Section 9 – NUTRITION OF THE RICE PLANT

NUTRITION OF THE RICE PLANT

Fertilizers are substances that contain important minerals for plant growth. These are usually applied to the soil. There are about 16 essential plant nutrient elements required for the completion of the life cycle of a plant. Nitrogen (N), phosphorus (P), and potassium (K) are the fertilizer elements needed in large amounts for such processes as manufacture of food (starch, fats, and protein), reproduction, growth development, and maintenance of life (Yoshida 1981, Mikkelsen 1979).

Nutrient functions and deficiency symptoms

Nitrogen

• accelerates growth of the rice plant, enabling it to produce more straw and grains;
• gives dark green color to the plant and increases the size of leaves and grains;
• increases protein content;
• increases plant height and tiller number
• improves the general quality of the plant.

The rice plant requires a large amount of nitrogen at the early and mid-tillering stages to maximize the number and development of panicles. Nitrogen absorbed at the panicle initiation stage may increase spikelet number per panicle (De Datta 1981, Tanaka 1966). Basal application before planting, followed by last harrowing, has proven to be good for early plant growth and establishment.

Nitrogen deficiency symptoms

• stunted growth with limited number of tillers;
• narrow, short and erect yellowish-green leaves, except young leaves which are a bit greener; and
• death of old leaves when they become light straw-colored.

Phosphorus

• stimulates root formation and development,
• promotes rapid growth of plants,
• hastens maturity,
• induces flowering and seed formation, and
• promotes good grain development and gives higher food value to the rice.

Phosphorus deficiency symptoms

• stunted plants with limited number of tillers;
• short, droopy, and dark green leaves;
• older leaves turn brown and die, while young leaves stay healthy looking; and
• reddish or purplish color in some varieties and general chlorosis of the plant.

Potassium

• increases vigor and disease resistance in plants,
• helps in protein production,
• strengthens the straw,
• induces plump development of grains,
• increases effect on root development necessary for plant growth,
• helps in the formation and transfer of starch, sugar, and oil (De Datta and Barker 1975).

Potassium deficiency symptoms

• stunted plants and slightly reduced tillering;
• short, droopy, and dark green leaves;
• yellowish at the interveins of lower leaves starting from the tip, eventually dries up to light brown color;
• sometimes, brown spots develop on dark green leaves; and
• long thin panicles develop with irregular necrotic spots.

Other elements: functions and deficiency symptoms

There are 16 essential elements in rice, divided into major and minor categories. The major elements-C, H, O, N, P, K, Ca, Mg, and S-are needed in higher amounts than the minor elements Fe, Mn, Cu, Zn, Mo, B, and Cl. But all essential elements must be available in optimum amounts for the plants to develop fully.

Deficiencies in the other elements common in the field present obstacles to growing of high-yielding varieties, especially in new open land. Symptoms, which are the plants’ way of communicating, show up when plants are deficient in a nutrient or when they suffer from toxicity due to an element. A systematic method of observing the major plant parts (e.g., height, tillers, leaves, and roots) is advisable if visible symptoms are sed to diagnose nutritional disorders (Tanaka 1966, Yoshida 1981).

For example, reduced tiller number is a common symptom of nutrient deficiency or element toxicity. In leaves, chlorosis, necrosis (brown spots), and orange discoloration are common symptoms of deficiency or toxicity.

Deficiency symptoms

Zinc

Zinc deficiency in lowland rice usually appears within 2–3 weeks after transplanting or sowing (Tanaka 1966, Yoshida 1981).

The midribs of the younger leaves, especially at the base, become chlorotic. Brown blotches and streaks appear on the lower leaves; stunted growth is then observed. Tillering may continue. The size of the leaf blade is reduced, but the leaf sheath is not affected so much. In the field, uneven growth and delayed maturity are signs of zinc deficiency. Rice plants near the bunds and drains mature earlier than those in the more poorly drained interior of the field where there is less zinc.

