Complete Farming Guide
Soybeans Farming Guide
Glycine max
420 million tonnes — the world's most important oilseed
Soybeans are the dominant source of protein for global animal agriculture and a cornerstone of the vegetable oil market. Unlike other major crops, soybeans fix their own nitrogen through symbiotic bacteria in root nodules, returning 40-60 lb N/acre to the soil for the following crop. Brazil recently overtook the US as the world's largest producer, reflecting soybean's explosive expansion into tropical savannas. Soybeans contain 35-40% protein and 18-20% oil, making them uniquely valuable as both a protein and energy source.
Quick Facts
Growing Season
Spring/Summer
Growth Period
100-150 days
Optimal Temp
20-30°C (68-86°F)
Water Needs
450-700mm
Top Producer
Brazil
Yield / Acre
45-55 bushels (US avg); 25-35 bu (global avg)
Soybeans Growth Stages — What to Do at Every Stage
Germination & Emergence (VE-VC)
Days 0-10What to do
Plant when soil temperature reaches 10°C (50°F) at 2-inch depth, but 13°C (55°F) is ideal for rapid emergence. Inoculate seed with Bradyrhizobium japonicum if the field has not grown soybeans in 3+ years. Plant at 1-1.5 inch depth, 140,000 seeds/acre.
Watch for
Seedling diseases (Phytophthora, Pythium) in cold, wet soils. Slugs in no-till fields feeding on cotyledons. Poor emergence from planting too deep.
Vegetative Growth (V1-V6)
Days 11-45What to do
Apply post-emergence herbicide early (V2-V3) before weed canopy closes. Scout for soybean aphids — populations can double every 2 days in July. Nodulation should be visible by V2 — dig up roots and look for pink nodules (pink = active nitrogen fixation).
Watch for
Yellow plants with no root nodules indicate failed nitrogen fixation — may need emergency nitrogen application. Bean leaf beetle feeding on leaves (round holes). Sudden death syndrome (SDS) symptoms beginning on lower leaves.
Flowering (R1-R2)
Days 46-70What to do
Soybeans begin flowering and will continue for 3-4 weeks. Apply foliar fungicide at R3 if disease pressure warrants (white mold, frogeye leaf spot). Ensure adequate moisture — yield components are being set.
Watch for
White mold (Sclerotinia) on stems starting at the soil line — white, cottony growth with hard black sclerotia inside stems. Spider mite damage during hot, dry periods — stippled, yellowing leaves starting at field edges.
Pod Development (R3-R5)
Days 71-100What to do
This is the peak water-demand period. R1-R5 is the most yield-sensitive window for irrigation. Apply insecticide for stink bugs if thresholds are reached (9 per 25 sweeps in R3-R5). Maintain scouting intensity.
Watch for
Soybean cyst nematode — stunted, yellowing patches that mimic nutrient deficiency. Dig roots and look for tiny white or yellow cysts (smaller than a pinhead) on roots. Stink bug feeding on developing pods causes flat, shriveled, and discolored seeds.
Seed Fill & Maturity (R6-R8)
Days 101-150What to do
Seeds are filling rapidly (R6) and moisture demand remains moderate. No further fungicide applications. Plan harvest timing — R8 (full maturity) occurs when 95% of pods have reached mature color. Harvest at 13% moisture.
Watch for
Green stem syndrome — stems stay green even though pods are mature. This delays harvest and increases shattering risk. Pod shattering in dry, hot conditions — prioritize harvest of shatter-prone varieties.
Common Soybeans Diseases — Identification Guide
Soybean Cyst Nematode (SCN)
Heterodera glycines
What you will see
Stunted, yellowing plants in irregular patches, often near field entry points or in sandy areas. Above-ground symptoms mimic nutrient deficiency or drought. Roots show tiny (0.5mm) white, yellow, or brown cysts attached to root surfaces — visible with a hand lens. Yield losses of 10-30% can occur without visible foliar symptoms.
Conditions that favor it
Present in most soybean-growing regions. Thrives in light, sandy soils with pH above 7.0. Populations build over consecutive soybean crops. Soil sampling is the only reliable way to detect SCN before economic damage.
Sudden Death Syndrome (SDS)
Fusarium virguliforme
What you will see
Interveinal chlorosis (yellowing between leaf veins) on upper leaves, progressing to interveinal necrosis (brown dead tissue between green veins). Leaves eventually fall off, leaving petioles attached to the stem — the "petiole retention" symptom is diagnostic. Roots may show blue-gray spore masses on the taproot surface. Often appears suddenly in R3-R4.
