Soybean yield increases have not kept pace with those of corn in farmers’ fields in recent years. Consequently, growers are re-examining soybean management practices to look for possible areas of improvement. One such area is soybean fertilization. This is especially true if fertility levels have not been maintained in some fields due to higher crop yields and higher costs for nutrients. In addition, corn and soybean nutrient needs, though similar, are not identical. For these reasons, growers may benefit from a review of basic soybean fertility principles.
Managing soybean fertility should begin with regular soil testing. Soil testing is a valuable and inexpensive tool for ascertaining the nutrient and pH status of a particular field and guiding field input decisions. Soil tests should be taken every three to four years and sent to reputable labs for analysis; testing in shorter intervals is more costly and unlikely to provide further useful soil information. Instructions for collecting, handling and shipping samples are available from your state Extension service or soil test lab.
Standard soil tests generally measure pH, buffer pH (or lime test index, which is buffer pH x10), available phosphorus (P), exchangeable potassium (K), calcium (Ca), magnesium (Mg), cation exchange capacity (CEC) and organic matter (OM). Note that soil test values may be reported in either pounds per acre (lb/acre) or parts per million (ppm). To convert ppm into lb/acre, multiply ppm by 2.
Micronutrient soil tests will report amounts of the other nutrients (at a greater cost), but these quantities do not always correlate well with nutrient application recommendations in agronomic crops. Tissue testing in-season is recommended for identifying and confirming micronutrient deficiencies indicated by soil tests.
When diagnosing a suspected in-season nutrient deficiency, take soil and tissue samples from a normal area of the field in addition to the affected area. Comparing these reports will give additional insight into possible nutrient issues and increased confidence in diagnosis and confirmation of a nutrient deficiency.
Most of these nutrients are more available within than beyond this optimal pH range. Note also that some nutrients become less available as pH rises above 6.8, so “blanket” applications of lime above this point may be detrimental to the crops. Another reason for keeping pH in the desired range is to support beneficial microorganism activity in the soil, particularly Bradyrhizobium japonicum, which is responsible for fixing nitrogen in soybean root nodules.
To raise soil pH, the lime test index or buffer pH value is used to determine the amount of liming material necessary to reach a target pH. Low values of lime test index or buffer pH indicate that more liming materials are necessary to raise pH by a point than in a soil with higher lime test index.
Phosphorus (P) and potassium (K) are macronutrients, or nutrients that are needed in relatively large quantities compared to others. The content of both P and K in soybeans is high relative to that of corn and wheat (Table 1).
Table 1. Removal of P and K by various crops. From Vitosh, et al., 1995.
A 60 bu/acre soybean crop would remove about 48 lbs P2O5 and 84 lbs K2O from the soil in the grain. This is 33% less phosphorus but 55% more potassium than a 200 bu/acre corn crop removes in the grain.
Soil testing should be used to determine the levels of plant- available P and K. These nutrients must be in high enough quantities in the soil to prevent plant deficiencies, but once this critical level is reached, additional yield gains with higher levels of P and K are generally not realized. University studies put the critical level for P somewhere in the range of 30 to 40 pounds per acre (Table 2). The critical level of K is more variable, and depends on the CEC of the soil (Table 2).
Symptoms of K deficiency include yellowing of the margins of the leaves, starting with the older, lower leaves (Figure 2).
Figure 2. Potassium deficiency in soybeans. Note the yellowing pattern beginning at the leaf margins. Photo courtesy of Robert Mullen, Ohio State University.
Phosphorus deficiency symptoms include stunting, spindly stems and dark-green to bluish coloration of plant tissue.
Fertilizing on a crop-removal basis with P and K can be a good agronomic practice once the critical levels are reached. In research studies, banding P and K usually does not produce consistently better yield results than broadcast applications. Fertilizers should not be placed with the soybean seed due to risk of seedling injury and stand loss from salt-induced stresses associated with fertilizers.
Consistent field history of Mn deficiencies may guide the decision-making for a soil-applied Mn amendment, but foliar applications are also appropriate for treatment with a Mn product. Test soybean tissue (20 topmost trifoliate leaves with stems or petioles removed) to confirm Mn deficiency before application. Research reports that less than 21 ppm in the tissue indicates low Mn, and crop response may result from foliar Mn treatment from growth stage V6 to R1.
If Mn products are to be tankmixed with glyphosate, use an EDTA-chelated formulation of Mn and add the product to the tank last (after water, AMS and glyphosate). For more information on manganese requirements, see the Crop Insights on Manganese Fertility in Soybean Production.
Iron (Fe) - Iron deficiency chlorosis (IDC) symptoms are similar to those of manganese, with interveinal chlorosis and severe stunting (Figures 4 and 5), and are usually associated with native high pH soils. This condition is yield-limiting in many soybean fields in the northern and western U.S. Corn Belt including western Minnesota, the Dakotas, Nebraska, Kansas, Iowa and other states. Primary management practices include variety selection, iron chelate treatments and increased soybean seeding rates.
Variety Selection - Because soybean varieties vary widely for tolerance to IDC, variety selection is the first and most important step in managing this problem. Pioneer Hi-Bred has a significant research effort to screen its soybean varieties in areas with IDC (Figure 5). This effort allows Pioneer to rate existing varieties as well as identify new varieties that can help growers overcome yield losses to IDC. Pioneer® brand varieties are rated on a 1 to 9 scale where 1 indicates poor tolerance and 9 indicates excellent tolerance. If growers are planting into an area with a history of IDC, they should select varieties with an IDC score of 6, 7 or 8.
Figure 4. Interveinal chlorosis due to iron deficiency (left). Note the similar appearance to Mn deficiency (Figure 3) and stunted growth (right).
Iron Chelate Treatments – Historically, soil-applied iron chelates could not be justified due to cost, and the value of seed- and foliar-applied treatments was also questionable in Pioneer and University studies. However, a relatively new soil-applied Fe-EDDHA, Soygreen®, is less expensive at about $10 to $15/acre. Pioneer and University studies have shown a profitable response of Soygreen, but in Pioneer studies, Soygreen did not increase yields of tolerant varieties with a rating of “7”.
Increased Plant Population – Another management practice that has resulted in higher yields in IDC areas is increased plant population. Scientists suggest increasing plant density to 200,000 plants/acre in 30-inch rows in chlorotic field areas. Using GPS systems to map affected fields, and variable-rate seeding equipment to vary seeding rates in affected vs. non-affected field areas could increase the efficiency of this management strategy.
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