Sulfur

Sulfur deficiency symptoms are similar to those of nitrogen, making it difficult to distinguish between the two elemental deficiencies if only visual symptoms were used. Yellowing starts on the leaf sheath and proceeds to the leaf blades; the entire plant becomes chlorotic at the tillering stage. Plant height is reduced, tiller number is lower, and panicles are fewer and shorter.

One distinct difference between sulfur and nitrogen deficiencies is that the former produces general chlorosis in the whole plant, but the older leaves do not dry quickly, in contrast to nitrogen deficiency where there is intense chlorosis. Sulfur deficiency delays growth and development, whereas nitrogen deficiency hastens flowering and maturity.

Calcium

In cases of acute calcium deficiency, the growing tips of the upper leaves become white, rolled, and curled. In extreme cases, the plant is stunted, and the growing point dies.

Magnesium

Height and tiller number are little affected when the deficiency is moderate. Leaves are wavy and droopy due to the expansion of the angle between the leaf blade ant the leaf sheath. Interveinal chlorosis, occurring on lower leaves, is characterized by an orange-yellow color. There are varieties that do not show leaf symptoms, but plant height and tillering may be affected at the early growth stage.

Iron

The entire leaves become chlorotic, then whitish. If iron supply is suddenly cut off, newly emerging leaves become chlorotic.

Manganese

Plants are stunted but have normal number of tillers. Interveinal chlorotic streaks spread downward from the tip to the base of the leaves, which later become dark brown and necrotic. The newly emerging leaves become short, narrow, and light green.

Boron

Plant height is reduced. As in the case of calcium deficiency, the tips of emerging leaves become white and rolled. The growing point may die in severe cases, but new tillers continue to be produced.

Copper

The leaves appear bluish-green, and then become chlorotic near the tips. New leaves appear needlelike with the basal end developing normally.

Detecting nutrient deficiency symptoms

Nutrient deficiency/toxicity symptoms in rice plants can be assessed by observing the color of the leaves, stems, and roots; plant height; tillering habit; and root system development.

The best time to observe visible symptoms is during the early stages of symptom development. It is of little value to observe plants when the disorder has become so severe that the plants are almost dead.

Iron toxicity symptoms may appear 12 days after trans-planting and these persist until flowering if the disorder is very severe.

• Plant height. Stunted growth is a common symptom of deficiency or toxicity.
• Tillers. Reduced tiller number is also a common symptom.
• Leaves. Chlorosis, necrosis (brown spots), and orange discoloration are common deficiency/toxicity symptoms
• Roots. When shoot growth is impaired, root growth is also poor. Roots are white when young and actively growing. When roots are long and old and if the soil contains adequate iron, these are normally brown because of iron oxide deposits on the root surface.

The most common deficiencies in wetland rice are those of N, P, Zn, and S; in upland rice, N, P, and Fe deficiencies are prevalent. Mn toxicity occurs on acid upland soils, whereas excess Fe depresses growth in acid wetland soils. The disorders are diagnosed by plant symptoms, plant analysis, and soil tests. Deficiencies are corrected by applying the missing element to the soil or plant, whereas toxicities of acid soils are alleviated by liming. The occurrence, symptoms, critical plant limits, soil tests for wetland and dryland soils, and soil amelioration techniques are discussed in (Ponnamperuma and Lantin 1985).

Types of fertilizers

Fertilizers are either organic or inorganic substances. Organic fertilizers are mineral nutrients needed by the plant that come from plant and animal matters such as rotten leaves, plant residues, and animal manure, whereas inorganic fertilizers are those that are formulated and commercially manufactured.

Organic fertilizers

These are composted plant waste and animal manure with very small amounts of plant nutrients. Its use is in contributing more to better soil structure (because of the organic matter) than providing essential elements.

The composition of compost mostly coming from plant waste ranges from 0.07 to 1.07% N, 0.03 to 0.5% P2O5, 0.09 to 2.22 K2O, and 39.6 to 93.2% H2O; it has a pH of about 5.9–9.4 (76). In Japan, suggest organic fertilizers are used at the rate of 10–30 t/ha as a supplement to inorganic fertilizers in order to attain rice yield of 7–10 t/ha.

