Experimental design: A full factorial randomized using a complete block design will be implemented at two sites for two field seasons. The soybean phase of a soybean-corn rotation will be evaluated in each of both years (i.e., plots will be altered to avoid continuous soybean). The second year is proposed in order to provide an additional site year to account for year-specific weather effects, yielding a total of four site-years. Treatment factors of P source, timing, placement, and rate will be tested to update 4R best practices for soybean P management.
Source (3x): MAP (11-52-0), DAP (18-46-0), TSP (0-46-0)
Placement and timing (3x): fall broadcast vs spring broadcast vs spring banding. Application in the spring relative to the fall is hypothesized to enhance influence P use efficiency (PUE) as well as N loss risk in the MAP control, and banding is hypothesized to increase PUE17 (Note: fall banding not realistic since banding is performed immediately before or concurrently with sowing.)
Rate (3x): Two P application rates, plus a 0 P fertilizer check or control, will be determined based on background soil P test values, to be 75%, 100%, 125% of soil test-based recommendations.
All treatments will be replicated in quadruplicate to enable statistical evaluation and on the same total P rate, informed by pre-plant soil P tests. The number of experimental unit plots are 3 × 3 × 3 × 4= 48 plots, plus 4 replicate plots of the 0 P control, for 52 plots per site. Each experimental unit plot will be 72 ft long × 38 ft wide, accommodating 30 rows 15” spaced soybean in width to ensure accuracy yield measurements that may be compromised with small plot (e.g., <8 soybean row width) research18.
Field trial locations: Extensive baseline soil testing will be performed prior to field trial implementation to confirm already identified P-responsive site and to inform site-specific P application rates based on Illinois recommendations19. Preliminary testing by PI Margenot has corroborated two P-responsive sites in central and southern Illinois sites with 16 mg kg-1 and 11 mg kg-1 Mehlich III-extractable P, respectively, both of which are below the threshold of 25 mg kg-1 considered adequate for soybean yield in the Illinois Agronomy Handbook20, 21. Baseline soil P tests at each site and yield targets will be considered for site-specific P rates. Due to the absence of recent glaciation events (Quaternary) 22, 23 soils in southern Illinois are generally more weathered (Alfisols) relative to soils in central and northern Illinois (Mollisols) 22, 24-26. The distinct soil types between unglaciated and glaciated merit a regional approach to soil P management.
1. Central IL: University of Illinois Crop Science Research and Education Center (“South Farms”). The university’s on-campus research farm in Urbana, IL is situated on the Drummer soil series (fine-silty, mixed, superactive, mesic Typic Endoaquolls), with an extent of 1.5 million acres. This soil type is representative of the organic matter-rich loess-derived soils of the glaciated region of central and northern Illinois, and with related soil series (e.g., Flanagan, Caitlin) has an immediate translatability to over 5.2 million acres. This site will be featured at the annual UI Agronomy Day in August.
2. Southern IL: University of Illinois Extension Ewing Demonstration Center. Located in Franklin Co. in southern Illinois, the Ewing Demonstration Center is situated in the Big Muddy watershed, a major P priority watershed for the state27. The soil type (Cisne series; fine, smectitic, mesic, Mollic Albaqualf) is representative of soils throughout the unglaciated region and has a mapped extent of over 0.5 million acres28. Considering related series (e.g., Cowden, Hoyleton, Wynoose, Huey), findings at Ewing Demonstration Center have translatability to over 1.6 million acres in southern Illinois. This site will be featured in the annual Ewing Field Day in July.
General management: Tillage by chisel plow to 6-8” depth will be performed in the fall after corn harvest, with secondary tillage by a cultivator before soybean planting in the spring. Soybean will be planted in April-June, as spring rains permit, in 15” rows with a target population of 150,000 plants ac-1. Baseline soil test K and micronutrients will be used to evaluate background fertilization of K and other potential non-P constraints, in order to ensure that differences among treatments reflect the tested 4Rs of P. Weeds will be controlled utilizing pre-emergence and post-emergence herbicide as needed.
