From: Plant Nutrient Management in Hawaii’s Soils, Approaches for TropicalPlant and Nutrient Subtropical Management Agriculture in Hawaii’s Soils J. A. Silva and R. Uchida, eds. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, ©2000
Chapter 17
S. A. El-Swaify
Definitions and forms of salinity (K+), magnesium (Mg2+), and calcium (Ca2+) and the Ð 2Ð All natural waters contain soluble salts. The concen- anions chloride (Cl ), sulfate (SO4 ), and carbonate in Ð tration of the salts determines whether the water is of the form of bicarbonate (HCO3 ). Boron, as non-disso- high quality (drinkable or usable for irrigation without ciated boric acid, can also be present in significant quan- need for special precautions) or of low quality (brack- tities in certain situations. Data on the total concentra- ish or saline). Water in the soil also contains soluble tion of these constituents, their relative abundance (par- salts (sometimes called free or nonattached salts). The ticularly that of Na+), and their effect on soil pH are amount of salts in the root zone (or the salt concentra- used to categorize the quality of water for irrigation, tion in the soil solution) determines whether the soil is determine the suitability of soil for cultivation, select “normal” or “salt-affected” (saline, sodic, or saline- crops that are most adaptable for use, and assess the sodic). need for soil reclamation. Salinity becomes a concern when an “excessive” amount or concentration of soluble salts occurs in the Sources of salinity soil, either naturally or as a result of mismanaged irri- Salts in soil and irrigation water may be either gation water. Worldwide, salt-affected soils are most ¥ naturally present as products of geo-chemical weath- abundant in arid regions, and in irrigated lands the for- ering of rocks and parent materials mation of salt-affected soils is the most important pro- ¥ derived directly from sea water by flooding, spray, cess of chemical soil degradation. In Hawaii, salinity or intrusion into groundwater resources is more of a concern in irrigated shoreline areas where ¥ caused by irrigation mismanagement, particularly groundwater supplies are influenced by intrusion of sea when internal soil drainage is impeded. water. Sediments dredged from salt-laden deposits are also chronic problems, particularly when used as fill The hazard of excessive salt accumulation in irrigated on which vegetation is attempted. In Hawaii, salinity soils increases when leaching of salts from the root zone is generally a cyclical problem related to water avail- is not adequately provided for. In this respect, soil sa- ability from rainfall; it is aggravated in dry years and linity avoidance may run counter to the concept of relieved in wet years. Fortunately, most of Hawaii’s “water use efficiency” if water management is not soils have physical and structural qualities that impart wisely practiced. In such situations, excess irrigation behavioral resilience and prevent salts from imposing is not “wasteful.” It is a necessity, because irrigation irreversible soil degradation. must be applied above and beyond the growing crop’s The most common salts in water and soil solutions consumptive use in order to meet leaching require- are composed of the cations sodium (Na+), potassium ments. As expected, the higher the salinity level, the
151 College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
more leaching is required. Preventing salinization is Salinity measurements are made on soil-water ex- more feasible, logistically and economically, than re- tracts, the most commonly used being the saturated claiming salt-damaged soils. soil extract (or saturation extract) obtained from a satu- Inorganic fertilizers are also salts, but they are sel- rated soil paste. After the prepared paste is allowed to dom considered a salinity hazard because salt build-up stand overnight, its pH is measured, and the extract is in the field must exceed certain levels before plants are then obtained by filtration over a vacuum. The electri-
adversely affected. However, situations may arise cal conductivity of this saturation extract (ECe, ex- wherein growers of high-value horticultural crops in pressed as mmho/cm or dS/m at 25°C) is the standard controlled cultures, pots, or greenhouses may attempt means of quantifying soil salinity. Other water extracts to “maximize” production through excessive fertiliza- (e.g., from 1:1 or 1:5 soil:water mixtures) can be used, tion. Also, a significant water quality hazard may arise but these do not lend themselves as well to compari- when the use-efficiency of applied fertilizers is low and sons with other data reported in the soil science litera- excess nonutilized nutrients, notably nitrates and phos- ture, to standard interpretation in relation to actual field phates, can be leached or otherwise transported into salinity, or to making direct use of available crop re- water resources. sponse and tolerance data.
