Soybean

The soybean was domesticated in East Asia, most likely in northern China, from its wild ancestor Glycine soja. Archaeological and genetic evidence points to the Yellow River basin as a primary center of domestication.

• Domestication estimated to have occurred between 5,000 and 9,000 years ago, during the Shang Dynasty period or earlier
• Wild progenitor Glycine soja is still found across China, Korea, Japan, and parts of Russia
• Introduced to Europe in the early 18th century and to North America by the mid-18th century as a forage and soil-improving crop
• Samuel Bowen is credited with growing the first soybeans in North America in 1765, in Savannah, Georgia
• Today, major producing nations include Brazil, the United States, Argentina, China, and India
• Brazil surpassed the United States as the world's largest soybean producer in the early 2020s

The soybean is an erect, bushy annual herbaceous plant, typically growing 30–130 cm tall, though some cultivars can exceed 2 meters.

Stem & Growth Habit:
• Stems are erect to semi-erect, covered with fine brown or gray pubescence (hairs)
• Growth habit varies from determinate (stem terminates in a flower cluster after reaching a set height) to indeterminate (continues vegetative growth alongside flowering)
• Branches arise from leaf axils; branching density varies by variety

Roots:
• Taproot system, penetrating up to 1.5–2 m deep
• Lateral roots spread widely in the upper soil layers
• Root nodules housing nitrogen-fixing Bradyrhizobium bacteria are a defining feature — typically spherical, 2–5 mm in diameter
• A single plant may develop 50–200+ nodules under favorable conditions

Leaves:
• Trifoliolate (three leaflets per leaf), arranged alternately on the stem
• Leaflets are ovate to lanceolate, 6–15 cm long, with entire margins
• Leaves exhibit nyctinastic movement — they fold downward at night and open during the day
• Covered with fine trichomes (hairs) on both surfaces

Flowers:
• Small, papilionaceous (butterfly-shaped), borne in axillary racemes of 3–15 flowers
• Color ranges from white to pale purple or violet
• Self-pollinating (cleistogamous) — flowers typically self-fertilize before fully opening, resulting in a naturally outcrossing rate of less than 1%

Fruit & Seeds:
• Pods are curved or straight, 3–8 cm long, covered with fine pubescence
• Each pod contains 2–4 seeds (rarely up to 5)
• Seeds are nearly spherical to ellipsoid, 5–11 mm in diameter
• Seed coat color varies widely: yellow, green, brown, black, or mottled, depending on cultivar
• Hilum (seed scar) color is an important cultivar identification trait and ranges from colorless to black
• A single plant may produce 50–200+ pods

Seeds contain approximately:
• 18–22% oil
• 35–42% protein
• 15–20% carbohydrate
• 5–6% ash/minerals

Soybean is a warm-season crop adapted to temperate and subtropical climates, requiring specific conditions for optimal growth.

Climate & Temperature:
• Optimal growth temperature: 20–30°C
• Germination requires soil temperatures of at least 10°C, with optimal germination at 15–30°C
• Frost-sensitive; killed by temperatures below −2°C
• Most commercial cultivars require 90–150 frost-free days to maturity

Photoperiod (Day Length):
• Soybean is a classic short-day plant — flowering is triggered when night length exceeds a critical threshold
• Cultivars are classified into 13 maturity groups (000 to X) based on photoperiod sensitivity and latitude adaptation
• Group 000: adapted to very short growing seasons at high latitudes (~55°N, e.g., southern Canada)
• Group VIII–X: adapted to low-latitude tropical regions (~20°N–20°S)

Soil:
• Prefers well-drained, fertile loam soils with a pH of 6.0–6.8
• Tolerant of slightly acidic to slightly alkaline conditions (pH 5.5–7.8)
• Poorly tolerant of waterlogged or saline soils

Nitrogen Fixation:
• Through symbiosis with Bradyrhizobium japonicum and related rhizobial species, soybean can fix 50–300 kg of atmospheric nitrogen per hectare per growing season
• This biological nitrogen fixation reduces or eliminates the need for synthetic nitrogen fertilizer in soybean cultivation
• Residual fixed nitrogen benefits subsequent crops in rotation systems (e.g., corn following soybean may require 25–50 kg/ha less N fertilizer)

Water Requirements:
• Requires 450–700 mm of water over the growing season
• Most critical water-demanding period is during flowering and pod-fill (R1–R6 growth stages)
• Drought stress during pod-fill can severely reduce yield

Soybean is cultivated as a row crop on a massive global scale, with modern agronomic practices maximizing yield and efficiency.

Site Selection:
• Full sun — requires at least 6–8 hours of direct sunlight daily
• Well-drained, loamy soils with pH 6.0–6.8
• Avoid fields with a history of soybean cyst nematode (Heterodera glycines) or other persistent soilborne pathogens

Soil Preparation:
• No-till or minimal-till systems are increasingly common, reducing erosion and preserving soil moisture
• Soil test before planting to determine nutrient needs — phosphorus and potassium applications may be necessary
• Inoculation of seed with Bradyrhizobium japonicum is recommended for fields where soybean has not been recently grown

Planting Time:
• Plant after soil temperatures reach and consistently maintain 10–15°C
• In the Northern Hemisphere, typical planting windows range from late April to early July depending on latitude and maturity group
• Row spacing: 15–76 cm (narrow rows of 15–38 cm can increase yield by improving light interception)
• Seeding rate: 250,000–400,000 seeds per hectare, adjusted for germination percentage and row spacing
• Planting depth: 2.5–5 cm

Watering:
• Supplemental irrigation is beneficial during critical growth stages, particularly flowering (R1–R2) and seed-fill (R5–R6)
• Overwatering or waterlogging promotes root diseases such as Phytophthora root rot

Fertilization:
• Nitrogen fertilizer is generally unnecessary due to biological nitrogen fixation
• Phosphorus (P₂O₅): 40–80 kg/ha based on soil test
• Potassium (K₂O): 60–120 kg/ha based on soil test
• Micronutrient deficiencies (e.g., iron chlorosis in high-pH soils) may require correction

Common Problems:
• Soybean cyst nematode (Heterodera glycines) — the most damaging pathogen/pest, causing estimated annual losses of over $1 billion in the U.S. alone
• Sudden death syndrome (caused by Fusarium virguliforme)
• White mold / Sclerotinia stem rot (Sclerotinia sclerotiorum)
• Herbicide-resistant weeds (e.g., Palmer amaranth, waterhemp)
• Aphids, bean leaf beetles, and stink bugs

Propagation:
• Exclusively by seed — commercial propagation uses certified seed to maintain genetic purity and disease-free status