Salt Leaching, Pecan South
Enviado por rochaanny • 22 de Noviembre de 2012 • 5.294 Palabras (22 Páginas) • 438 Visitas
28 PECAN SOUTH
Pecans, along with almonds
and walnuts, are among the
most salt-sensitive tree crops
currently grown under irrigation.
Many growers are not aware that
salts are affecting tree growth, nut
yields and quality, because symptoms
of salt-affected trees are difficult to
differentiate from water stress (Fig.
1). Diagnosis of salt problems and
general approaches for minimizing
soil salinization have been previously
discussed (Miyamoto, 2006: Miyamoto
et al., 1986). The present article
provides additional information on
practices of salt leaching during
routine irrigation as well as salt
leaching irrigation for restoration.
The objective of salt leaching is
to keep root zone soil salinity below
a level that negatively impacts tree
growth and production. The threshold
salinity of irrigated pecans is in the
range of 2 to 3 dS m-1 when measured
in the soil saturation extract
(Miyamoto et al., 1986), an official
method of measuring soil salinity
(Rhoades and Miyamoto, 1990). In
areas rich in gypsum, trees may
tolerate higher levels of soil salinity,
probably by 1 or 2 dS m-1. Calcium
and sulfate ions are less harmful to
pecan trees than sodium and chloride
ions (Miyamoto et al., 1985).
There are 2 ways to approach
the task of salt leaching. The first
approach is to leach following each
irrigation as a maintenance practice.
The second approach is to let salts
accumulate in the orchard during
the growing season and correct by
flushing during the dormant period.
The latter takes into account the
reality that; soil salinity levels vary
widely even in a small orchard, and
it is more convenient to leach during
the dormant period. Once any part
of the orchard begins to be salinized,
growers need to carry out salt
leaching irrigation for restoration.
Needless to say, the cause of soil
salinization has to be identified
prior to deciding the method of salt
leaching for restoration. This subject
is discussed in the second part of this
article.
Minimizing Soil Salinization
Theories:
Salts are carried into irrigated
fields through irrigation water. The
quantity amounts to at least several
tons per acre annually, depending on
the salt content of the water and the
irrigation rate. If there is no drainage,
salts accumulate in the order of 20 to
50 tons/acre in 10 years. Leaching
salts is therefore critical to health
of the orchard. There are 2 concepts
in orchard salinization; one concept
assumes that irrigation management
is the key to prevent salinization,
and the other assumes that soil
type controls soil salinization. Both
concepts are correct, depending on
circumstances.
Irrigation-Based Approach:
This approach is cited in numerous
publications under the common name
of “leaching equation”, and is based
on the idea of providing the drainage
required to maintain the salt balance
in the root zone each time we irrigate
(Rhoades, 1974). The leaching
equation has several assumptions:
a) the site soil is permeable enough
to allow necessary water infiltration
and drainage, and b) the salt carried
into the field is being leached
quantitatively, and c) crops respond
to the mean salinity of the root zone.
The applicable equation to compute
the leaching requirement is
LR = (Dw - ET) / Dw = ECw / ECd (1a)
= ECw/ [2 (n + 1) ECe - nECw] (1b)
where LR is the leaching requirement,
DW the depth of irrigation, ET the
consumptive use, ECw the
...