Measuring and expressing salinity Soil parameters related to salinity The concentration of total dissolved salts (TDS) may be In addition to total salt concentration, the ionic com- determined in many ways (Table 17-1), but it is com- position of various constituents and the pH are also monly done either by separating and weighing the salts or determined by conventional laboratory procedures. measuring the electrical conductivity of the soil solution. Major cations which occupy the soil’s exchange com- 1. Direct gravimetric method. A specified volume plex (exchangeable Na+, K+, Ca2+, and Mg2+) are deter- of an aqueous solution is dried, and the salts contained mined after extraction in ammonium acetate or other in it are weighed and expressed as milligrams per liter appropriate extracts. Other soluble ionic species sus- (mg/L, equivalent to parts per million, ppm) or as grains pected of contributing to specific crop stress or toxic- per gallon (gpg; 1 gpg = 17.1 ppm). ity problems are similarly determined. Both the con- 2. Electrical conductivity. Inorganic salts are elec- centrations of individual ions and their relative propor- trolytes that, in aqueous solutions, are capable of con- tions to one another are calculated to allow systematic ducting an electric current. The higher the salt concen- data interpretation and evaluation of management op- tration, the higher the electrical conductivity (EC). Mea- tions. The most important parameter is the sodium ad- sured EC values for irrigation water are identified as sorption ratio (SAR) for water solutions and exchange- + ECw). Such measurements are made by specially de- able sodium percentage (ESP) for soils. SAR = Na ÷ signed conductivity meters and sensors, and they can [(Ca2+ + Mg2+) ÷ 2]1/2, where all ionic concentrations be taken either in water or directly in the soil (in situ). The standard unit for EC in the older literature was Table 17-1. millimohs per centimeter (mmho/cm); this is equiva- Conversions among various units expressing salinity. lent to decisiemens per meter (dS/m) in the SI unit sys- tem used in current literature. Because electrical con- Electrical conductivity of water ductance varies with temperature, measurements must 1 mho/cm = 1000 mmhos/cm (millimohs per cm) be corrected for the prevailing temperature and reported 1 mmho/cm = 1000 µmhos/cm (micromohs per cm) 1 dS/m = 1 mmho/cm at the standard “room” temperature of 25°C. Determinations of ECw and pH for irrigation water Total salt concentration in water are made on freshly collected, representative samples 1 g/L = 1 ppt (part per thousand) following filtration. Sample storage time should be 1 ppt = 1000 ppm (parts per million) minimized to avoid changes in water composition. For 17.1 ppm = 1 gpg (grain per gallon) soils, samples must be carefully collected to be truly 1 mmho/cm = 640 ppm (approximately) representative, because salt distribution in field soils 1 mmho/cm = 10 meq/l (milliequivalents per liter) 1 ppm = 0.00136 tons/acre-ft usually varies considerably over both space and time.
