Published in Southeast Farm Press 24(8): 31; March 19, 1997.
(© Copyright 1997 Intertec Publishing Corporation. Reprinted with permission.)
Higher Soybean Yields Still Possible
by M. Ray Tucker, Chief, Soil Testing Section, N.C. Dept. Agriculture
The average yield of soybeans in North Carolina over the past 40 years is 25 bushels per acre. A record low was set in 1943 with nine bushels per acre. The record high was set in 1994 with 31 bushels per acre.
Although a record 31-bushel yield is much better than nine, it still falls short of the genetic potential for soybean production. Indeed, under proper management soybeans are capable of producing more than 50 bushels per acre.
The long-term yield average of 25 bushels per acre raises questions about the cause of consistently low yields and what changes in current production practices may be needed to raise the state-wide average.
Growers producing high soybean yields across the nation have ranked weed control, row spacing and varieties as the most important considerations for optimizing soybean production.
Soil fertility was not considered a major yield-limiting factor.
However, in North Carolina, fertility is indeed of major importance. Sandy coastal plain soils have limited nutrient-holding capacity and are prone to leaching and fluctuations in soil pH.
Here, moisture stress certainly plays a major role, but other factors are also involved. Sandy coastal plain soils require a higher level of management for lime and fertilizer to sustain yields that exceed the statewide average.
Research has shown the single most yield-limiting factor across the Southeast is soil acidity (low pH). Soil test summary data from the NCDA Agronomic Division show the same evidence across North Carolina. These data indicate that growers are not addressing the lime needs of soybeans, as well as other crops like peanuts and cotton.
Optimum soybean yields cannot be achieved without adequate lime. Soybeans are more sensitive to high levels of soil acidity than most other field crops. The optimum pH for soybeans on sandy and clay-textured soils ranges from 5.8 to 6.2. Yields on mineral soils decline as soil pH decreases below pH 5.5. For organic soils, optimum soybean yields can be achieved at pH 5.0.
Research has shown a 15-bushel yield increase on acid soils with an application of one ton of lime per acre. At the current market price of $7.06 per bushel, an increase of 15 bushels per acre would generate additional income of $105.60 per acre.
Subtracting lime cost ($30 per ton), the profit from lime is $75 per acre. Under these conditions, lime will return $2.50 for each dollar invested.
Optimizing other production practices would no doubt enhance profits even further.
Another factor that limits soybean yields across North Carolina is low potassium. Potassium deficiency restricts grain development, which reduces the size and weight of beans, thus lowering yields.
Most extremely sandy-textured soils do not have the capability to hold potassium against leaching and show little or no accumulation from long-term potash application.
In such cases, annual applications are the best way to supply enough potassium to sustain good soybean production.
Split applications of potash during the early growth stage on very sandy-textured soils reduce leaching.
Soybeans require large amounts of potassium. A crop yielding 50 bushels per acre removes about 100 pounds of potassium. Most of the potassium utilized by soybeans is taken up within 60–100 days after emergence. Therefore, adequate potassium must be provided within the first 100 days of growth. At maturity, soybean seeds contain 60 percent of the total potassium within the plant.
Soybean production guidelines developed through research provide management information on a variety of factors that influence yields.
Variety selection, row spacing, insect and disease management, nematode control, lime and fertilizer needs, tillage practices, maturity groups, seeding rates and planting dates are all important.
Good soybean production requires optimizing all of these factors.
Soybeans respond well to recommended lime and fertilizer application. Such amendments increase profits for growers who carefully manage their soybean land. The resulting higher yields more than pay for production costs.
The first step toward higher soybean yields is to have your soil tested and apply the recommended amount of lime and fertilizer.