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Fertilizing
Soybean does best in fertile soil and makes good use of carryover fertilizer. Response to application of commercial fertilizer has been inconsistent under North Dakota growing conditions. If a soil test of the field or response in other crops indicates distinctly low phosphate availability, a band application by planter attachment of 10 to 30 pounds of phosphate per acre may be beneficial.
Fields that have no prior soybean history may benefit from additional nitrogen fertilizer if the soil test shows less than 60 pounds per acre available in the top 24 inches at planting time. High nitrogen fertility circumvents the benefits of rhizobium bacteria as the bacteria will not convert atmospheric nitrogen when soil nitrogen is readily available to the plant.
Soybean without nodules or with ineffective nodules will respond to nitrogen like any other crop. Since legumes have the ability to fix nitrogen, inoculating soybean seed just before planting is important, especially on fields that have not recently been planted to soybean. Crop response to phosphorus and potassium are not always noticeable in the year of application.
Approximately 60 percent of the phosphorus and 50 percent of the potassium taken up by soybean plants is removed from the field when the seed is harvested. One bushel of soybean contains about three-fourths of a pound of P2O5 and over a pound of K2O per bushel.
Starter fertilizer is best placed in a band 2 inches to the side and 2 inches below the seed. "Pop-up" fertilizer, a small amount of fertilizer placed in direct contact with the seed, should not be used on soybean because stand loss from fertilizer toxicity can result.
Micronutrient deficiency in soybean can be a problem on high pH soils. Iron chlorosis generally occur on high-lime soils and is the most commonly reported trace element deficiency in North Dakota. The typical iron deficiency (chlorosis) symptom is yellow leaves with green veins.
Suggested foliar treatments to correct iron deficiency are (a) 20 pounds of ferrous sulfate in 100 gallons of water applied at 10 to 20 gallons per acre, or (b) 0.10 to 0.15 lbs/A of iron as iron chelate applied in water carrier at the second trifoliolate leaf stage. These treatments will usually eliminate deficiency symptoms but may not result in a profitable yield increase. NDSU research showed that application at the fourth trifoliolate stage never resulted in a soybean yield increase. However, research in southern Minnesota showed economical yield increases from foliar application of iron. Selecting varieties tolerant of high-lime soils can help counteract iron chlorosis. Some varieties are quite susceptible to iron chlorosis. Soybean has not responded to zinc fertilizer in eastern North Dakota and western Minnesota sites where the soil zinc level was above 1.0 ppm.
Soybean response to fertilizer has varied considerably. Experience on your farm is your best guide. Leave an unfertilized check strip for comparison. The use of foliar fertilization is not a substitute for a good soil fertility program of soil applied fertilizers and micronutrients. Consult NDSU Extension circular SF-719, "Fertilizing Soybean," at http://www.ext.nodak.edu/extpubs/plantsci/soilfert/sf1164w.htm for additional information.
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