152 Soil and Water Salinity Plant Nutrient Management in Hawaii’s Soils are in milliequivalents per liter. ESP = (exchangeable Saline soils have been conventionally defined as + Na ÷ CEC) x 100, where both values are in those with ECe values of 4 dS/m or higher; saline sodic milliequivalents. High sodium concentrations (such as soils are saline soils that also have high percentages of in sodic soils) can be toxic to plants, may be destructive exchangeable sodium. The interpretation of values in to soil structure, and may lead to high pH levels (>8.5) the table is particularly suited for Hawaii and is some- and, ultimately, the development of alkaline conditions. what more liberal than for temperate areas, whose soils are less resistant to the effects of salt and sodium than Interpreting salinity measurements are Hawaii’s well weathered and well structured soils. Data on total salinity and ionic composition should be Saline soils can be reclaimed merely by leaching ex- interpreted in view of cess salts. Sodic soils, however, must also be amended ¥ existing or intended agricultural use with a calcium source to displace excess sodium and ¥ the characteristics of the soil in question allow the recovery of favorable soil structure. ¥ the availability and quality of water supplies It is important to note that the permissible levels of ¥ irrigation management plans salinity and ionic constituents are lower for irrigation ¥ prevailing climatic conditions water than for soils because, following irrigation, salts concentrate in the crop root zone as a result of water The basic evaluation criteria are electrical conductiv- loss by evaporation and evapotranspiration. A crude rule ity and sodium abundance in waters and soils. The stan- of thumb is that such concentration is in the order of 2Ð dards given in Table 17-2 are suggested for Hawaii. 4 times. Certain “minor” chemical constituents of irrigation water or soil solution can also impose severe restric- Table 17-2. tions on plant growth when present in excessive Salinity standards for agricultural situations in Hawaii. amounts; these are called toxic or specific-ion effects Measurement Interpretation (see Tables 17-3 to 17-6). Table 17-4 shows similar in- formation for a number of “trace” elements. Boron is Irrigation water the minor constituent of most concern for toxicity in ECw value salt-affected soils (Table 17-6). (dS/m at 25°C) < 0.75 Little or no hazard to crops (acceptable) Plant tolerances of and responses to salinity 0.75 Ð 3.0 Increasing hazard (intermediate) > 3.0 Severe hazard (excessive) High concentrations of soluble salts in the root zone impose physiologic stresses on growing plants. These SAR value stresses may be caused by a salt present in soluble (or < 10 Acceptable, free) form (osmotic stress). They may also be due to 10 Ð 25 Intermediate toxic or specific-ion effects, or to nutritional imbal- > 25 Excessive ances. Most such stresses can be specified quantita- Soil tively. Quantifying a crop’s sensitivity to salt requires that we define ECe value (dS/m at 25°C) ¥ the threshold salinity level below which the crop’s < 2 Little no effect on the growth and yield performance is unaffected by salinity of plants, ¥ the incremental decline in yield per unit increase in 2 Ð 4 Affects only very sensitive plants salinity above the threshold level 4 Ð 8 Affects many plants ¥ the salinity level at which the crop ceases to grow. 8 Ð 16 Affects tolerant plants > 16 Affects even very tolerant plants Such data are obtained experimentally and are used ESP value to plot crop tolerance diagrams that relate relative crop < 10 Acceptable yield to the electrical conductivity of the soil’s satura- 10 Ð 35 Intermediate tion extract (EC ). In the appendix to this chapter, the > 35 Excessive e schematic lines in the key show how such data are dis-
153 College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
Table 17-3. Relative tolerance of exchangeable sodium by some crops and grasses.
Toleranty Semitolerant Sensitive ESP level for 50% reduction in yield: >35 15 Ð25 ≤15
Karnal grass (Diplachna fusca) Wheat (Triticum vulgare) Cowpeas (Vigna sinensis) Rhodes grass (Chloris gayana) Barley (Hordium vulgare) Gram (Cicer arietinum) Para grass (Brachiaria mutica) Oats (Avena sativa) (20Ð40)z Groundnut (Arachis hypogaea) Bermuda grass (Cynodon dactylon) Raya (Brassica juncea) Lentil (Lens esculenta) Rice (Oryza sativa) (20Ð40)z Senji (Melilotus parviflora) Mash (Phaseolus mungo) Sugarbeet (Beta vulgaris) Barseem (Trifolium alexandrinum) Mung (Phaseolus aurus) Alfalfa (40Ð60)z Sugarcane (Saccharum officinarum) Peas (Pisum sativum) Barley (40Ð60)z Bajra (Pennesetum typhoides) Maize (Zea mays) Beets (40Ð60)z Cotton (Gossypium hirsutum) Cotton, at germination (Gossypium hirsutum) Carrots Dwarf red kidney bean Avocado (2Ð10)z Onion Red clover Green beans (10Ð15)z Tomatoes (40Ð60)z Lemon Corn Wheat (40Ð60)z Lettuce Tall fescue Radish Peach Strawberry
yCrop yields are affected seriously if the ESP is more than about 55, 35, and 10 for tolerant, semitolerant, and sensitive crops, respectively. Tolerance in each column decreases from top to bottom. The grasses listed are highly tolerant and some, like Karnal grass, will grow even in soils of ESP 80Ð90. zFigures for tolerance limits under nonsaline conditions.
