The disorders are: 1. Iron Toxicity 2. Aluminum Toxicity 3. Boron Toxicity 4. Sulphide Toxicity 5. Toxicity of Organic Acids 6. Salinity.
Nutritional Disorder # 1. Iron Toxicity:
The iron concentration in soil solution increases due to reduced soil under land submergence. Toxicity, usually, occurs on soils with low pH and high organic matter. Nutritional disorders associated with iron toxicity are bronzing in Sri Lanka, Akagare Type I in Japan and Akiochi in Korea.
Iron content is usually, but not always-, high in plants showing bronzing. Plants exhibiting bronzing symptoms are usually low in potassium. Akagare Type I is remedied by the application potassium. But plants affected by Agakare Type I are, generally, high in iron content leading to belief that bronzing and disorder may be similar. Toxic effect of iron can be minimised by applying lime and phosphatic fertilisers.
Nutritional Disorder # 2. Aluminum Toxicity:
Toxicity of aluminum may be a problem in upland rice on acid soils and on lowland acid sulphate soils. Its toxicity retards root growth leading to reduced nutrient uptake and decreasing drought tolerance.
Phosphorus content of shoots is usually higher than those of roots and decreases with increased aluminum concentration. Younger seedlings, less than two weeks old, are more susceptible than 2-5 weeks seedlings.
Toxicity of aluminum under drought would increase plant water stress due to decrease in rooting depth. Thus, aluminum toxicity is considered an important soil factor affecting drought tolerance of rice. Orange yellow interveinal chlorosis, which may become necrotic indicate aluminum toxicity.
Nutritional Disorder # 3. Boron Toxicity:
Boron toxicity symptoms include chlorosis at the tips of older leaves, especially along margins, followed by appearance of large dark brown elliptical spots in the affected parts, which ultimately turn brown and dry up. More than 3.0 ppm hot water soluble boron in soil is toxic. High potassium aggravates boron toxicity. Provision of adequate drainage minimises accumulation of boron.
Nutritional Disorder # 4. Sulphide Toxicity:
Under submerged conditions, sulphate is reduced to sulphide, which inhibits respiration and oxidative power of the roots and retard uptake of nutrients. Occurrence of sulphide toxicity in rice depends on the concentration of sulphide in soil solution and oxidising power of rice roots. Sulphide toxicity occurs mainly in sandy, well drained degraded soils and on poorly drained organic soils.
Nutritional Disorder # 5. Toxicity of Organic Acids:
Organic acid toxicity may occur in organic soils and poorly drained soils and when large quantities of fresh organic material such as green manures and straw are incorporated. Concentration of organic acids increases with flooding, reaches a peak and then decrease. Organic acids such as formic, acetic, propionic and butyric acids occur in submerged soils. High concentration of organic acids impairs root elongation, respiration and nutrient uptake by rice plant. They are toxic at concentrations of 10-2 to 10-3 M.
Nutritional Disorder # 6. Salinity:
In rice, salinity injury varies with salt concentration, kind of soil, variety and developmental stage. Rice plant is tolerant to salinity during germination but very sensitive at 1 to 2 leaf stage. Its tolerance increases during tillering and elongation and decrease at flowering. Electrical conductivity of less than 2 mmho cm-1 appears to be ideal for rice crop.