The Na requirement for lactating dairy cows is approximately 2 g kg^1 dwt in forage [12], which is higher
than the average Na content of natrophobic pasture species [50]. In contrast, the K content (20 to 25 g
kg^1 dwt) in natrophobic species is usually adequate or in excess of animal needs. An adequate Na con-
tent can increase the acceptability of forage to animals and enhances daily feed intake [170].
C. Agricultural Production Systems—Secondary Salinization
Secondary salinization associated with irrigated agriculture is a serious problem, threatening the long-
term sustainability of many production areas [171]. Because of the inherent limitations associated with
most engineering approaches, large tracts of cropland are becoming saline despite the enormous resources
committed to these projects. Nearly 2.12 million ha of irrigated cropland out of total of 14 million ha un-
der irrigation in Pakistan became saline after only a few years of irrigation [172]. Similarly, 40% of the
irrigated cropland in Iraq and Iran has been affected by secondary salinization [173], as has nearly a third
of the irrigated cropland in India [174]. Secondary salinization is a constant threat to the agriculture-based
economy of California, where irrigated agriculture plays a critical role in the production of fruits and veg-
etables [175]. According to estimates of the Food and Agriculture Organization (FAO) and the United
Nations Educational, Scientific, and Cultural Organization (UNESCO), as much as half of all the exist-
ing irrigation systems of the world are under the influence of secondary salinization, alkalization, and wa-
terlogging. This phenomenon is common not only in established irrigation systems but also in areas where
irrigation has only recently began. Worldwide, nearly 10 million ha of irrigated lands are abandoned
yearly because of secondary salinization resulting from irrigated agriculture [176].
The long-term survival of irrigated agricultural production systems depends on tackling salinity prob-
lems in an integrated manner. There needs to be a proper balance of nutrient management and irrigation
coupled with the biological option of genetic improvement of the salinity tolerance of crops. Most crops
discriminate against Na uptake, favoring K instead (see Chapter 44 by Subbarao and Johansen), which
leads to buildup of Na salts in soils as irrigation continuously delivers some Na salts, although at low con-
centrations. Integrating crops that have the ability to take up significant amounts of Na into the cropping
systems could help bring about a more balanced flow of Na through these irrigated systems. This could be
particularly relevant to high-intensity irrigated systems, where vegetable crops are grown under intensive
fertilizer and irrigation management. Use of crops such as red beet, celery, pac-choi, Swiss chard, and
horseradish coupled with judicious limitation of K application to facilitate Na uptake could help to bring
a positive shift in the salt balance of these regions. As noted earlier, a major portion of K could be replaced
by Na without negative effects on crop growth rates. Our preliminary estimations indicate that red beet
could remove as much as 900 kg of NaCl ha^1 (about 60- to 80-day growing period), assuming that Na
levels in the plant (both tops and roots) reach a moderately high level of 50 g kg^1 dwt and a productivity
of about 70 Mt fresh wt ha^1 (of both tops and tubers) (I. Goldman, personal communication).
D. Closed Life Support Systems for Space
Advanced life support systems (ALSS) being studied for space travel must ultimately strive for self-suf-
ficiency in providing food, potable water, and a breathable atmosphere for humans. Such closed life sup-
port systems could form the basis for human colonies on the lunar or Martian surface [177]. Bioregener-
ative components of such systems would use plants to regenerate oxygen, food, and clean water through
photosynthesis and transpiration. However, to minimize resupply costs, waste materials need to be com-
pletely recycled to provide nutrients for sustained plant production. For example, inedible plant biomass
could be processed in bioreactors with the effluent nutrients used in the food production systems
[178–181]. Human wastes (both solid and liquid) would also need to be processed as a source of nutrient
and water inputs. If the requirements of plants and humans were the same, then nutrient cycling from one
component to another would not be a problem. However, some elements such as Na are needed in rela-
tively high levels for human metabolism but are absorbed and utilized by plants in only limited amounts.
This discrepancy in Na metabolism between humans and plants could pose a threat to the system’s long-
term equilibrium if cycling is incomplete and external Na is used to meet the metabolic requirements of
humans (Figure 6).
In a functional bioregenerative system, human urine would be one of the waste products recycled
back to the plant production systems as a source of water and nutrients, especially N. Nearly 900 mmol
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 377