played and define, quantitatively, various categories of tural breakdown occurs, it can exacerbate salt effects crop tolerance. These categories also appear on the spe- on crops through increased surface crusting, germina- cific tolerance curves for various crop groups shown. tion inhibition, and reduced permeability, porosity, and For crops not included in these figures, additional lit- aeration. erature search or new research will be needed to re- Salinity in the root zone is represented analytically trieve available information. as a single value for the site in question, but it actually fluctuates greatly in time and space. Continuous moni- Additional information toring of representative locations in affected irrigated The points in the following paragraphs relate to crop- fields is more informative than a “snapshot” measure- ping and managing salt-affected soils. ment. Technology now exists that allows continuous Crop species as well as cultivars differ in their re- monitoring of salinity directly in the field. sponse to salinity. Multiple stresses on growing plants are not uncom- Crop growth stages, beginning with germination, mon in salt-affected soils. Most important are the ad- can display different sensitivities to salt stress. Gener- ditive effects of salt and water stress (drought). ally (but not always), younger stages are more sensi- Water management is the most readily available tive. modifier of salt stress in the root zone. The amount, Different salts, cations, and anions vary in their frequency, and method of irrigation collectively deter- effects on plants and soils, so that waters or soil solu- mine the quantity, status, and distribution of salts in tions with similar EC values may not have similar ef- soil. In the absence of sufficient leaching, salts move fects if their ionic compositions are different. Specific upward by capillarity in response to evaporation gra- ion toxicity occurs most commonly due to excessive dients, and they accumulate in different locations on Ð Ð + boron, Cl , HCO3 , Na , and other ions (Tables 17-3Ð the soil surface. Therefore, seedbed design and seed 6). placement can be done to minimize exposure of seeds Soil structure and other physical properties may and roots to salt, particularly in the early, sensitive also be sensitive to certain ionic constituents. If struc- stages of growth (see Figure 17-1). Applying irriga-
154 Soil and Water Salinity Plant Nutrient Management in Hawaii’s Soils
Table 17-4. Trace element tolerances (mg/liter) for Table 17-5. Crop sensitivity to chloride in soil solution.z irrigation waters. Tolerance Continuous use, Short-term use, Low Medium High Element all soils fine-textured soils 20% yield reduction at (mg/liter) Aluminum 1.0 20.0 Arsenic 1.0 10.0 < 19 20 Ð 24 > 24 Beryllium .5 1.0 Boron 0.75 2.0 Avocado (14) Strawberry (20) Gardenia Cadmium 0.005 0.05 Lemon (15) Rice (23) Wheat, young (25) Chromium 5.0 20.0 Navy bean (18) Alfalfa (23) Tomato (39) Cobalt 0.2 10.0 Dallis grass (19) Sorghum (23) Cotton (50) Copper 0.2 5.0 Kidney beans (24) Flax (50) Lead 5.0 20.0 Corn, young (70) Lithium 5.0 5.0 Barley (50) Manganese 2.0 20.0 Molybdenum 0.005 0.05 zIn irrigation water, problem thresholds for chloride-sensitive crops are (in Nickel 0.5 2.0 me/liter chloride content): no problems, <3; moderate problems, 3Ð9; and Selenium 0.05 0.05 severe problems, >9. Source: Doneen 1958. Vanadium 10.0 10.0 Zinc 5.0 10.0
Figure 17-1. Bed shapes and salinity effects at different levels of soil salinity at planting time. The pattern of salt build-up depends on bed shape and irrigation method. Seeds sprout only when they are placed so as to avoid excessive salt build-up around them.