Conditions that favor it
Cool, wet conditions during planting promote early root infection, but symptoms appear during reproductive stages (R3+) under warm, wet conditions. Compacted, poorly drained soils. SCN co-infection makes SDS worse.
White Mold (Sclerotinia Stem Rot)
Sclerotinia sclerotiorum
What you will see
White, cottony fungal growth on stems, usually starting at the nodes near the soil line. Stems become bleached and dry, and hard, black sclerotia (survival structures, 2-10mm) form inside the stem pith. Wilting and death of individual plants or groups of plants in the canopy.
Conditions that favor it
Cool temperatures (15-25°C) during flowering, high humidity, dense canopy that retains moisture, and narrow row spacing. Fields with a history of white mold have sclerotia in the soil that germinate during flowering.
Frogeye Leaf Spot
Cercospora sojina
What you will see
Circular to angular leaf spots with dark brown-red borders and gray-tan centers, resembling a frog's eye. Spots are 3-5mm in diameter. Severe infections cause premature defoliation. Can also appear on stems and pods. Resistant strains to common fungicides (QoI resistance) are now widespread.
Conditions that favor it
Warm (25-30°C), humid conditions with frequent rainfall or dew. Spreads from infected residue. Worst in continuous soybean and reduced-till systems.
Phytophthora Root and Stem Rot
Phytophthora sojae
What you will see
Seedling damping-off (pre- and post-emergence death) in early season. In older plants, a characteristic chocolate-brown lesion on the lower stem that extends from the soil line upward, sometimes one-sided. Plants wilt and die with leaves remaining attached. Roots are severely rotted.
Conditions that favor it
Saturated, poorly drained soils. Occurs after heavy rainfall events that waterlog the field for 24+ hours. Clay soils with compaction layers. Soybean fields with a history of Phytophthora.
Common Soybeans Pests — Identification & Damage
Soybean Aphid
Damage
Sucks sap from leaves and stems, causing stunted growth, leaf curling, and reduced pod fill. Heavy infestations produce copious honeydew that supports sooty mold growth, turning leaves black. Transmits soybean mosaic virus and alfalfa mosaic virus. Populations can explode from 0 to 10,000+ per plant in 2-3 weeks under favorable conditions.
How to identify
Tiny (1.5mm) light green aphids with black-tipped cornicles ("tailpipes"). Found on undersides of leaves and new growth. Use a hand lens. Economic threshold: 250 aphids per plant with 80% of plants infested and population increasing.
Bean Leaf Beetle
Damage
Adults feed on leaves (round holes between veins), pods (scarring and entry holes for pod diseases), and seedling cotyledons. Pod feeding is most economically damaging — creates entry points for secondary fungal infections and seed quality problems. Also vectors bean pod mottle virus.
How to identify
Small (6mm) beetle, highly variable in color — tan to red with or without spots, but always with a distinctive black triangle behind the head. Very active and drops from plants when disturbed. Threshold: 25% defoliation before R1, 15% during R1-R5.
Stink Bugs (Green, Brown, Brown Marmorated)
Damage
Pierce developing pods and feed on seeds, causing flat, shriveled, discolored seeds ("stay green" damage) and reduced oil and protein content. Heavy feeding can cause pods to abort entirely. Late-season damage reduces seed quality more than yield.
How to identify
Shield-shaped bugs, 12-17mm. Green stink bug is solid green; brown stink bug is dull brown; brown marmorated stink bug has alternating light/dark bands on the antenna. Use a sweep net — economic threshold is 9 stink bugs per 25 sweeps during R3-R6.
Spider Mites (Two-Spotted Spider Mite)
Damage
Feed on leaf undersides, sucking cell contents and causing stippling (tiny yellow dots), then bronzing, then leaf death. Infestations start at field edges and move inward. Severe infestations during pod fill can cause 40-60% yield loss. Drought stress dramatically increases mite reproduction.
How to identify
Tiny (0.5mm) yellow-green mites with two dark spots, found on leaf undersides. Look for fine webbing. Hold a white sheet of paper under a plant and tap — mites appear as tiny moving dots. Stippled leaves feel rough/sandy to touch. Threshold: heavy stippling on lower 1/3 of canopy.
Soybean Looper
Damage
Larvae consume leaf tissue, leaving only the veins (skeletonizing) in severe cases. Each larva can consume 100+ square centimeters of leaf area. Defoliation during R1-R5 directly reduces yield by reducing photosynthetic capacity for pod and seed fill.
How to identify
Green larvae (30mm) that move in a looping motion (inchworm gait). Distinguished from other green larvae by having only 3 pairs of prolegs. Found on lower to mid-canopy leaves. Threshold: 15% defoliation during reproductive stages (R1-R5).
