Introduction
The growth of the rice plant in any medium (soil, sand, water) depends on the availability of sunlight, water, and various chemical elements. Sixteen elements are recognized as essential in rice nutrition: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, boron, zinc, molybdenum, chloride. Among these, carbon, hydrogen, and oxygen are absorbed directly out of the air and water; the rest must be present in the soil.
Three elements, the so-called primary elements - nitrogen, phosphorus, and potassium - are ingested by the rice plant in unusually large quantities and are therefore particularly important in producing a high yield. This chapter provides a brief, non-technical outline of the role outline of the role of nitrogen, phosphorus, and potassium in the growth of the rice plant.
Note: In considering the effects of individual elements, the relative amounts of other elements present is important for example, nitrogen alone produces certain effects, but the effects may be quite different if there is a proper balance between nitrogen, phosphorus, potassium, and other elements.
The chemical form in which a nutrient element is present in the soil is also important, since the availability of a nutrient to the plant varies with the roots ability to extract the nutrient element from the chemical compound in which it occurs
I. Nitrogen
Rice grown under high management requires large amounts of nitrogen (n) One crop consumes approximately 20-25 kg of nitrogen for every ton of yield, making nitrogen the single most important rice nutrient. Nitrogen's fundamental importance as a primary nutrient element is augmented by the fact that many improved rice varieties cultivated around the world have been bred to show a marked response to the application of nitrogenous fertilizers.
Nitrogen increases the vigor and enhances the growth of the rice plant. When absorbed during the vegetative phase, nitrogen:
- helps synthesize the chlorophyll necessary for photosynthesis (as evidenced by a marked "greening" of the leaves)
- promotes rapid leaf, stem, and root growth (as evidenced by an increase in the height, size, and number of tillers, as well as an increase in the size of leaves)
- speeds growth, thus enabling seedlings to grow fast enough to avoid many seedling blights
When absorbed during the reproductive and ripening phases, nitrogen:
- promotes development of the panicle (as evidenced by an increase in the number of spike-lets)
- stimulates nutrient absorption and assimilation ( as evidenced by an increase in size and number of filled grains
- increases the protein content of the grains, thus improving the quality of the crop
Nitrogen deficiency in rice can be recognized by:
- yellowish, color of the leaves, particularly of younger leaves
- small size of plants
- low number of tillers
- straightness, stiffness Or upper leaves
II. Phosphorus
Like all cereal grains rice requires a considerable amount of phosphorus (P) for vigorous growth and high yield.
Although in general response -to phosphorous in irrigated rice i<; less marked than response to nitrogen, phosphorus is none the less a very important nutrient - one crop consumes approximately 15 kg of phosphorous for every ton of yield.
Phosphorus is particularly important to the rice seedling d ring the time it is recovering from transplanting shock. Phosphorus greatly stimulates root development in the young plant, thus increasing its ability to absorb nutrients from the soil.
When absorbed during the vegetative phase, phosphorus:
- increases the number of root hairs, thus facilitating the uptake of other nutrients (enables the seedling to recover rapidly from transplanting shock)
- stimulates extensive root growth, thus increasing the plant's resistance to drought
- promotes tillering by facilitating nitrogen absorption - promotes early flowering and ripening (can be exploited to offset the effects of late planting)
During the reproductive phase, the phosphorus intake of rice decreases considerably.
When absorbed during the ripening phase, phosphorus:
- increases the protein content of the grains thus improving the food value of the crop invigorates the germination power of the seed (as evidenced by an increase in the germination rate of the seed produced)
Phosphorus deficiency in rice can be recognized by:
- small size of plants
- short, underdeveloped root systems
- low number of tillers
- bluish green color of the leaves
- purple color of the lower part of the culms (Note some traditional varieties have net rally purple culms.)
III Potassium
Because of the presence of potassium (K) in most irrigation water, the response of rice to potassium is often not as marked as the responses to nitrogen and/or phosphorus, except in unusual situations (e.g. when certain toxicities are offset by potassium). Nevertheless, potassium should not be overlooked as an important nutrient element, since each crop requires approximately 15 kgs. of potassium for every -ton of yield.
When absorbed during the vegetative phase, potassium:
- strengthens cell walls, thus making the plant physically stronger and enabling it to withstands the adverse effects of bad weather
- increases the plant's resistance to penetration by disease organisms
When absorbed during the reproductive and ripening phases potassium:
- increases the plant's resistance to diseases affecting the panicle and grains
- increases the protein content of the grains, thus improving the quality of the crop
- increases the size and weight of the grains
Potassium deficiency in rice can be recognized by:
- deep, dark color of the leaves (spreading from the tips)
- irregular dead spots on the leaves and panicles
- droopiness of leaves, resulting in reduced photosynthesis and consequent slower growth
- unusual susceptibility to disease and pest attack
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