Soil salinity at planting time (millimohs) 4816
Single-row bed
Seeds fail to germinate Seeds germinate
Double-row bed
Seeds germinate
Salt accumulation
Sloping bed
(Source: Bernstein et al. 1955.)
155 College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
tion water in excess of the crop’s consumptive use Table 17-6. Sensitivity to boron in irrigation water.z forces the salt to move downward and eventually out Sensitive Semitolerant Tolerant of the root zone. Installation of subsurface drainage (1 ppm) (2 ppm) (4 ppm) provisions may be necessary to dispose of such excess water and leached salts. Navy bean Sunflower Asparagus Sprinkler irrigation with marginal water may be Pear Potato Date Palm Apple Cotton Sugarbeet more harmful than other irrigation methods, because Grape Tomato Alfalfa many crops are sensitive to salts that directly contact Persimmon Sweet pea Broad bean their leaves. Sea spray in shoreline areas may damage Orange Radish Onion certain vegetation. Avocado Field pea Turnip Grapefruit Barley Cabbage Lemon Wheat Lettuce Interpreting salinity data Corn Carrot Examples of analytical data and its interpretation are Milo sorghum shown in Tables 17-7 and 17-8. Bell pepper According to the information on irrigation water Sweet potato Lima bean in Table 17-7, the Koolau well water, with a very low ECw and an SAR value of less than 2, is uncondition- zSensitivity based on symptom appearances and not necessarily yield data. ally suitable for use in irrigating any crop suitable for Source: USSL 1954. growing in the area, using any method of irrigation. Indeed, this water—from the chemical perspective— is considered excellent for human consumption as well. On the other hand, the basaltic aquifer sample is of
“moderate” quality based on the ECw value and the SAR
Table 17-7. Examples of analytical data for irrigation water in Hawaii.
Cations (ppm), [meq/liter] Anions (ppm), [meq/liter] ECw
Sample source (dS/m) Na Ca Mg K Cl SO4 HCO3 SiO2
Basal Koolau well 0.205 20 8.0 6.0 2.0 22 5.5 65 36 [0.88] [0.44] [0.5] [0.05] [0.61] [0.12] [1.1] N.A.
Basaltic aquifer, 2.42 432 31 52 19 767 112 76 79 sea-water intruded [18.8] [1.55] [4.33] [0.49] [21.3] [23.3] [1.25] N.A.
Table 17-8. Examples of analytical data for soils (plow-layer depth) in Hawaii.
pH in Exchangeable cations (meq/100g) z ECe CEC saturated Sample source (dS/m) (meq/100 g) paste Na Ca Mg K
Irrigated Molokai series, Kunia, Oahu 1.17 11.3 6.8 0.46 5.1 0.33 1.32
Irrigated Molokai (?) series, Lahaina, Maui 6.80 21.7 7.0 6.1 0.7 5.5 0.4 zCation exchange capacity as measured by extraction with pH 7 AmOAc.