Another source of organic fertilizers is the green manure crops that should be seeded before and/or after rice. These are Sesbania aculeata, Crotolaria juncea, and Phaseolus aureus. Each one has a potential yield of 3-9 t/ha of green matter harvested about 8 weeks after seeding or just before flowering; this is timed for soil incorporation before rice planting (De Datta 1981)

There are various methods of compost making. Plant materials, wastes, and residues are often mixed with animal manure placed in a shallow pit or raised above the ground into a heap. It is covered with caked mud to protect it from direct sunlight; at times, a portable shed is provided. The compost is watered intermittently and mixed once a week to hasten decomposition. Four to five weeks is enough for composting prior to incorporation into the soil before the planting of rice (Gorrez and Golden 1966).

Azolla is another source of nitrogen for wetland rice farming. It has been used as organic fertilizer in China, Korea, and Vietnam. In the Philippines, it can supply about half of the nitrogen fertilizer requirements per hectare as recommended in past national rice programs. Azolla is a tiny aquatic fern that grows in ponds, ditches, canals, and other water surfaces. Its importance as a fertilizer is derived from its symbiotic relationship with the blue-green algae, Anabaena azollae, which live in the cavities of azolla leaves and which fix nitrogen from the atmosphere. When azolla is directly grown in rice paddies and then incorporated into the soil as green manure, its nitrogen content is released upon decomposition and is used by the rice crop.

Inorganic fertilizers

Inorganic fertilizers are one of the most effective production inputs for rice. Several combinations of nitrogen, phosphorus, and potassium fertilizers or single-element fertilizers are available. The number on the packaging bag refers to the percentage by weight of mineral nutrients in the fertilizer. Fertilizer 24-12-12 means that a 100-kg bag contains 24% nitrogen, 12% phosphorus, and 12% potassium, along with inert ingredients. In a urea bag, the label 45-0-0 means 45% nitrogen and zero phosphorus and potassium. Fertilizer 16-20-0 has 16% nitrogen, 20% phosphorus, and 0% potassium. The rest of the materials in the bag are filler substances and may contain calcium or sulfur.

Timing of fertilizer application

Applying the correct amount of fertilizer at the time when the crop needs it most is one sure way to increase yield. Split application of nitrogen one dose as basal before transplanting and another at panicle initiation is best for obtaining high grain yields, particularly in the case of short- and medium-season varieties. On coarse soil, nitrogen should be applied in three doses: one-third basally incorporated, another third applied at 20-30 days after transplanting, and the remaining third applied at panicle initiation stage. All the phosphorus and potassium should be incorporated into the soil before transplanting.

Nitrogen applied before transplanting increases tillering, which in turn increases panicle number. The number of panicles per unit area is a major factor that determines grain yield. The formation, maintenance, and death of tillers depend, to a great extent, on the nitrogen content of the plant (Ponamperuma 1955). Nitrogen applied during panicle initiation increases the number of spikelets (grains) per panicle per unit area, and this increases yield.

Management of fertilizers to reduce nitrogen losses

The most common method of applying nitrogen fertilizer to lowland rice in Asia is by broadcasting it into the floodwater 2–4 weeks after transplanting. Under such conditions, the plant’s recovery of nitrogen fertilizer seldom exceeds 40%; even with the recommended practice of best split (two-thirds of N broadcast and incorporated into the soil before transplanting and the rest applied at panicle initiation). Considerable research has been done to identify the causes of these losses and the following management strategies were found to minimize losses, enhance plant recovery, and increase yield (De Datta 1981).

1. Broadcasting fertilizer on saturated soil with no free water on the surface and then mixing it with the soil, followed by flooding in 2–3 days.
2. Broadcasting fertilizer on dry soil and flooding immediately so the water carries urea into the soil.
3. Physically placing the fertilizer into the soil (commonly referred to as deep placement at 5–10 cm under soil surface), which is potentially the most practical method of reducing N losses.

