riceDoctor

Salinity

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Whitening and browning of leaves (IRRI).

Diagnostic summary

  • affects respiration and photosynthesis processes
  • decreased biological N2 fixation and soil N mineralization

  • affected leaves with white tips
  • some leaves with chlorotic patches
  • stunting
  • reduced tillering
  • patchy field growth

  • important throughout the growth cycle of the rice plant
  • associated with poor irrigation practice or insufficient irrigation water, alkaline soils in inland areas, increase in the level of saline groundwater, and intrusion of saline seawater in coastal areas
  • may be accompanied by P deficiency, Zn deficiency, Fe deficiency, or B toxicity

 

Full fact sheet

  • Tips of affected leaves turn white
  • Chlorotic patches appear on some leaves
  • Plant stunting and reduced tillering
  • Patchy field growth
  • Symptoms first manifest themselves in the first leaf, followed by the second, and then in the growing leaf
  • Salinity or sodicity may be accompanied by P deficiency, Zn deficiency, Fe deficiency, or B toxicity

Further effects on rice growth:

  • Reduced germination rate
  • Reduced plant height and tillering
  • Poor root growth
  • Increased spikelet sterility
  • Excess Na uptake decreases 1,000-grain weight and total protein content in grain, but does not alter major cooking qualities of rice
  • Decreased biological N2 fixation and soil N mineralization

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Patchy field (IRRI).

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White leaf tips(IRRI).

Plant and soil can be tested to confirm salinity.

Increased Na content in rice plants may indicate salinity injury, which may lead to yield loss. The critical concentration of salt (NaCl) in leaf tissue at which toxicity symptoms appear, however, differs widely between varieties. Varieties showing the greatest tolerance for salt within plant tissues are not necessarily those showing the greatest overall phenotypic resistance to salinity.

The correlation between Na:K ratio and salinity tolerance has been established; however, no absolute critical levels in plant tissue are known. A Na:K ratio of <2:1 in the grain may indicate salt-tolerant rice varieties.

The Na:Ca ratio in plant tissue does not seem to be a good indicator of salinity. No effects on growth or NaCl concentration in the shoot were found over the range of Na:Ca ratios (5-25:1) commonly found in the field.

On soil, EC in saturation extract or soil solution: For rice growing in flooded soil, EC is measured in the soil solution or in a saturation extract (ECe). For upland rice grown at field capacity or below, EC in soil solution is about twice as great as that of the saturation extract. A rough approximation of the yield decrease caused by salinity is:

Relative yield(%) = 100 - [12(ECe   - 3)]

  • ECe   <2 dS m-1 optimum, no yield reduction
  • ECe  >4 dS m-1 slight yield reduction (10-15%)
  • ECe  >6 dS m-1 moderate reduction in growth and yield (20-50%)
  • ECe  >10 dS m-1 >50% yield reduction in susceptible cultivars

Exchangeable Na percentage (ESP):

  • ESP <20% no significant yield reduction
  • ESP >20-40% slight yield reduction (10%)
  • ESP >80% 50% yield reduction
     

Sodium adsorption ratio (SAR):

  • SAR >15 sodic soil (measured as cations in saturation extract)
     

Irrigation water has:

  • pH 6.5-8, EC <0.5 dS m-1 high-quality irrigation water
  • pH 8-8.4, EC 0.5-2 dS m-1 medium- to bad-quality irrigation water
  • pH >8.4, EC >2 dS m-1 unsuitable for irrigation
  • SAR <15 high-quality irrigation water, low Na
  • SAR 15-25 medium- to bad-quality irrigation water, high Na
  • SAR >25 unsuitable for irrigation, very high Na

Notes:

  • Measurement of EC as an indicator of salinity is rapid and simple. EC alone, however, is insufficient to assess the effects of salinity on plant growth because salt concentrations at the root surface can be much greater than in the bulk soil. In addition, EC only measures the total salt content, not its composition. Na and B must be considered as well. Salinity is highly variable in the field, both between seasons and within individual fields. Individual EC values must be treated with caution unless they are based on representative soil samples.