156 Soil and Water Salinity Plant Nutrient Management in Hawaii’s Soils of about 7, but is “hazardous” based on the ClÐ con- centration exceeding 9 meq/liter. If this water is to be Appendix 17-1. Salt tolerances of various crops; com- pare yield response lines for various crops with the gen- used for irrigation, a predetermined leaching require- eralized zones of salinity tolerance shown in the key ment (LR) will be needed to avoid accumulating ex- (adapted from Maas and Hoffman 1977). cessive amounts of salt in the root-zone. Quantitative formulas, which allow computing LR values, combine Key: Salinity tolerance zones information on the water’s quality and crop tolerance 100 to salinity and show that sensitive crops will require higher leaching requirements than tolerant crops. 80 Considering the soil data in Table 17-8, the com- 60 UNSUITABLE FOR CROPS M position of the Kunia sample reflects the long history MODERATELY
SE O S TO TO 40 E D of irrigation with the very high quality “ditch water” N LERANT N ER LERANT SITIVE S commonly used by plantations in the Pearl Harbor ba- ITIV A TE 20 sin. The ECe and pH values are not limiting to crop E LY
growth, and the ESP value of less than 5% is well within Relative crop yield (%) 0 the acceptable range for both crops and soils. The 05101520 25 30 35
Lahaina sample, on the other hand, has a “moderately ECe (millimhos / cm) high” ECe value, which is expected to bring about salt stress to most crops. The ESP value of about 28% is 100 close to the upper limit of the intermediate quality GRAIN CROPS range. Unless the crop of interest is tolerant of both 80 these excesses, it is necessary to have a management strategy that maximizes salt leaching and displacement 60 of exchangeable sodium by calcium. The most com- mon amendment for the latter purpose is agricultural 40 gypsum (CaSO4.2H2O). The required application can
w barley corn be determined after deciding the value by which ESP rice h e a exceeds the desired level at the soil depth to be treated 20 t (in this case, 28 Ð 5 = 23). The gypsum amount that is chemically equivalent to this excess sodium can then 0 051015 20 25 30 35 be calculated (a detailed example is provided in El-
Swaify et al. 1983). Most of Hawaii’s soils are struc- 100 turally tolerant to exchangeable sodium, so that soil SUGAR, FIBER & permeability will generally remain favorable during the OILSEED CROPS 80 reclamation process.
References 60
sugarbeet Bernstein, L., M. Fireman, and R.C. Reeve. 1955. Control of salin- soybean sugarcane cotton ity in the Imperial Valley, California. U.S. Dept. of Agric., Agric. 40 Res. Svc. 41(4). peanut flax El-Swaify, S.A., S.S. Arunin, and I.P. Abrol. 1983. Soil saliniza- tion: development of salt-affected soils. In: Natural systems for 20 development: what planners need to know. MacMillan, NY. Chap. 4, p. 162Ð228. 0 Maas, E.V., and G.J. Hoffman. 1977. Crop salt tolerance: evalua- 051015 20 25 30 35 tion of existing data. In: H.E. Dregne (ed), Managing saline ECe (millimhos / cm) water for irrigation. Proc., Intl. Salinity Conf., Texas Tech Univ., Lubbock, TX. p. 187Ð198.
157 College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa
Appendix 17-1, continued. 100 FORAGE CROPS legumes Key: Salinity tolerance zones birdsfoot trefoil 100 80
80 alsike, ladino, re common vetch 60 b ro a 60 UNSUITABLE d a b lfa FOR CROPS e M MODERATELY a lfa SENSITIVE O n S T TO OLERANT 40 E D 40 d straw N E LERANT S R b IT A e T rs IV b E cow berry clover sesbaniae 20 ig e E L m Y 20 tre Relative crop yield (%) c pe lo fo a v
Relative crop yield (%) e 0 il r 051015 20 25 30 35 0 ECe (millimhos / cm) 0 246810 12 14 16 2018 100 100 FORAGE CROPS ROOT CROPS grasses 80 80
60 60
w 40 ild wheatgrass, fairway 40 ry w e h garden beef ea wheatgrass, tall , b sweet potato e tgra a s potato rd u d ss, crested ca radish 20 le a o n 20 n rro s g io s ra s n t s 0 0 051015 20 25 30 35 024 68101214 100 100 FORAGE CROPS FRUIT & NUT CROPS grasses 80 80 apricot, blackberry, & boysenberry date
60 60 grapefruit & orange ryegrass strawberry grape hardinggrassryegrass orchardgrass meadow foxtail fescue, tall 40 almond & plum 40 lo ve corn barley g ra ss 20 peach 20
0 0 024 68101214 0 246810 12 14 16 18 20 22 24 100 ECe (millimhos / cm) VEGETABLE CROPS
80
60
b spinach 40 e cucumbercabbagetomatobroccoli a n p e p p e 20 r lettuce
0 024 68101214
ECe (millimhos / cm)
158 Soil and Water Salinity