Sampling & analyses: Soils will be sampled at preplant, to test soil N and P response to fall-application, and at harvest, to evaluate residual soil test N and P. Soybean aboveground biomass and grain yield will be collected to develop a mass balance, calculate PUE, and calculate yield and thus profitability outcomes. Final plant biomass will be estimated at R8 before leaf drop, by destructively harvest two 1 m2 subplots in the non-grain harvested rows. Pods will be removed as sub-samples, and all biomass will dried at 65 °C until constant moisture. The number of pods per plant, seeds per pod, and seed size will be recorded to understand yield components of soybean. Pod samples will be shelled and grain weighed. At harvest, subsamples (2 lb) from the small plot combine will be used for grain composition analyses. Total N, P, S and other mineral elements (K, Mg, Ca, Na, Fe, Mn, Cu, Zn, B) concentration in non-grain and grain biomass will be determined. Above ground biomass data will be used to normalize for potential differences in stand count and above ground biomass growth, to obtain a total amount of P as well as other nutrients taken by the crop on a per acre basis. This will serve as the basis to calculate agronomic PUE as a function of P source, timing, placement and rate, enabling a comprehensive update 4Rs of P for soybean in Illinois.
To monitor N loss reductions under the 4Rs of P management for soybean, resin lysimeters will be installed beneath the plow layer. This will enable quantification of hypothesized benefits of the 4Rs of P, in particular placement, timing and source, for soybean production to contribute to achieving state N loss reductions. Leaching of N is the major pathway of N loss in Illinois29-31, and can be quantified in our state’s soil types by resin lysimeters4, 43, 44. Resin lysimeters will be installed in April prior to P application and soybean planting, and harvested in November; at that time, another set of lysimeters will be installed and harvested in April prior to installation of spring lysimeters. In each plot, three resin lysimeters will be positioned between the center crop rows at 12” depth. This approach has been successfully used to measure soil N leaching in central Illinois cropping systems7, 32, 33. Resin lysimeters will be custom-made as a 5.1 cm diameter polyvinylchloride pipe containing a 1.2” layer of ion exchange resin with dual anion and cation affinity to absorb nitrate and ammonium dissolved in soil solution34, 35. The ion exchange resin is held in place between a layer of washed sand and a permeable nylon membrane (<150 µm)35. At lysimeter harvest in April and November, the resin will be removed, and N extracted (2 M KCl) for colorimetric quantification as nitrate-N and ammonium-N at Dr. Margenot’s lab at the University of Illinois Urbana-Champaign.
Yield for each experimental plot will be recorded by harvesting the middle 8 (of 30) soybean rows. Operational and input costs of each of the four components of P source, timing, placement, and rate will be used for an economic and ultimately profitability assessment of the 4Rs for P management.
Specific field trial outcomes that will inform Deliverables will be determined as follows:
Yield and grain analysis will be performed to determine productivity outcomes and to calculate P use efficiency (PUE) of the 4Rs, respectively. The P harvest index (HI), the percentage of total nutrient accumulation partitioned to grain (as seeds), will be quantified1. Pod number, seeds per pod, and seed weight will be evaluated in order to explain yield components in response to P source, timing, placement, and rate.
To help explain crop growth and yield response, soils (6” depth) will be measured for P availability by Bray and Mehlich III extraction in conjunction with above-ground biomass harvest at three growth stages21, 37.
To test the hypothesized water quality advantage of TSP over MAP or DAP as a non-N source, N loss risk will be measured as inorganic N leachate monitored by resin lysimeters38 as well as soil test N. Yield-scaled N loss mitigation for soybean production for each of the 4Rs, and their interactions, will also be calculated.
An economic evaluation will be conducted using data on P uptake, and PUE results of the field trial will be used to calculate per acre costs in order to assess cost efficiency as well cost gains or losses of TSP relative to MAP.
Statistical analyses: A general linear model (GLM) in the form of two-way analysis of variance (ANOVA) will be used to test for significant main and interactive effects of P source (x4) and P rate (x4), each tested as fixed effects. Replicates, one per each of the four blocks, will be treated as a random effect in the model. For all response variables, assumptions of normality of residuals and homogeneity of variances will be tested, and data will be transformed as appropriate to meet assumptions of ANOVA. In the absence of significant (a = 0.05) interactions. Site effects will be tested as an additional factor in a three-way ANOVA, with assumptions of the GLM first evaluated as described above. Post-hoc means separation of soil and crop responses will use Tukey’s HSD.
VII. Project Deliverables
(1) Yield and profitability outcomes of placement, timing and source of P for soybean for the glaciated and unglaciated regions of Illinois
(2) Updated recommendations on P management specific to soybean, which will be integrated into forthcoming update of the Illinois Agronomy Handbook by PI Margenot
(3) Gauging water quality co-benefits of P management practices that support soybean yield increases
(4) Communication of findings by updating of the Illinois Agronomy Handbook for soybean production recommendations, ISA-sponsored webinars and in-person talks by PI Margenot, and extension publications via the University of Illinois Extension bulletins.