The following points highlight the six major types of nutrient toxicity symptoms in rice. The types are: 1. Nitrogen 2. Iron 3. Aluminium 4. Manganese 5. Boron 6. Sulphide.
Nutrient Toxicity Symptom: Type # 1. Nitrogen (N):
Plants look overly green; may be healthy, but also may be lodged at maturity (especially in direct-seeded rice); may have thin stems and increased disease (e.g., bacterial leaf blight, sheath blight, blast) or insects (leaf-folder); patchy pattern resulting from uneven application across the field.
It is used where fertilizers are relatively cheap and where farmers don’t understand the amount of nitrogen required relative to their yield goals.
Nutrient Toxicity Symptom: Type # 2. Iron (Fe):
Tiny brown spots on lower leaves, starting from the tips and spreading toward the base; the spots are generally combined on interveins and leaves turn orange-brown and die); leaves usually remain narrow and often green; and in severe cases the entire leaf becomes purplish-brown; dark brown to black coating on the root surface and many dead roots; fresh uprooted rice hills have many black roots.
Occur generally in lowland rice soils with permanent flooding during crop growth.
Nutrient Toxicity Symptom: Type # 3. Aluminium (Al):
Orange-yellow interveinal chlorosis on leaves which may become necrotic in serious cases; poor stunted growth; yellow to white mottling of interveins is followed by leaf tip death and leaf margin scorch; stunted and deformed roots in susceptible cultivars.
Occurs on acid upland soils and acid sulphate soils and associated with strong P fixation and P deficiency.
Nutrient Toxicity Symptom: Type # 4. Manganese (Mn):
Stunted plants and limited tillering; brown spots on the veins of the leaf blade and leaf sheath, especially on the lower leaves. More common in acid soils where Mn availability is very high.
Nutrient Toxicity Symptom: Type # 5. Boron (B):
Chlorosis at the tip of the older leaves especially along the margins, followed by the appearance of large dark brown elliptical spots in the affected parts, which ultimately turn brown and dry up; necrotic spots prominent at panicle initiation. Vegetative growth is not markedly depressed
Common in arid and semiarid regions; occur in soils formed on volcanic parent material, associated with the use of irrigation water pumped from deep wells and some coastal saline soils
Nutrient Toxicity Symptom: Type # 6. Sulphide:
Interveinal chlorosis of emerging leaves; coarse, sparse, dark brown to black root system; Freshly uprooted rice hills often have poorly developed root systems with many black roots (stains of Fe sulphide) unlike healthy roots, which are covered with a uniform and smooth orange-brown coating of Fe3+ oxides and hydroxides.
Increased occurrence of diseases, such as brown spot (caused by Helminthosporium oryzae), because of unbalanced plant nutrient content caused by H2S toxicity. Associated with low-Fe soils; occur in well-drained sandy soils, degraded paddy poorly drained organic soils, and acid sulphate soils.