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Whitening and browning of leaves (IRRI).
Further effects on rice growth:
Patchy field (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)]
Exchangeable Na percentage (ESP):
Sodium adsorption ratio (SAR):
SAR >15 sodic soil (measured as cations in saturation extract)
Irrigation water has:
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:
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:
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:
Examples of salt-affected soils include:
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:
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:
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:
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:
Foliar application of K, particularly if a low-tolerance variety is grown on saline soil. Spray at the late tillering and panicle initiation stages.
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.