7.4: Crop Salt Tolerance

The salt tolerance of a plant is defined as the plant’s capacity to endure the effects of salt. Crop salt tolerance is not an exact value because it depends on many factors. Although salt tolerance cannot be stated in absolute terms, relative crop response to known salt concentrations under typical conditions can be predicted. For a more complete reference on crop salt tolerance see Maas and Hoffman (1977).

Figure 7.7. Relative grain yield of corn grown in the Sacramento-San Joaquin Delta of California as a function of soil salinity (adapted from Hoffman et al., 1983).

The salt tolerance of a crop can be described by plotting relative crop yield as a continuous function of soil salinity (Figure 7.7). For most crops, this response function follows a sigmoidal relationship where crop yield is not reduced significantly as salinity initially begins to increase but, as salinity increases further, yield is reduced rather rapidly. Then, as salinity reaches high levels, crop yields, although low, do not decrease as rapidly as at moderate concentrations. For practical purposes this sigmoidal relationship for crop salt tolerance can be represented by two straight lines, one line is a tolerance plateau with a slope of zero and the other line is concentration dependent and its slope indicates the yield reduction per unit increase in salinity.

Figure 7.7 shows the “two straight lines” model fitted to actual field data for corn grain yield. The point at which the two straight lines intersect designates the salt tolerance threshold which is the maximum soil salinity that does not reduce yield appreciably below that achieved under nonsaline conditions. For soil salinities exceeding the threshold, relative yield (Yr) in percent can be estimated from:

Y_r = 100 – S(EC_e – T) for EC_e > T

where: T = salt tolerance threshold expressed in EC_e units of dS/m,

S = slope expressed in % per dS/m, and

EC_e = the mean salt concentration in units of electrical conductivity of saturated soil extracts taken from the crop root zone.

The threshold and slope values provide general guidelines about salt tolerance for crop management decisions. Irrigators need to know the level of soil salinity that initiates yield reduction (T, threshold) and the rate at which yield is reduced at salt levels greater than the threshold (S, slope). Typical ears of corn from the experimental results plotted in Figure 7.7 are shown in Figure 7.8. The top row of ears were grown using nonsaline irrigation water; the bottom row with irrigation water having an EC of 8 dS/m.

Figure 7.8. Example ears of corn produced with irrigation water having no salt (top) and with salt concentrations equal to one-fourth the salt concentrations of sea water (bottom).

Crops differ significantly in tolerance to soil salinity. The relative salt tolerances of major crops are given in Table 7.2. The table gives the salt tolerance threshold (T) and the percent yield decline (S). These two values can be inserted into the salt tolerance equation (Equation 7.3) to predict relative crop yield (Yr). Qualitative ratings for ease in comparisons among crops are also given in Table 7.2. The qualitative salt tolerance ratings are sensitive (s), moderately sensitive (ms), moderately tolerant (mt), and tolerant (t). These qualitative ratings can be seen in Figure 7.9.

Table 7.2. Salt tolerance of major crops (adapted from Maas and Hoffman, 1977).

Crop	Salt Tolerance Threshold, T (dS/m)	Percent Yield Decline, S %/(dS/m)	Qualitative Salt Tolerance Rating ^[a]
Grain Crops
Barley	8.0	5.0	t
Corn	1.7	12	ms
Cowpea	4.9	12	mt
Rice	3.0	12	s
Sorghum	6.8	16	mt
Soybean	5.0	20	mt
Wheat	6.0	7.1	mt
Fiber, Sugar and Oil Crops
Cotton	7.7	5.2	t
Flax	1.7	12	ms
Peanut	3.2	29	ms
Sugar beet	7.0	5.9	t
Sugar cane	1.7	5.9	ms
Grasses and Forage Crops
Alfalfa	2.0	7.3	ms
Bermuda grass	6.9	6.4	t
Clover	1.5	12	ms
Fescue	3.9	5.3	mt
Orchard grass	1.5	6.2	ms
Ryegrass	5.6	7.6	mt
Trefoil, birdsfoot	5.0	10	mt
Vegetables and Fruit Crops
Asparagus	4.1	2.0	t
Bean	1.0	19	s
Cabbage	1.8	9.7	ms
Carrot	1.0	14	s
Celery	1.8	6.2	ms
Corn, sweet	1.7	12	ms
Lettuce	1.3	13	ms
Potato	1.7	12	ms
Strawberry	1.0	33	s
Sweet potato	1.5	11	ms
Tomato	2.5	9.9	ms
Woody Crops
Almond	1.5	19	s
Apricot	1.6	24	s
Blackberry	1.5	22	s
Date palm	4.0	3.6	t
Grape	1.5	9.6	ms
Grapefruit	1.8	16	s
Guayule	15	13	t
Orange	1.7	16	s
Peach	1.7	21	s
Plum	2.6	31	s
^[a]s = sensitive; ms = moderately sensitive; t = tolerant; mt = moderately tolerant

Figure 7.9. Division boundaries for qualitative salt tolerance ratings of crops (adapted from Maas and Hoffman, 1977).

A handy guide to classify potential crop damage from increasing salt levels in irrigation waters is given in Table 7.3. The reader is cautioned, however, that the use of saline water depends upon the crop, soil, climate, geology, and management practices. Thus, this classification is only a rough guide.

Table 7.3. Classification guide for saline irrigation water.

Irrigation Water	Salt Concentration (ppm or mg/L)	Electrical Conductivity (dS/m)	Crop Problems
Fresh	< 300	< 0.5	none
Slightly saline	300–600	0.5–1	rare
Moderately saline	600–1,200	1–2	occasional
Saline	1,200–2,400	2–4	common
Highly saline	2,400–4,800	4–7	severe

Example 7.2

A saline area of a field has an average salt concentration of 3,000 mg/L. Calculate the relative yield of corn in this salt-affected soil. If the nonsaline portion of the field produces 180 bushels per acre, what is the actual yield of the saline area in the field?

Given: C = 3,000 mg/L

Nonsaline corn yield = 180 bu/ac

S = 12% / (dS/m) and T = 1.7 dS/m (Table 7.2 for corn)

Find: Relative (Y_r) and actual (Y_a) corn yields in the saline area

Solution

\(EC = \dfrac{C}{640}=\dfrac{3,000}{640}=4.7 \text{ dS/m}\) (Eq. 7.2)

\(Y_r=100-S(EC_e-T)\) (Eq. 7.3)

\(Y_r=100-12(4.7-1.7\)

\(64\%\)

\(Y_a=Y-r\times Y_{max}\)

\(Y_a = 0.64 (180\text{ bu/ac})\)

\(Y-a = 115 \text{ bu/ac}\)