After incubation, the germinating seeds are sown immediately on prepared seedbeds to ensure good seedling stand. A delay of more than one day would result in poor growth and low percentage of seedling stand. It is therefore important that the seedbed be prepared in advance so that incubated seeds are sown on time.
There are three general methods of raising seedlings: the dapog seedbed (adapted in the Philippines), the wetbed, and the dry bed methods (practiced in Asia). Four major operations are involved in raising seedlings (De Datta 1981, IRRI 1977):
- preparation of the seedbed,
- soaking, incubation, and sowing,
- nursery management of seedlings, and
- pulling of seedlings for transplanting.
Dapog Seedbed
Land preparation for the dapog seedbed is similar to that used for the wetbed or dry bed method. After three plowing and one to two harrowing, a slightly raised bed (4-6 cm high) with a width of 1-1.5 m and a convenient length of about 25-30 m is prepared. The surface of the seedbed should be smooth and level. This dapog seedbed area is adequate for planting a 1-hectare rice field. The entire surface of the seedbed is covered with banana leaves (with midribs removed) with no holes or breakage and which overlap each other on all sides. A plastic sheet material can also be used for the same purpose. For both surfaces, place strips of 1-inch-wide banana bracts along the periphery of the seedbed and secure them by piercing through the banana bracts into the soil with thin bamboo pegs. This is to prevent the seeds from spreading outside the seedbed itself, while the banana leaves and plastic sheets spread over the bed will prevent the roots of the seedlings from getting in contact with the soil, thus, facilitating the separation and lifting of seedlings that are ready for transplanting. The nutrients required for seedling growth are obtained solely from the endosperm.
The dapog seeding rate is 80 – 100 kg seeds per 30 – 35 sq.m2 seedbed, which is suffi-cient to plant 1 hectare. The germinating seeds are spread at 3,000 gm/sq.m2. The growing seedlings are ready for trans-planting between 9 and 14 days after seedbed sowing.
This method is labor-and space-saving, easy to manage, has a short nursery period, and poses no transporting problem as seedlings can be rolled up with no severe shock manifesting on the young seedlings.
Wetbed Method
The seedbed should be prepared and sowed 25-35 days before the growing seedlings are transplanted in the main field. The nursery seedbed is plowed and harrowed until the soil with organic matter is well mixed; it is then leveled and puddled. A raised bed, 4-6 cm high, 1-1.5 m wide, and 400-500 m long, is prepared and then seeded with about 50–80 kg of seeds, evenly spread at 100 gm/sq.m2. As practiced in most Asian countries, the seedlings are ready for transplanting 20-25 days after seedbed sowing, enough for a 1-hectare rice field.
Drybed method
The drybed method is practiced in areas where water is not sufficient. Nursery preparation is the same as in the wetbed method, except that the seed-bed is prepared dry and cannot be puddled. Canals are cons-tructed around the bed to keep the soil moist and to catch rainwater for the seedlings. As in the wetbed method, the rate of seeding is 50-80 kg with a plot dimension of 400 to 500 sq.m2. Seedlings are ready for transplanting 20-35 days after seedbed sowing. This method is commonly used under rainfed condition where water supply is limited.
Regardless of the seedbed method used, it is always good practice to control insects and maintain good management in the nursery. The operational cost is almost negligible and the area to be treated is smaller, compared with doing the same job in the main field at a higher cost and resulting oftentimes in less effective control.
Fertilization of the Seedbed
Applying fertilizer in the nursery is most often not practiced because farmers always select a fertile nursery area. When long-maturing varieties (135-150 days) are nitrogen-deficient, apply 1.0 kg of ammonium sulfate or 0.5 kg of urea per 100 sq.m2 of the seedbed as a topdress. This is done 1-2 weeks after sowing. Seedlings should not be pale yellow in color, which might suggest nitrogen deficiency. If a second topdressing is required, this must be done 1 week before pulling the seedlings for transplanting in the main field. The same kind of fertilizer and the same fertilization rate should be used. (Consult the chapter on “Nutrition of the rice plant” for other nutrient deficiency symptoms.)
Uniform sowing of seedbed
Always aim for a smooth, leveled seedbed for all kinds of soil preparation and evenness of sowing. Seeds should barely sink into the soil and not be buried too deeply. Unevenness of leveling can result in growth suppression on high and low areas, leading to patchy and irregular plant establishment.
Sow pregerminated seeds immediately and uniformly at the rate of 3,000 gm/sq.m2 for the dapog method and 100 gm/sq.m2 for both the wetbed and drybed methods. Care should be taken in manually using a small wooden board to shift or push the seeds in any direction to attain uniform and even sowing of seeds.
An ideal seedbed when sowed (wetbed and drybed) would allow the seeds to barely sink into the soil but not buried completely. The puddled wetbed is left to settle for 1 day with only a thin film of water left on the bed surface. Controlled shallow flooding of 2-3 cm after sowing will produce short, sturdy seedlings with good root systems. The nursery should be set up away from shady places where there are houses or big trees. Fertilizers may be used if the nursery period is longer under upland conditions, or the seedbed is located in cold places, or seedlings are grown in poor soil.
Nursery Irrigation Management
During the first week of growth of the seedlings, admit water into the seedbed just enough to saturate the soil. When the shoots are 2-3 cm high, seedbeds may be kept submerged with shallow water, depending on the height of the seedlings, to control weeds. A high water level is undesirable because it results in taller seedlings, instead of the ideal short and strong seedlings with good roots.
For dapog seedbeds, the germinating seeds are splashed daily with water until the soil reaches saturation point. The seeds are then gently pressed down with a wooden board to maintain root contact with the seedbed surface. The same procedure is repeated every morning and afternoon thereafter for 4-5 days; this ensures uniform seedling growth. The pressing action on the germinating seeds prevents the drying up of seedlings since the roots are in contact with water on the surface of the seedbed. After this period, the seedbed is continuously irrigated to a depth of 1-2 cm until transplanting time.
For wet beds, add enough water to saturate the soil during the first week and irrigate continuously to a depth of 2-3 cm, until 1 day before transplanting.
The dry bed is dependent on rainwater and frequently used in upland rainfed areas where irrigation is not possible.
Maintenance and Care
Plant protection in the nursery is a “must.” Check for insect infestation, egg mass, insect excreta, and other symptoms of damage. As pointed out earlier, pest and disease control mea-sures applied in the nursery are many times easier, cheaper, and more effective than treat-ing the main field. Spray the seedlings with recommended insecticides against stem borers, planthoppers, leafhoppers, leaf-eating insects, and those feeding below ground level. Rats also feed on growing seedlings and dig up the seedbeds. Weeds, which are easy to identify in the seedbed, compete with rice for sunlight, nutrient, and water. The plant competition for nutrients, water, and sunlight results in poor seedling growth; it takes longer for the seedlings to recover when transplanted in the main field.
Water is a major component of the rice plant and is involved in the manufacture of carbohydrates and minerals, which are utilized by the different plant parts. Without irrigation or adequate water in the leaves, pores close, water cannot pass out, and air cannot enter, thereby severely retarding plant development (Vergara 1979). However, deep flooding results in tall plant growth and poor root systems due to lack of air in the soil.