Agriculture Reference
In-Depth Information
soil. This has been demonstrated for copper. Increased transpiration capacity
has also been measured in leek plants with mycorrhizal roots.
The symbiosis of plants and AM fungi dates from the earliest stages of land
plant evolution, and even pre-dates the evolution of true roots (Raven
et al.
,
2005). The extent of the ramification through soil of the external hyphae of
AM and other types of mycorrhizae and their importance in the biogeo-
chemical cycling of mineral nutrients, have gained increasing recognition in
recent years (Read and Perez-Moreno, 2003; Leake
et al.
, 2004). The meagre
root system of allium crops (see Chapter 2) is almost certainly inherited from
wild ancestors that were in symbiosis with AM, which increased the nutrient-
foraging capacity of their roots. Some spectacular increases in growth of both
directly sown and transplanted allium crops have been recorded following root
inoculation with AM in low-P soils.
However, the relevance of all this to modern allium vegetable production
has been questioned (Stribley, 1990; Ryan and Graham, 2002). These high-
value crops are normally grown in soils rich in P, and since fertilizer inputs as a
whole represent only about 5% of variable production costs (Nix, 2006),
farmers have little incentive to risk loss of yield by failing to maintain soil P at a
level sufficient for maximum yield. Nevertheless, several reports indicate that
AM do give significant benefits in some allium production situations. In Israel,
the destruction of native soil AM by solarization or methyl bromide fumigation
(see 'Root Diseases') lowered chive yields on commercial farms, but yields could
be restored and even increased by transplanting seedlings inoculated with AM
(Wininger
et al.
, 2003). In soils of moderate to high P status in Denmark, leeks
sown in soils which had previously grown crops or green-manures that were
hosts to AM had higher initial P uptake, faster seedling growth and faster root
colonization by AM than leeks sown following rotational pretreatments that
lowered soil AM populations - for example, after brassicas, which are non-
mycorrhizal (Sorensen
et al.
, 2005). On nutrient-rich peat soils in the UK,
module-raised, AM-inoculated onion transplants gave fewer thick-necked
plants than non-inoculated seedlings (Snellgrove and Stribley, 1986). A
similar effect was observed in a P-rich soil with direct-sown onions given
ammonium phosphate liquid 'starter fertilizer' (see Chapter 6), and this was
attributed to increased seedling growth rates acting via the chain of causation
shown in Fig. 4.33.
The significance of AM for allium crop production may increase in future.
First, because there is a growing demand for 'organic' vegetables in some
countries and, in this production system, soluble P fertilizers are banned;
consequently, soil under organic management tends towards lower levels of
available P (de Melo, 2003). Secondly, in conventional agriculture there is
increasing emphasis on developing lower input and more 'sustainable'
production systems (Sorensen
et al.
, 2005) in order, among other things, to
minimize water pollution from P-rich soils. Thirdly, sources of readily available
fertilizer P are finite and may become more expensive and in short supply in