  • From EC, the osmotic potential of the saturation extract can be estimated as:

    • Osmotic potential (MPa) = EC × 0.036

  • If the samples do not contain much gypsum, EC measurements can be converted as follows:

    • EC  = 2.2 × EC1:1 EC1:1 measured in 1:1 soil:water suspension
    • ECe   = 6.4 × EC1:5 EC1:5 measured in 1:5 soil:water suspension

No other deficiency exhibits these symptoms but salinity.

Plant growth on saline soils is mainly affected by high levels of soluble salts (NaCl) causing ion toxicity, ionic imbalance, and impaired water balance. On sodic soils, plant growth is mainly affected by high pH and high HCO3- concentration. The major causes of salinity or sodicity are as follows:

  • Poor irrigation practice or insufficient irrigation water in seasons/years with low rainfall.
  • High evaporation. Salinity is often associated with alkaline soils in inland areas where evaporation is greater than precipitation.
  • An increase in the level of saline groundwater.
  • Intrusion of saline seawater in coastal areas (e.g., Mekong Delta, coastal India)

Salt-affected soils (~11 million ha in South and Southeast Asia) are found along coastlines or in inland areas where evaporation is greater than precipitation. Salt-affected soils vary in their chemical and physical properties, but salinity is often accompanied by P and Zn deficiency, whereas Fe toxicity is common in acid sulfate saline soils.

Salt-affected soils can be grouped into:

  • saline soils (EC >4 dS m-1, ESP <15%, pH <8.5)
  • saline-sodic soils (EC 4 dS m-1, ESP >15%, pH ~8.5)
  • sodic soils (EC <4 dS m-1, ESP >15%, pH >8.5, SAR >15) 

Examples of salt-affected soils include:

  • saline coastal soils (widespread along coasts in many countries)
  • saline acid sulfate soils (e.g., Mekong Delta, Vietnam)
  • neutral to alkaline saline, saline-sodic, and sodic inland soils (e.g., India, Pakistan, Bangladesh)
  • acid sandy saline soils (Korat region of northeast Thailand)

Salinity is defined as the presence of excessive amounts of soluble salts in the soil (usually measured as electrical conductivity, EC). Na, Ca, Mg, Cl, and SO4 are the major ions involved. Effects of salinity on rice growth are as follows:

  • Osmotic effects (water stress)
  • Toxic ionic effects of excess Na and Cl uptake
  • Reduction in nutrient uptake (K, Ca) because of antagonistic effects

The primary cause of salt injury in rice is excessive Na uptake (toxicity) rather than water stress, but water uptake (transpiration) is reduced under high salinity. Plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Growth rate, however, is reduced. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratios in the rice plant and reduced K transport rates.

Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase. Rice tolerates salinity during germination, is very sensitive during early growth (1-2-leaf stage), regains tolerance during tillering and elongation, but becomes sensitive again at flowering.

Several factors affect the tolerance of different rice varieties to salinity:

  • Transpiration rate and potential for osmotic adjustment.
  • Differences in nutrient uptake under Na stress. Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
  • Efficient exclusion of Na+ and Cl-. Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
  • Rapid vegetative growth results in salt dilution in plant tissue.

Rice is more tolerant of salinity at germination, but plants may become affected at transplanting, young seedling, and flowering stages. Thus, this problem occurs throughout the growth cycle of the rice crop.

Salinity can be a major problem in localized areas - tending to occur in low coastal regions and semi-arid inland saline areas.

