stream for regulating osmotic pressure within the cells and body fluids, where it protects against ex-
cessive loss of water [11]. In contrast, the principal electrolyte for plants is K, and even in ecosystems
where there is a predominance of Na, plants exhibit a strong preference for K. Because of this contrast
in electrolyte requirements, insufficient Na is available in the edible portions of most plants for herbi-
vores. Therefore the dietary requirements of herbivores for Na must be met from external supplements,
such as salt licks.
It is possible under the appropriate management conditions (such as low K and high Na in the root-
ing media) for many crop plants to accumulate significant concentrations of Na in their edible tissues. Un-
der such conditions, a number of vegetative crops can supply a significant portion of an animal’s dietary
requirements for Na. Such an increase in plant tissue Na would be especially desirable in meeting the
metabolic Na requirements of grazing animals where it is difficult to supply Na through external supple-
ments [12]. For example, in intensively farmed areas of New Zealand, the Na levels in pastures are in-
sufficient to meet the metabolic requirements of grazing animals. Thus, improving Na levels in pasture
crops could play an important role in meeting the dietary Na needs, which in turn could substantially en-
hance their appetite, daily food intake, and weight gain [12]. In our present industrial society, there are
relatively few instances in which human Na requirements are not met from diet. This is mostly because
of Na additions (both for taste and as a preservative) to most processed foods.
Problems of secondary salinization associated with irrigated agriculture can partially be related to the
long-term affect of continued discrimination against Na during nutrient uptake by plants. This secondary
salinization could be limited if the appropriate management procedures were applied, such as leaching,
restrictive K fertilization, and biomass removal, along with using genetic strategies to secure efficient
plants for Na mobilization. This chapter summarizes the current level of understanding of Na metabolism
in plants. Although Na is not a major nutrient in most plants, there is some degree of utilization of Na in
many if not all plants. The high degree of similarity between K and Na (physical and chemical properties)
and the extensive use of Na by a number of salt-tolerant plants suggest that there is a potential for Na sup-
plementing or replacing K in many, if not all non–K-specific monovalent plant functions. However, to in-
crease significantly Na utilization by plants requires the implementation of suitable nutrient management
practices and/or appropriate genetic strategies. Such a utilization could be useful in the management of
certain saline agricultural systems and the management of low-Na pastures. Sodium cycling is also a
problem in other biosystems including the bioregenerative life support systems being developed by the
National Aeronautics and Space Administration (NASA) for space. In such a closed system all waste
products must be recycled, including Na-containing wastes, to grow plants that provide food, O 2 , and
clean water for the system (see Chapter 48 by Wheeler et al.).
364 SUBBARAO ET AL.TABLE 1 Chemical Characteristics and Comparison of Sodium and
Potassium Concentrations in Soils, Natural Waters, and Plants
Sodium Potassium
Atomic number 11 19
Atomic weight 22.9 39.5
Concentration in lithosphere (ppm) 28.3 25.9
Mineral soils: (% as the oxides)
Tropical 0.01–0.5 0.1–2
Temperate 0.01–1.0 0.1–4
Soil solution (mM) 0.4–150 0.2–10
Soil solution in field soils (mM) — 0.08–1.6
Seawater (mM) 480 10
Rivers of North America (mM) 0.4 0.04
Rivers of Australasia (mM) 0.13 0.04
Plant foliage
Glycophytesa 0.2–2.0 15–50
Halophytesb 25–154 10–33aGrown in 5 mM K 1 mM Na (g kg (^1) dwt).
bGrown in 5–8 mM K 295–340 mM Na (g kg (^1) dwt).
Source: Ref. 6.