Salt Leaching, Pecan South

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 salinity

of irrigation water, ECe the mean

salinity of the root zone measured in

the soil saturation extract (Rhoades

and Miyamoto, 1990), and n is an

empirical coefficient. Typically, n =

2 in sandy soils, and n = 1 in clayey

soils (Rhoades, 1974; Miyamoto et al.,

1986).

The consumptive use varies

with tree sizes, weather conditions

(Miyamoto, 1983: Miyamoto,

1985), as well as floor management

practices (Prichard et al., 1990). We

assumed a typical annual use rate

of 40 inches, and the threshold soil

salinity (ECe) of 2.5 dS m-1 (Miyamoto,

2006: Miyamoto et al., 1986). The

computed leaching requirements and

the irrigation needs are shown in

Table 1. The leaching requirement

increases with increasing salinity

of irrigation water, and so does the

irrigation water requirements. This

type of estimate may be found in

many other publications.

The question is how reliable is

the estimate? In the New Mexico

section of the Middle Rio Grande,

salinity of the water ranges typically

from 400 to 600 ppm (or 0.6 to 0.8

dS m-1), and we seldom observe soil

salinity exceeding 2.5 dS m-1. In the

El Paso Valley, it usually ranges from

700 to 850 ppm or 1 to 1.2 dS m-1, and

salt problems are increasingincrease

downstream, especially in clayey soils.

The leaching requirement computed

by Eq (1) is small, ranging from 2 to

11 percent for the prevailing salinity

of irrigation water used in the region

(Table 1). Nonetheless, the equation

shows that growers in the El Paso

Valley need to better use a greater

quantity of water than the folks

upstream. This seems to make sense.

However, it does not necessarily

indicate that Eq (1) is reliable, but

simply means that the LR is small

when salinity of irrigation water is

low. In reality, the errors involved

in estimating the consumptive use

or measuring the check-in flow are

usually larger than the difference in

irrigation quantity needed to meet

the prescribed leaching fraction at a

low salinity range (Miyamoto, 1983).

In other words, Eq (1) is essentially

a conceptual model, and soil salinity

has to be checked through soil salinity

monitoring.

There are also some questions

about Eq (1) at a high salinity range

(>2

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