17.2  Growth and cropping responses

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Table 17.3


Figure 17.2 Soil salt restricts plant growth so that crop yield is reduced, but species differ in sensitivity. These four broad categories of salt tolerance were delineated by the USDA Soil Salinity Laboratory, Riverside, from a statistical analysis of an extensive survey of published data on yield and soil salinity (measured as electrical conductivity (ECE) of a saturated extract and expressed here as deciSiemens per metre (dS m-1)). Crops representative of each category are listed in Table 17.3. (Based on Maas and Hoffman 1977)

Salts dissolved in soil water inhibit plant growth because (1) salt reduces water uptake, and (2) excessive salt becomes toxic and causes further reductions in growth. To exist in a saline soil, plants must take up water but exclude salt.

Extensive research in California during the 1970s (USDA Salinity Laboratory, Riverside) provided baseline data on comparative salt tolerance for a wide range of crop plants (Table 17.3; Figure 17.2). Statistical analysis of this far-ranging survey of crop plants showed that (1) yield did not generally decrease significantly until a salinity threshold had been exceeded, and (2) that yield generally decreased linearly with further increase in salinity. Some deviations from linearity occurred as relative crop yield dropped below 20–30%. The yield–salinity relationship (Figure 17.2) becomes steeper, and threshold salinity decreases from ‘tolerant’ to ‘sensitive’ cat-egories. Representative crops in each category (Table 17.3) highlight a number of horticultural species as sensitive or moderately sensitive, compared with cereals and coarse grains that are either moderately tolerant or tolerant.

For survey purposes (Table 17.3; Figure 17.2), soil was regarded as saline if electrical conductivity of a saturated extract was more than 4–5 dS m–1, equivalent to about 40–50 mM NaCl, and sensitive plants such as lupin are greatly reduced at this level of salinity. By contrast, tolerant plants such as barley withstand 8 dS m–1 (equivalent to about 80 mM NaCl) while specialised halophytes grow under highly saline conditions, with NaCl concentrations reaching or even exceeding that of sea water, which is about 500 mM (Sections 17.3, 17.4).