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United States Department of Agriculture

Agricultural Research Service

Research Project: Optimizing Soybean Nutrition in the Mid South

Location: Crop Genetics Research Unit

2013 Annual Report


1a.Objectives (from AD-416):
Identify optimal potash and phosphate requirements for soybean grown in rotation with corn. Determine the influence of Bradyrhizobia inoculation on soybean yield and nutritional status in multiple production systems. Define best fertilizer management strategies for wheat/soybean double crop systems.


1b.Approach (from AD-416):
Plant soybean corn on lighter textured soils commonly used for corn or cotton production. Apply multiple rates of potash or phosphate to both corn and soybean with other essential nutrients applied according to soil test results. Maintain plot integrity throughout study. Conduct tests at multiple locations when possible. Inoculate soybean with multiple available inoculants and plant soybean in rotation with cotton, corn and soybean to determine when seed inoculants are needed to optimize yield. Grow Maturity Group IV and V soybean following wheat. Investigate multiple fertilization strategies and timing for optimal input.


3.Progress Report:

Field trials were conducted at numerous locations within the state of Mississippi, both on research stations and in producer fields, to evaluate soybean response to potash (K) and phosphorus (P) fertilization. Attempts were made to identify sites that have a range of soil test P and K values (i.e. low to high) prior to crop establishment. Each experiment was arranged as a randomized complete block design with no less than 6 replications of each P or K rate plus an unfertilized control. Plant tissue was collected at the flowering (R1 to R2) stage of soybean growth and development from one of the top three nodes with fully expanded leaves from no less than 20 plants within each plot, followed by analysis for elemental concentration. Soybean yield was harvested with a small plot combine at maturity and reported at a standard moisture content of 13.0%. To determine if sites were responsive to potash fertilization, single degree of freedom contrasts comparing the mean yield of soybean receiving any rate of K fertilization against the mean yield of the untreated control were used. Each site year was analyzed independently. Following identification of yield responsiveness, soybean yield data will be transformed into a relative yield. Relative soybean yield and tissue concentrations will be analyzed to identify a soil test critical value for both the Lancaster and Mehlich-3 soil test extractants, using a multiple model approach to determine best fits when enough data points are collected.

Activities initiated to fulfill this research objective during 2012 included field trial testing at six locations within the Mississippi Delta region. Individual trials were placed in Bolivar, Humphreys, and Washington Counties with two trials occurring at the Delta Research and Extension Center at Stoneville, MS, and the remainder in production fields. Of the six harvested trials, only two positive responses to potash fertilization were observed. At the Bolivar County responsive site, soybean yield increased with increasing potash rate. For soybeans cultivated at the Bolivar Co. site approximately 120 lbs K2O were needed to maximize soybean yield. Similarly at the Humphrey County site, soybean yield also increased positively with increasing potash rates (p-value = 0.0654). Soybean yield at the Humphrey’s site was lowest (60.58 bushels/acre) with No Potash application and greatest (70.45 bushels/acre) when 160 pounds potash/acre was applied. In general, soybean yield increase from potash application at the responsive sites ranged from 7-10 bushel greater when potash was applied compared to the untreated control plots. Soybean tissue K concentration from trifoliate leaves collected at the full flowering (R2) stage of growth and development followed a similar trend as the soybean yield data. Tissue K concentration was only positively influenced by potash fertilization on the sites where yield responses to potash were observed. At the Bolivar County site, tissue K concentration increased by approximately 0.3 percent when potash fertilizer was applied compared to untreated plots. Similar to soybean yield, soybean tissue K concentration increased with increasing potash application rates. Soybean tissue K at R2 at the Humphrey County site also increased with increasing K rate. Overall soybean tissue K concentrations were greater at the Humphrey County site than at the Bolivar County Site. Large gains in overall tissue concentration were not observed, but were significant. It is important to note that neither responsive site showed widespread visual potash deficiency symptomology. The lack of visual symptoms is important in interpreting the tissue concentration data. At the two responsive sites, soil test K was not extremely low and would perhaps fall at or close to the critical level for which a potash recommendation may be made. These 2012 data gave a good start to revising soybean fertilization recommendations for potash in Mississippi. These preliminary data, although limited, suggest that soybean yield increases may not be observed at soil test K levels above 150 part per million (ppm).

Research was established at the Delta Research and Extension Center near Stoneville, MS, during 2012 on multiple soil textures which included a Commerce very fine sandy loam, and two sites on a Bosket very fine sandy loam. Prior to seeding, composite soil samples (0-6-inch depth) from each replicate were taken to characterize soil chemical properties. Soil samples were oven-dried, crushed to pass through a 2-mm sieve, and analyzed for nutrient availability (Mehlich-3), water pH (1:2), and organic matter. Before seeding, plots were flagged to establish plot boundaries. Phosphorus (40 pounds/acre) was applied as triple super phosphate, and potassium (100 pounds/acre) was applied as muriate of potash as a blanket application to each experimental area. Experimental plots contained four, 35 foot long rows of soybean with a row spacing of 40 inches. The trial included five inoculant source treatments (Excaliber-SA, Graph-EX-SA, Optimize , Primo-CL, Vault-HP). Each inoculant was applied to Pioneer ‘94B73’ seed previously treated with Trilex 6000, except for the untreated control which did not receive inoculation or a seed treated fungicide. Each seed treatment was applied at the manufacture’s specified rate within 5 days of seeding.

Each experiment was a completely randomized design of 5 inoculants plus 2 standards (untreated and Trilex 6000) and included four replications. Each trial defined by the previous crop cultivated was analyzed separately. Mean separations for plant population and grain yields were performed by Fishers Protected Least Significant Difference method at a significance level of 0.10 when appropriate. All statistical analyses were performed with SAS version 9.2 (SAS Institute, Cary, NC).

The 2012 growing season produced many challenges with excessive drought, however lower than normal nighttime temperatures provided excellent environment for fruit formation. Soybean yields in all trials were acceptable, with no catastrophic failures. In two of three trials soybeans had not been in rotation for at least 5 years. Due to past history in Mississippi a greater yield increase was expected to have occurred. However, soybeans have been cultivated in areas adjacent to the two cotton sites, therefore some population of rhizobia could have been introduced into the fields with implements and general farming practices. Future research is needed to determine the benefit of inoculants with regard to increased plant population. In instances were numerical differences existed between inoculants and the untreated control, it appears it would be economically feasible to utilize an inoculant. Future research is also needed across multiple sites and years, and planting dates to determine the ultimate benefit of soybean inoculation in the Mississippi Delta.


Last Modified: 7/28/2014
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