarization and fumigation
treatments adversely affect indigenous AM fungi, and inoculation with AM
fungi has been shown to enhance plant growth and yield following these
treatments (Wininger et al., 2003). Other situations in which AM fungus
inoculation would be beneficial are after a long period of bare fallow
(Thompson, 1987), following a nonmycotrophic crop such as rapeseed, or in
soils with low available P.
The inoculum production method described here also may be used to
proliferate indigenous AM fungi. Therefore, farmers may be able to
re-introduce to their production fields the AM fungi adapted to their soils.
Doing so may only require adding soil from an uncultivated part of the farm
(e.g., fence row or natural wooded area), to the compost. No pre-inoculated
plants would be required. Further, a contributing factor to the high yields
found when native soils are first put under cultivation is the soil
biological community (Aleman, personal communication), important members of
which are the AM fungi.
We have developed a method for on-farm production of hundreds of millions of
propagules of AM fungi within raised bed enclosures containing compost and
vermiculite. More research is needed to develop formulae to predict the
proper dilution ratio for optimum inoculum production given the nutrient
analysis of specific composts and explore the production of indigenous AM
fungi in similar enclosures.
Dept of Soil Science
NC State University
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