Until recently, researchers thought that the major mechanism for nitrogen losses in lowland rice was nitrification or denitrification in aerobic/anaerobic layers of lowland rice soils. This view has recently been challenged, particularly as the results of a series of field experiments and other studies have shown that ammonia volatilization and, in special cases, runoff can be major modes of N losses of up to 50% of the fertilizer applied to the floodwater (De Datta 1973).

Fertilizer recommendations

Results of several years of extensive applied research on different types of soil throughout the Philippines suggest that the need for nitrogen fertilizer is two to three times more than phosphorus fertilizer in phosphate form. In most cases, no potassium is needed. The response of rice to fertilization under lowland condition depends on several factors. These are varietal type, season, spacing, soil characteristics, fertility level, timing of cultural practices, water management, and, the most important as far as the farmer is concerned, profitability (Villegas 1977).

For all high-yielding varieties in the Philippines, a fertilization rate of 60-30-0 or 30-30-0 kg NPK/ha is adequate for the wet season; in the dry season, when irrigation is available, 80-30-0 up to 120-30-0 kg NPK/ha is sufficient (79). When potassium is needed, a moderate application of 30 kg K2O/ha is recommended. For low-fertilizer-responsive varieties, 40-30-0 and 60-30-0 kg NPK/ha can be applied in the wet and dry season, respectively.

The recommendations on fertilizer rates for dry- and wet-season croppings in the Philippines are contained in a Masagana 99 document (MA 1985). They include the application of zinc sulfate (at 100 kg ZnSO4/ha) when soil pH is in the 6.8 to 7.9 range. In severe cases (soil pH ≥8.0), 25–30 kg/ha is applied before the last harrowing (MAF 1985).

Calculations of needed fertilizer materials

It is best to know the nutrient status of the soil through soil analysis or fertilizer trials to determine the kind and amount of various fertilizer elements required (Casem 1968, Villegas 1977, Xuan and Ross 1972). Fertilizers are important but expensive inputs in rice production. Wasting these major elements?i.e, using amounts in excess of what are actually needed means losses in investment. Thus, using the correct rate or dosage of the appropriate material is critical.

Fertilizer materials (FMs) are classified according to the number of elements present in a fertilizer bag. Single-element fertilizers contain only one kind of nutrient element. Ammonium sulfate, urea, and superphosphate are single-element fertilizers. Incomplete fertilizers (also called mixed fertilizers) contain two fertilizer elements, e.g., nitrogen and phosphorus. The term ‘mixed’ refers to two or more of the major fertilizer elements supplied by two or more fertilizer materials. An example of an incomplete fertilizer is diammonium phosphate (DAP), which contains 16% N and 20% P2O5. Complete fertilizers contain all the three major elements (nitrogen, phosphorus, and potassium); examples are 14-14-14, 12-24-12, and 24-12-12.

Computation of fertilizer requirements

The basic data needed in computing for FM requirements are a) the recommended rate (RR) expressed in kg of nutrient per unit area, b) the analysis (A) of the selected fertilizer nutrients expressed in percentage, and c) the area to be fertilized in terms of hectares or square meters. Like the numbering system used to express fertilizer analysis, RR is given in terms of kg of nutrient element per hectare, arranged in NPK (nitrogen, phosphorus, potassium) order.

RR 90 + 60 + 30 means that, in 1 hectare (10,000 m2), 90 kg nitrogen, 60 kg phosphorus, and 30 kg potassium must be applied. The formula for calculating the amount of FM to satisfy the given RR is:

FM = RR (kg nutrient/ha) x area (ha) ÷ % nutrient in FM. Proceed by calculating the amount of FM, given the RR and area, using different fertilizer combinations.