Varieties that tolerate salinity are available, but their use does not substitute for proper water and irrigation management. Breeders will unlikely be able to produce varieties with ever-increasing tolerance of salinity. A variety adapted to present levels of salinity may not survive if salinity increases because water management practices have not been corrected. Rice is a suitable crop for the reclamation of both sodic and saline soils. On sodic soils, rice cultivation results in a large cumulative removal of Na caused by mobilization of insoluble CaCO3. On saline soils, cultivation practices lead to the loss of salts by leaching. Management of salinity or sodicity must include a combination of measures. Major choices include the following:

  • Cropping system: In rice-upland crop systems, change to double-rice cropping if sufficient water is available and climate allows. After a saline soil is leached, a cropping pattern that includes rice and other salt-tolerant crops (e.g., legumes such as clover or Sesbania) must be followed for several years.
  • Varieties: Grow salt-tolerant varieties (e.g., Pobbeli, Indonesia; IR2151, Vietnam; AC69-1, Sri Lanka; IR6, Pakistan; CSR10, India; Bicol, Philippines). This is a short-term solution that may result in increased salinity over the longer term if other amelioration measures are not implemented.
  • Seed treatment: In temperate climates where rice is direct seeded, coat seed with oxidants (e.g., Ca peroxide at 100% of seed weight) to improve germination and seedling emergence by increased Ca and O2 supply. Alternatively, treat rice seeds with CaCl2 to increase seed Ca2+ concentration.
  • Water management: Submerge the field for two to four weeks before planting rice. Do not use sodic irrigation water or alternate between sodic and nonsodic irrigation water sources. Leach the soil after planting under intermittent submergence to remove excess salts. Collect and store low saline rainwater for irrigation of dry-season crops (e.g., by establishing reservoirs). In coastal areas, prevent intrusion of salt water.
  • Fertilizer management: Apply Zn (5-10 kg Zn ha-1) to alleviate Zn deficiency. Apply sufficient N, P, and K. The application of K is critical because it improves the K:Na, K:Mg, and K:Ca ratios in the plant. Use ammonium sulfate as N source and apply N as topdressing at critical growth stages (basal N is used less efficiently on saline and sodic soils). In sodic soils, the replacement of Na by Ca (through the application of gypsum) may reduce P availability and result in an increased requirement for P fertilizer.
  • Organic matter management: Organic amendments facilitate the reclamation of sodic soils by increasing the partial CO2 pressure and decreasing pH. Apply rice straw to recycle K. Apply farmyard manure.

The following are options for treatment of salinity:

  • Saline soils: Salinity can only be reduced by leaching with salt-free irrigation water. Because rice has a shallow root system, only the topsoil (0-20 cm) requires leaching. Cost, availability of suitable water, and soil physical and hydraulic characteristics determine the feasibility of leaching. To reduce the level of salinity in affected soils, electrical conductivity in the irrigation water should be <0.5 dS m-1). Where high-quality surface water is used (EC ~0), the amount of water required to reduce a given ECe to a critical-level ECc can be calculated as follows:

    • Aiw=Asat[(ECe  /ECc)+1]
    • where Aiw represents the amount of irrigation water (in cm) added during irrigation and Asat is the amount of water (cm) in the soil under saturated conditions. For example, to lower an initial ECe   of 16 dS m-1 to 4 dS m-1 in the top 20 cm of a clay loam soil (Asat = 8-9 cm), about 40 cm of fresh water is required. Subsurface drains are required for leaching salts from clay-textured soils.
  • Sodic soils: Apply gypsum (CaSO4) to reduce Na saturation of the soil (ESP, Na:K ratio). Because of complex chemical and physical interactions, it is difficult to calculate the exact amount of gypsum required. The amount of Ca2+ contained in gypsum required to reduce the ESP to a target level can be estimated as follows:
    • Ca (kg ha-1) = (ESP0 - ESPd) × CEC × B × D × 20.04
    • where ESP0 is the original and ESPd is the target ESP value (% of CEC), CEC is in cmolc kg-1, B is the bulk density (g cm-3), and D is the soil depth (m) to be reclaimed.
  • Foliar application of K, particularly if a low-tolerance variety is grown on saline soil. Spray at the late tillering and panicle initiation stages.

Source:

Dobermann A, Fairhurst T. 2000. Rice. Nutrient disorders & nutrient management. Handbook series. Potash & Phosphate Institute (PPI), Potash & Phosphate Institute of Canada (PPIC) and International Rice Research Institute. 191 p.