Soybeans Nutrient Management — NPK Guide
Nitrogen (N)
NOT needed — soybeans fix 100-300 lb N/acre from the atmosphere via root nodules
Phosphorus (P)
30-50 lb P2O5/acre based on soil test
Potassium (K)
60-80 lb K2O/acre — soybeans are heavy K users
Application Timing
Inoculate with Bradyrhizobium at planting if field is new to soybeans. P and K are applied pre-plant or at planting. Potassium deficiency is common and shows as yellow-brown leaf margins on lower leaves during pod fill. If nodulation fails (check roots at V2-V3 for pink nodules), emergency application of 50-75 lb N/acre at R1 may be needed.
Soybeans Irrigation Schedule
Total Water Requirement
18-22 inches (450-560mm) total crop water use
Critical Stages
R1-R5 (flowering through seed fill) is the most water-sensitive window. A single week of drought stress during R3-R5 can reduce yields by 5+ bushels/acre. Peak water use is 0.25-0.30 inches/day during R3-R5.
Irrigation Frequency
Irrigate to maintain soil moisture above 50% of available water capacity. During R3-R5, maintain 60%+ to protect against brief heat waves. Irrigation can usually stop after R6 (full seed).
Soybeans Economics — Cost, Yield & Profit
Cost per Acre
$550-662/acre (US average)
Yield per Acre
48-55 bushels/acre (US average)
Revenue per Acre
$650-750/acre at $12-14/bushel
Profit per Acre
$50-150/acre
Regional Context
Soybean margins are modest but more stable than corn because input costs are lower (no nitrogen purchase needed, lower seed costs). Soybeans in rotation with corn typically add $20-40/acre in nitrogen credit to the following corn crop. In South America, soybean profitability is higher due to lower land costs and double-cropping with safrinha corn. Premium markets for food-grade, non-GMO, and organic soybeans command $2-5/bushel above commodity price.
Frequently Asked Questions — Soybeans Farming
Do soybeans need nitrogen fertilizer?
No — this is one of the most remarkable features of soybeans. Through a symbiotic relationship with Bradyrhizobium japonicum bacteria in their root nodules, soybeans fix 100-300 lb of atmospheric nitrogen per acre per season. The key is ensuring good nodulation: inoculate seed with fresh Bradyrhizobium inoculant if the field has not grown soybeans in the past 3-5 years. At V2-V3, dig up plants and check root nodules — they should be pink inside (pink = active hemoglobin, meaning nitrogen fixation is occurring). White or green nodules are inactive. If fewer than 10 nodules per plant or nodules are not pink, nitrogen fixation is failing and you may need emergency N application. After harvest, soybeans leave 40-60 lb N/acre in the soil for the next crop.
What is soybean cyst nematode and how do I check for it?
Soybean cyst nematode (SCN) is the #1 yield-limiting pathogen of soybeans globally, causing over $1 billion in losses annually in the US alone. The challenge is that SCN often causes 10-30% yield loss with no visible above-ground symptoms — the field just produces less than expected. To check for SCN: (1) collect soil samples from 6-8 inches deep in a zig-zag pattern across the field in fall or early spring, (2) submit to a nematode testing lab — results are reported as eggs per 100cc of soil, (3) economic threshold is 200 eggs per 100cc. Management includes rotating to non-host crops (corn, wheat), using SCN-resistant varieties (check for PI 88788 or Peking resistance sources), and maintaining soil pH below 7.0 when possible.
When should I spray fungicide on soybeans?
The most effective fungicide timing for soybeans is R3 (beginning pod) — this is when fungicides provide the greatest return on investment because they protect the canopy through the critical R3-R6 seed fill period. However, not every field needs a fungicide application. Consider spraying if: (1) you have a history of white mold, frogeye leaf spot, or other foliar diseases, (2) the field is in narrow rows (15 inches or less) with a dense canopy, (3) weather is wet and humid during flowering, (4) your variety is susceptible to key diseases. Skip fungicide if the canopy is thin, the weather is hot and dry, or you have resistant varieties. In fields with white mold risk, target the R1-R2 (flowering) timing to prevent sclerotia germination. Always use a fungicide with multiple modes of action to manage resistance.
AI-Powered Crop Monitoring
Monitor Your Soybeans Fields with Cropple.AI
Cropple.AI detects soybean stress patterns invisible from the ground — early iron deficiency chlorosis, SCN damage zones, and sudden death syndrome patches — using multi-spectral satellite imagery. Get automated spray recommendations based on weather-driven disease risk models and growth-stage-specific management alerts.
No credit card required. Cancel anytime.