1. Combination of single-fertilizer materials

RR = 90 + 60 + 30
Area = 1 ha

FM available:
= 21-0-0 ammonium sulfate (AS) (21% nitrogen)
= 0-20-0 ordinary superphosphate (OSP) (20% phosphorus)
= 0-0-60 muriate of potash (60% potassium)

Therefore: (90 kg N/ha x 1 ha x 100) ÷ 21 as ammonium sulfate
= 450 kg AS (21% nitrogen)

and, (60 kg P2O5/ha x 1 ha x 100) ÷ 20 as ordinary superphosphate 
= 300 kg OSP (20% phosphorus)

further, 30 kg KCL/ha x 1 ha x 100 ÷ 60 as muriate of potash 
= 50 kg KCl (60% potassium)

Therefore, one must apply the following to satisfy the RR of 90 + 60 + 
30 (kg NPK/ha) ? 450 kg AS, 300 kg OSP, and 50 kg KCl

1. Combination of incomplete and single-fertilizer materials RR = 90 + 60 + 30
   Area = 1 ha FM available: = 21-0-0 as AS (21% nitrogen)
   = 16-20-0 as DAP (16% nitrogen and 20% phosphorus)
   = 0-0-60 as KCl (60% potassium) Therefore,
   RR (kg nutrient/ha) x area (ha)
   Amount of FM = —————————————–
   % nutrient in FM 60 kg P2O5 x 1 ha x 100 DAP ———————————-
   20
   = 300 kg DAP (represents 60 kg of P2O5)

Find the amount of N in 300 kg of DAP. It has 16% N, therefore: 300 kg DAP x 0.16 (16 over 100)
= 48 kg N

The RR for N is 90 kg; since 48 kg will be supplied by the 300 kg DAP, 90 (RR) – 48
= 42 (kg N balance required)

The balance can be satisfied by using AS.

    42 kg N/ha x 1 x 100
AS = -----------------------------
    21
    =   200 kg AS

    30 kg K2O/ha x 1 x 100
KCL = --------------------------------
    60
    =   50 kg KCL (muriate of potash)

Therefore, with incomplete and single fertilizer materials, the RR of 90 + 60 + 30 kg NPK/ha can be satisfied by applying the following:
= 300 kg DAP,
= 200 kg AS,
= and 50 kg KCL.

1. Combination of complete, incomplete, and single-fertilizer materials
   RR = 90 + 60 + 30
   FM = Complete (14-14-14)
   DAP = (16-20-0)
   AS = (20-0-0)
   Area = 1 ha
   RR (kg nutrient/ha x area (ha)
   Amount of FM = —————————————
   % nutrient in FM 30 kg NPK/ha x 1 ha x 100 14-14-14 = ————————————
   14
   = 214.3 kg (representing 30 kg each of NPK/ha)

Complete fertilizer (14-14-14) carries the three major fertilizer elements (NPK). Therefore, 90 + 60 + 30 RR minus 30 + 30 + 30 kg NPK/ha from 214.3 kg (14-14-14) = 60 + 30 + 0, which is the balance of NP required. This could be satisfied partly by using incomplete fertilizer that can supply both nutrients.

    30 kg P2O5 x 1 ha x 100
DAP     =   ---------------------------------
    20
    = 150 kg DAP (16-20-0)

Find the amount of N in 16-20-0 to determine if there is still a balance for N.

Therefore, 150 kg (16-20-0) x 0.16 (16 over 100) = 24 kg N. The N balance still required as per RR is 60 kg N (balance of N after applying 214.3 kg of 14-14-14) minus 24 kg N (N from 150 kg of 16-20-0 = 36 kg N.

Apply AS to satisfy the balance required for N.

AS  = 36 kg N/ha x 1 ha x 100 ÷ by 21 (% N)
    = 171.4 kg AS

Therefore, to satisfy the RR 90 + 60 + 30 kg NPK/ha using a combination of single, incomplete, and complete FMs, apply the following amount:
214.3 kg of 14-14-14
150.0 kg of 16-20-0
171.4 kg of 21-0-0