Irrigation may also be required during the flowering stage on sandy soils or during very dry years on medium and fine-textured soils. However, if water is applied during flowering, it is important to follow with adequate water during seed fill. Otherwise, more but smaller seeds will develop, reducing yields. Although soybean roots can reach depths of 5 to 6 feet, the largest concentration of roots and the majority of soil water extraction occur in the top 2 to 3 feet of the soil profile. Therefore, irrigation water should not normally penetrate below 3 feet.
Soybean produce highest yields on soils with good internal and surface drainage.
The most convenient way to time soybean irrigation is crop growth stage. Precipitation during the growing season, stored soil moisture prior to the growing season and irrigation system capacity combine to furnish water to the crop. Research has shown indeterminate soybean respond well to delayed irrigation. However, as rainfall and stored soil water decrease from east to west, delayed irrigation can reduce yields when compared to full-season irrigation. If necessary, irrigation began prior to flowering to supply water according to the water use of the crop.