Irrigation Scheduling

Irrigation depth

Irrigation depth varies depending upon the source of irrigation (water discharge), soil type, stage of crop growth, weather conditions and so on. When water discharge is low, farmers tend to apply light irrigation to cover a greater area per unit of time. In heavy soils, heavy irrigations are avoided as these can cause waterlogging or excessive moisture, leading to an adverse effect on crop growth and yields. A crop sown in suboptimal moisture conditions may require a light irrigation within a few days after sowing to improve germination, especially in light soils. Irrigation depth also varies depending upon the prevailing weather conditions. If there is wind, irrigation should be avoided or only a light irrigation should be applied, otherwise there may be lodging.

Irrigation methods

In the soybean, flood irrigation is a common practice in flat-bed-sown crop. However, where the crop has been sown on raised beds, furrow irrigation is applied. Some water-saving high-tech irrigation techniques such as sprinkler and drip irrigation are either not used or very rarely used in soybean. However, the choice of irrigation method is also determined by factors such as the source of irrigation, surface topography and soil texture.

Furrow- and flood-irrigated soybean crops show better growth, seed yields and net returns than non-irrigated crops, and both irrigation systems are equally good in all of these parameters (Table 9.2). Drip irrigation not only uses less water than sprinkler irrigation but also maintains a higher soil temperature, leading to a higher emergence rate and enhanced seedling growth (Wang et al., 2000). Surface and subsurface drip irrigation systems can be used. However, in the case of subsurface drip irrigation, at lower depth a high moisture content with low soil oxygen concentration may cause hypoxia. Oxygation (aerated irrigation water) can ameliorate hypoxia and increase soybean yields (Bhattarai et al., 2008). Although drip and

Table 9.2. Effect of irrigation method on plant characters, seed yield and net returns of soybean grown following rice at Stoneville, Mississippi, USA (average across years and cultivars from maturity groups IV and V) (adapted from Heatherly and Spurlock, 2000).

Irrigation method

Plant height (cm)

Seed weight (mg per seed)

Seed yield (kg ha-1)

Net returns ($ ha-1)

Non-irrigated (control)

ll

129

2eSB

20e

Furrow-irrigated

BS

148

3l20

34l

Flood-irrigated

Be

148

3l02

393

sprinkler irrigation systems are known for their irrigation water economy, these systems are not very popular among farmers for soybean crops due to their initial high costs.

Irrigation indices

Irrigation can be based on parameters such as crop stage, weather conditions and soil moisture status.

In the case of soil moisture status, irrigation is applied when the soil moisture is depleted to a certain level (Al-Assily and Mohamed, 2002). Irrigation may also be applied based on the crop's growth stage (de Costa and Shanmugathasan, 2002). Furthermore, a computer program method, tensi-ometer or gypsum block may be used for deciding when to irrigate (Thompson et al., 2002). Irrigation may be applied to the crop based on any parameter. However, the main objectives of all of these are that the crop yields are high and irrigation water use is efficient.

Effect of irrigation on soybean

As with other crop plants, soybean requires optimum soil moisture for high seed yields. If there is an adequate availability of moisture in the soil either due to rainfall or otherwise, the crop may not need any irrigation. However, in the absence of adequate soil moisture, irrigation has to be applied to obtain proper crop growth and high crop yields.

Soybean yields increase as irrigation increases (Gercek et al., 2009). Soybean is known to respond significantly to irrigation during flowering and seed formation (Bharambe et al., 2002). Similar yields can be obtained by applying a single irrigation at the R4, R5 or R6 stage (Sweeney et al., 2003), which are, however, about 20% higher than with no irrigation. The beneficial effect of irrigation on seed yields may be due to an increased number of seeds per plant or increased weight per seed. Irrigation at R4 increases seeds per plant, whereas irrigation at R5 or R6 increases weight per seed (Sweeney et al., 2003). In the case of a saturated soil culture (continuously irrigated), soybean root and nodule growth, nitrogenase activity, leaf conductance to water vapour and seed yields are higher than with conventional irrigation (Troedson et al., 1989); the seed yields are possibly higher due to greater photosynthesis, sustained nodules during pod growth and consequently a continuous nitrogen supply to the seeds.

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