Wetland Preparation
Lowland rice requires a well-puddled and thoroughly prepared soil in order to
- provide a surface that is weed-free and makes transplanting easier;
- mix organic matter such as straw, stubble, and weeds with the soil;
- prevent or minimize water seepage from the field during subsequent flooding.
Tillage operation varies according to water availability, topography, soil texture, farmer’s resources, and type of cultural management employed (De Datta 1981).
Wetland preparation has three phases: a) flooding the field to saturate and soften the soil; b) plowing two to three times at weekly intervals to turn over the soil and incorporate organic matter; and c) harrowing to break up the clods, puddle the soil, and level the field. Together, the three operations utilize one-third of the total water requirements in growing a crop of rice. The generally low weed population in transplanted rice is attributed to plowing (deep tillage), harrowing (breaking clods, pudding, and leveling), and to people actually trampling on the weeds during transplanting (De Datta and Barker 1978, D Datta et al 1969).
Wetland soil preparation requires shallow rototilling or plowing to prevent destruction of the hard pan. The depth of soil penetration is calibrated on the tractor, so that subsequent passes are also controlled. Destruction of the hard pan will make future machinery operation much harder; alternatives in land preparation should therefore be evaluated (author’s experience).
If land is not uniformly prepared, plant growth becomes uneven, causing differences in vegetative development and maturity from one section of the field to the next. When organic matter is not well decomposed by transplanting time, seedlings are likely to suffer from the effects of toxic decomposition products such as carbon dioxide, methane, and sulfides. For this reason, land preparation is started at least 3-4 weeks before transplanting to allow for complete decomposition and escape of toxic gases from the soil. The rice field should always be saturated or flooded from the first day of plowing up to two weeks before harvesting in order not to lose the nitrogen in the soil. Drying up and subsequent flooding of the field allow microorganisms known as anaerobes to multiply rapidly and act on the nitrate, reducing it to nitrogen gas, which then escapes into the atmosphere (author’s experience).
Dryland Preparation
Dryland preparation follows the same operation as wetland preparation, except that the former is solely dependent on rainfall availability for water use. These are the rainfed farms without a per-manent source of water during the crop-growing period. After the last harrowing and immediately following the onset of the rains, seedlings are transplanted. If seeds are used, these are drilled, sown in furrows, or broadcast.
In most Asian countries, land pre-paration is generally done with animal-drawn farm implements. Carabaos or buffaloes are stronger animals, while cows need to be harnessed in two or four and worked in tandem to pull a central equipment.
Machineries are costly and often beyond the budget of farmers. Thus, dryland preparation is not practiced, except in big rice farms. Weeds are harder to control and the water needs of the crop depend on the amount of rainfall. The crops are thus more susceptible to periodic drought and stress. The pest and disease situation is always serious and hard to manage. Substantial research and development work is needed to increase the present yield level of the larger dryland and upland areas in Asia planted to cereals.
Another alternative, when using tractor-drawn equipment to avoid destroying the hard pan or subsoil, is to irrigate the field 1-2 days before tilling the soil. The paddy is allowed to dry for about a week to encourage a new generation of weeds to grow; the procedure is repeated up to the third tilling operation. The granulated tilled soil produces soil particles about ?-1 inch in diameter, which is ideal for both direct broadcasting of pregerminated seeds and transplanting in shallow water (author’s experience).
Effect of Depth Tillage on Yield
In an experiment by Yoshida (1981) in a farmer’s field, grain yield, as well as nitrogen content of both straw and grain, increased with increasing soil depth from 4 cm up to 36 cm without using fertilizer. In another experiment, the effect of soil depth was studied on fertilized and unfertilized soil. The results showed that grain yield in the fertilized soil at 6 cm depth was comparable with that in unfertilized soil at 22–25 cm depth of tillage. These data indicate that good results can be obtained in shallow soil if fertilization is applied; likewise, deeper tillage will result in higher yield even without fertilization (IRRI 1976). The best judgment on how deep should the soil be prepared is related to variable factors that include use of animals or machinery, rice variety, fertilizer, and cropping season (author’s experience).