At this time of the year, soybeans have emerged or are emerging. Soybean plant development has two distinct phases. The first is the vegetative stage (V) which starts at emergence and leaf development (Table 1). The second phase in the development is the reproductive (R) stage which starts with flowering, followed by pod development, and ends when the seed is fully mature.
Correct production decisions using plant growth stages are important to optimize yield. Information about the development of the soybean plant and various pictures of all the growth stages can be found in the recently reviewed ‘Soybean Growth and Management Quick Guide,’ an NDSU extension publication (A1174).
Plant stages are determined by the leaf, flower, pod, and seed development within the pods. A leaf is considered fully developed when the leaf at the node directly above it (the next younger leaf) has expanded enough so that the three leaflets, which make up the “trifoliolate leaf”, are fully visible. Leaf development will continue during the reproductive phase and plant stature will still increase. The growth stages can overlap. On a field level a growth stage is reached when 50% or more of the plants are in, or beyond, the growth stage as indicated in Table 1.
Flowering of the soybean plant is initiated (after the longest day) on the third to sixth main stem node (a node is a part of the stem where a leaf is attached) and continues upward and downward from there. First flowers of timely planted soybean are usually found in the second week of July. Pollination and fertilization are usually accomplished before the soybean flower opens.
The critical growth stage in soybean for drought stress is from flowering to seed fill. Any stress from the R4-R6 (late pod development to early seed fill) will cause more yield reduction than if the drought stress takes place at another time in the plant’s development.
The seed is physiological mature when 95% of the pods on the plant have the mature color. The date of soybean maturity is associated with the soybean maturity group. Early maturing soybean may be mature by the middle of September.
(Source – http://www.farms.com/news/soybean-growth-and-development-93102.aspx)