Agriculture Reference
In-Depth Information
such non-pristine foliage. Although the total transpiration will be less than for a
well-grown crop, low LAIs have increased transpiration rates per unit leaf area
due to more micro-scale transfer of heat from dry soil surfaces to the leaves and to
a higher aerodynamic roughness.
IRRIGATION TECHNIQUES AND EXPERIMENTS
Onion crops are grown using furrow,
sprinkler and trickle irrigation. Furrow irrigation involves inundating furrows
adjacent to raised beds on which onions are planted, using systems of channels
and small sluices. If salinity is a problem, onions may be planted on the side of
raised beds, since salts reach a maximum concentration at the apex of the bed
after being drawn there by evaporation.
Various types of sprinklers are used on onions, including centre pivots,
moving gantries or fixed lines. If onions are directly sown, frequent light
sprinkler irrigation is often necessary to ensure rapid and even emergence by
keeping the seeds and seedlings hydrated and preventing the formation of a dry,
impenetrable crust on the soil surface. In areas prone to salinity it is important
to avoid a salty 'crust' developing after frequent light irrigation, as this is
particularly damaging to emerging seedlings (Miyamoto, 1989).
Beginning in the early 1990s, onion and garlic crops have been produced
using plastic 'drip-lines' to supply water and, often, dissolved fertilizer (N, and
sometimes N and K); the latter is termed 'fertigation'. Particularly in semi-arid
regions, these techniques have led to dramatic increases in yield. For example,
in the Treasure Valley of eastern Oregon, USA, yields above 115 t/ha of long-
storing, 'Sweet Spanish' onion bulbs can reliably be produced under drip
fertigation, up from 70 t/ha under furrow irrigation (Shock
et al.
, 2004).
Near Lleidda, in north-eastern Spain, yields of 90 t/ha of bulbs (equivalent
to 11-13 t/ha of dry matter) are produced, up from 50 t/ha under sprinkler
irrigation (Bosch-Serra and Currah, 2002). Garlic yields of 30-40 t/ha are
achieved under drip fertigation in Mexico, three times previous yields, and the
larger bulb sizes achieved have necessitated a re-evaluation of optimum
densities and clove sizes for planting (Castellanos
et al.
, 2004). Onion growers
using fertigation have received a price premium for the very large bulbs
produced. In Oregon the increasing sizes are reflected by a progression of
superlatives to name the grades marketed: i.e. bulb diameters 76-102 mm,
Jumbo, 102-108 mm, Colossal, > 108 mm, Super Colossal.
Drip-lines may be laid on the surface or buried 10-15 cm deep in the soil.
In the system described by Shock
et al.
(2004), drip tapes were installed at a
depth of 13 cm between double rows of onions spaced 0.56 m apart on beds
1.1 m wide. The drip tape had emitters spaced 30 cm apart, with a flow rate of
0.5 l/h. Fertilizers other than N were incorporated into the silt-loam soil before
bed formation in amounts determined from soil analysis. N fertilizer was
applied through the drip-tape as urea-ammonium nitrate on five occasions at
10 day intervals during the main period of leaf growth. Soil water potential was
measured by sensors 20 cm below the onion rows. The mean soil water
