Vegetable soybean, being a legume, fixes atmospheric nitrogen in the soil through the Bradyrhizobium bacteria, and normally it does not require nitrogen fertilizer application. However, depending upon a soil test and the amount of fertilizer applied to the previous crop, a starter nitrogen fertilizer of 25-30 kg N ha-1 can be applied at the time of sowing. Based on soil tests in Taiwan, the recommended fertilizer application is about 10 t ha-1 of compost, 60 kg N ha-1, 30 kg P ha-1 and 50 kg K ha-1. Half of the nitrogen fertilizer is applied as a basal dressing and the other half as a top dressing at the flowering and pod-formation stage. To ensure optimum size and good quality of seed, another dose of 20 kg N ha-1 can be given at the seed-filling stage (Chen et al., 1991). In Japan, compost is applied at 10-150 t ha-1. The nitrogen, phosphorus and potassium fertilizer rate is 30-40, 150, and 80-100 kg ha-1, respectively (Kamiyama, 1991). Lime is also applied at the rate of 1000 kg ha-1 (Kokubun, 1991). In trials conducted in Taiwan, the results showed that a basal dressing of 50 kg N ha-1, 30 kg P ha-1 and 20 kg K ha-1 at sowing, a top dressing of 70 kg N ha-1 and 10 kg P ha-1 at 15 days after sowing and a top dressing of 50 kg N ha-1 and 50 kg P ha-1 at the pod-initiation stage gave the highest yield and good-quality pods. Bra-dyrhizobium inoculation along with 20 kg N ha-1 increased both the number and weight of nodules (Hung et al., 1991). A fertilizer rate of 25 kg N ha-1 plus Bradyrhizobium inoculation was excellent for alluvial soil in Vietnam's Mekong Delta (Diep et al., 2002). If there is no rain following fertilizer application, irrigation is necessary for proper absorption of the nutrients. Potassium sulphate is better than potassium chloride. If the soil is deficient in micronutrients such as boron, zinc or molybdenum, these should be provided as chelates (NSRL, 2008).
For weed control, a pre-emergence herbicide such as Lasso (alachlor) or Pursuit (imazethapyr) is sprayed at 1.5 kg a.i. ha-1. Intercultivation is performed once or twice during the crop season to control weeds. Hand weeding is also performed to eliminate weeds when necessary. Weed control up to the R1 growth stage is extremely important, because the crop does not cover the ground at that time; after the R1 growth stage, the crop canopy covers the ground well and suppresses weeds. No Roundup Ready soybean gene has been found in the vegetable soybean cultivars grown in Taiwan (Cherng and Tay, 2002).
Optimum soil moisture (50% of the soil) is essential for good germination. Under optimum soil moisture and temperature the seed germinates and the seedling emerges in about 5-10 days. After the rice harvest, if the field is dry then it is irrigated, and when the soil moisture comes to the right stage, ploughing, ridging, furrowing and sowing are carried out. After seedling emergence, irrigation is given at 15- to 20-day intervals until the pods are well developed. The frequency and amount of irrigation depends on the type of soil, rainfall, drainage, season and crop duration to maintain proper soil moisture. On heavy clay soils with good water-holding capacity, usually three to four irrigations are sufficient; loamy and sandy loam soils require more frequent irrigations. Irrigation during initial flowering at the R1-R2 growth stages (Fehr et al., 1971) and early podding period accelerates pod filling and seed filling and increases the yield (Zhang, 2004). Insufficient moisture at the R1-R2 and R3-R5 growth stages induces flower and pod drop. Therefore, optimum moisture should be maintained during these critical stages to achieve a high yield and good quality (Kokubun, 1991).
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