TRANSPLANTING
A field is said to be ready for transplanting when it has been thoroughly plowed and harrowed, properly fertilized, and excess water drained off to about 1 cm. Transplanting is the most common method of establishing a rice
crop stand in Asia, where seedlings are first raised in the seedbed and later transplanted in the main field.
Advantages and disadvantages
Advantages
- Weeding and all other cultural operations are easier, faster, and more efficient.
- Proper plant spacing results in uniform growth and maturity of the crop, thus achieving higher yield.
- It gives the seedlings a head start over the weeds as the planter tramples the germinating weeds at planting time.
Disadvantages
- Labor cost can be expensive.
- Seedlings are exposed to possible injury during handling.
- Maturity is delayed by about 5-7 days compared with direct seeding of the same variety.
Methods of transplanting
Straight row transplanting
Good land preparation and efficient management from planting to harvesting is attainable when the field is planted in straight rows. Almost all manual and mechanical operations are easier to manage. Straight row planting has specific spa-cing between rows and hills. This is achieved by using wire and board markers, or a mechanical transplanter (IRRI 1966).
Random transplanting
This method is the most extensively used by farmers who rely on the rain to grow a crop of rice. There is no definite spacing of seedlings between hills and no alignment in any direction. The rainfed areas are much bigger than irrigated areas where straight row planting is practiced. Almost no advantages can be said about random transplanting. The following are the disadvantages:
- Improper spacing may reduce yield potential by 25-30%.
- Too close a spacing increases lodging and cost of transplanting.
- Spacing too far can result in low yields.
- Cultural operation is inefficient and harder to perform.
Age and condition of seedlings for transplanting
Seedlings between 25 and 30 days old (from sowing in the seedbed) are ready for transplanting at temperatures ranging from 16 °C to 33 °C for most of the short-duration HYVs. The physiological age of the seedling is just as important in subtropical or cold regions like the Himalayas. Seedlings with five to six leaves are ready for transplanting; this can mean from 25 to even 65 days of growing in the nursery, as it is affected by cold temperature (Mallick 1982). The yield in Nepal, even with old seedlings planted after winter, is still good.
It is common practice for farmers to transplant traditional or seasonal varieties that are between 35 and 50 days old when rain is scarce. This happens because the environmental factors at the time are not favorable. In general, transplant wetbed seedlings when they are 20-30 days old; for dapog, transplant seedlings 9-14 days after sowing. It is good practice to provide water in the seedbed 1 day before pulling the seedlings to soften the soil.
Handle seedlings with care during the pulling operation and when bundling or tying and transporting them to the field. Any undue stress results in slow recovery, reduced tillering, delayed maturity, and lower yield. Conditions are more critical under rainfed conditions where early plant development is necessary to enable plants to cope with extreme rainfall and sometimes severe dry weather (De Datta 1981).
Pulling and transporting seedlings
To soften the soil and facilitate washing of roots, irrigate the seedbed 1 day before pulling the seedlings. Uprooting is preferably done in the afternoon and the seedlings transplanted within 24 hours. Pull only a few seedlings at a time without twisting the plant and holding them at the base closest to the ground level and roots. Wash off the soil from the roots carefully and tie the seedlings into bundles of convenient size. Careful uprooting and washing of roots not only minimize replanting of missing hills but also result in faster plant establishment and improved tillering.
If seedlings are tall with droopy leaves, one-fourth to one-third of the upper parts of the leaves may be cut to reduce transpiration. It should be recognized, however, that by pruning the top, disease organisms can easily enter through the wound. Root pruning is not recommended because it may just decrease the yield.
While transporting seedlings to the main field, avoid rough treatment or any violent disturbance. Handle the seedlings gently and place them in water under shade if transplanting has not begun.
Optimum number of seedlings per hill
With two to eight seedlings per hill, no significant difference in grain yield was observed. When tillering is low, more seedlings per hill are often planted and this could be costly. Wetbed or drybed seedlings are planted, three to four per hill. For dapog, 6 to 10 seedlings per hill are used. The goal is to produce 350-400 productive tillers per square meter to achieve yields of about 4 t/ha. It is important to reserve some seedlings in the nursery for replanting of missing hills within 7-10 days after transplanting.
Depth of transplanting
The most preferable depth of transplanting dapog is 1-2 cm, while that of wetbed and drybed seedlings is 2-3 cm. If seedlings are planted too deeply, the old roots, for lack of air, would die and new roots would develop from a node closest to the soil surface. This is called “node rooting.” Deep planting disrupts the normal absorption of nutrients from the soil, thus delaying plant recovery and tillering by 1-2 weeks. On the other hand, very shallow planting will make it difficult to hold seedlings in place, especially when the wind is constantly blowing. It takes 2-4 days before new roots are formed.
Plant spacing
Spacing between hills and rows depends largely on soil fertility, amount of fertilizers used, age of seedlings, variety, and season. Planting the seedling closer than what is required increases the cost of seeds and transplanting operation, aside from the plants’ tendency to lodge. On the other hand, wider spacing does not provide the required population density per unit area, thus resulting in yield lower than necessary. Since many factors interact, spacing recommendation is made on the basis of field trials.
Pre- and post-transplanting care (zinc deficiency and iron toxicity)
The early stages of plant disorder or symptom development are the best time to take notice of visible problems. It is of little value to save plants when symptoms of the disorder have become so severe that the plants are nearly dying (Yoshida 1981).
Zinc deficiency symptoms in lowland rice usually appear within 2-3 weeks after transplanting or as soon as the first true leaves are formed. Chlorosis, which begins at the base of the new leaves, spread quickly to the entire leaf, which takes on a pale yellowish-green color. The severity of chlorosis increases with time and eventually dark irregular necrotic spots develop in the leaves, which become structurally weak. The affected plants usually die within 4-6 weeks from planting, and seedlings usually disintegrate in the water (Mikkelsen 1979). To solve this problem, dip the seedling roots before transplanting in a solution of 1 kg zinc oxide mixed in 40-50 liters of water (per hectare basis). Other methods of treatment include foliar application of zinc sulfate (0.1% aqueous solution) or applying various zinc compounds into the soil.
Symptoms of element toxicity normally appear in the lower leaves where the absorbed elements accumulate more. Iron, manganese, and boron toxicity symptoms would appear first in the lower older leaves (Tanaka 1966).
Iron toxicity symptoms would show tiny brown spots on the lower leaves, starting from the tips and spreading toward the base. Leaves usually remain green; in severe cases, the entire leaf looks purplish-brown.
Iron deficiency symptoms would show the entire young leaves becoming chlorotic and then whitish. If iron supply is cut suddenly, newly emerging leaves become chlorotic. A 0.1-0.2% aqueous solution of ferrous sulfate or ferrous citrate with a small amount of sticker may be applied (Tanaka 1966).
To ensure good crop establishment, these general suggestions for pre- and post-transplanting care should be followed:
- Treat seedlings if tungro and nutrient deficiency Symptoms are observed.
- Plant in straight rows with almost no water in the field.
- Plant at the right age and use correct spacing, number of seedlings per
hill, and proper depth. - Maintain the fields at saturation point until seedlings are established.
- Irrigate the field 25 days after transplanting at 35 cm depth.
- Plant missing hills within 10 days after transplanting.
- Control weeds by hand, use of herbicides, or through rotary weeder
within 25 days after transplanting.