The best soybean yields occur on well-drained, but not sandy, soils having a pH of 6.5 or above. The critical stage for soybean yield is in August and droughty soils that typically dry out in August will have disappointing yields. Soybeans have a very broad optimal planting date with optimum dates from about May 5-25 in the warmer regions in central and western New York. Soybeans can be successfully planted in late April or early May in these regions but final stands may be more erratic so an insecticide/fungicide seed treatment is recommended for late April and early May plantings. Mid to late Group II and early Group III varieties can be planted in these regions up until about May 20 and then just Group II varieties until June 1. If a wheat crop is to be planted after soybean harvest, then a late Group I vs. a Group II variety planted in late May will mature earlier and allow for a more timely wheat planting date. In the cooler regions in central and western New York and in Northern New York, optimum planting time is during the midlle two weeks of May. Early Group II and Group I soybean varieties should be planted at this time in these regions.
Although soybean yields decline with June plantings, high yields can still be achieved by planting early Group II or Group I varieties in central and western New York and early Group I and Group 0 varieties in Northern New York until about June 15. The earlier-maturing varieties, which tend to be short in stature, yield better at a row spacing of 15 inches or less. Soybean plantings after June 20 in central/western NY and after June 10 in NNY can be risky, even with Group 0 varieties, especially if the remaining part of the growing season is cool or if frost occurs before October 1.
It is important to place the soybean seed into the ground at a precise depth and in firm contact with the soil so choice of planting equipment is especially critical. A corn planter usually does a better job of planting than a grain drill, but soybeans typically yield about 5% less in 30-inch vs. 7.5 inch row spacing in New York even with lower final stands. In addition, modern drills have much better depth control than older grain drills.
Seeding rate depends on both row spacing and seed size. We recommend seeding rates, for seed not treated with insecticide or fungicide, of about 170,000 seeds per acre for 7.5 inch row spacing (~7.5 seeds per 3 ft.), 160,000 seeds/acre for 15-inch row spacing (about 14 seeds per 3 feet), and 150,000 plants per acre for 30-inch row spacing (~26 seeds per 3 ft.). If an insecticide/fungicide seed treatment is used, seeding rates can be reduced by 10,000 to 20,000 seeds per acre. Planting depth should be about 1.25 1 to 1.5 inches, depending on soil moisture conditions, and should not exceed 2 inches. Soybeans, however, can emerge reasonably well from a 2.5 inch depth, if soil crusting is not prevalent during actual emergence from the soil. Likewise, soybeans can be planted at the 1.0 inch depth, but the seed is susceptible to drying out, if conditions are dry after planting. We recommend the use of inoculum for soybean plantings in New York, especially on fields with a limited soybean history. On fields where soybeans have been grown for more than 20 years, however, inoculum may not be necessary. Likewise, the use of an insecticide/fungicide seed treatment is not necessary but can help stand establishment, especially on early-planted soybeans. Soybeans, however, can fill in the gaps very well and perfect stands are not required for maximum soybean yields.
Managing the Crop
Use soil test results to determine both lime and fertilizer requirements (see Table 6.3.1). Soybeans do not require supplemental nitrogen fertilizer if optimally fertilized for phosphorus and sulfur and at optimal pH because soybeans can fix nitrogen through a symbiotic relationship with Bradyrhizobium bacteria. If used, band-placed fertilizer should be at least 2 inches to the side and 2 inches below the seed. Do not place any fertilizer in contact with the soybean seed. Diammonium phosphates or urea should not be used in the fertilizer band. Do not use more than 40 pounds of potassium in the fertilizer band at planting either.
Soybean fields are not the best choice for maximizing the value of manure nutrients if there are other crops like corn or grass hay fields that can benefit from the nitrogen. However, manure addition does have benefits beyond nitrogen supply and there are legitimate reasons to apply manure to fields that will be rotated to soybeans. For example, manure supplies nutrients other than nitrogen and where low soil test levels suggest a potential response to phosphorus, potassium, sulfur, or micronutrients, manure is a reasonable choice to supply these nutrients. Furthermore, since harvest of soybeans will remove nutrients such as phosphorus, potassium, and sulfur from the field, manure can reduce the need for fertilizer purchases.
Nitrogen fixation in legumes is reduced but not eliminated when manure is applied. As nitrogen rates exceed 50% of expected crop removal, more nitrogen is vulnerable to loss. Therefore it is recommended to limit applications of manure to rates that supply no more than 50% of the expected N removal based on manure nitrogen credit estimates. Nitrogen credits from manure should not exceed 50, 75, or 100 lbs of nitrogen removed per acre for estimated soybean yields of 30, 45, or 60 bu/acre. Limiting rates to these levels will also reduce the risk of lodging.
It is recommended to inject or incorporate the manure shortly after application when there are concerns about the risk of manure runoff or nitrogen volatilization. Rates should be adjusted based on the application method. See nmsp.cals.cornell.edu/publications/factsheets/factsheet4.pdf.
In corn grain-soybean rotations, fall application of manure into corn stubble can help break down the corn crop residue but fall manure application in row crop systems (especially without a cover crop) can result in increased nitrate leaching over the winter and early spring. Similarly, surface applications of liquid manure without incorporation are likely to result in nitrogen volatilization losses.
Another consideration with the use of manure as a fertilizer source is the potential to over-apply phosphorus. Soybean removal of phosphorus varies, but is typically around 0.8 lbs P2O5 per bushel. Manure applications that add phosphorus beyond crop removal to soils that test very high in phosphorus is not encouraged as such applications will further increase soil test P levels over time, increasing environmental risk with no agronomic gain.
Disease pressure needs to be considered as well when using manure on soybeans; fields with a history of diseases like Pythium and white mold are at higher risk of increased disease pressure when manure is applied. Therefore, manure application to such fields is not recommended either.
The most frequent cause of disappointing soybean yields is drought in the month of August, when seeds within the pods are enlarging and filling. The crop is actually fairly drought resistant before that, but moisture stress in August causes pods to shed and seeds to abort. For this reason, it is recommended that soybeans not be grown on sandy or gravelly soils.
Lodging before harvest is commonly encountered, but modern combines are designed to handle lodged soybeans. At maturity, leaves have been shed and only stems and pods must be passed through the machine. Soybeans store safely at 14 percent moisture. They crack or break if handled roughly, especially when they are very dry, as they usually are when in storage during the coldest months.