Project Description:
To increase the sustainability of Illinois soy and improve soybean growers’ profitability, this work will tailor the 4Rs of P for regional soybean production. In addition to the 4Rs of P for central-north and southern IL production conditions, N-free P options will be evaluated in two-year field trials to quantify yield, P use efficiency, economics, and water quality benefits. Results will be used to position Illinois soy at the forefront of reducing N losses while identifying region-specific agronomic P practices to support profitable soybean production.

Summary/Project Scope:
I. Proposal Description and Background

This project will provide fundamental information on the sustainability of Illinois soy by addressing economically effective usage of phosphorus (P) with co-benefits to water quality. The work will evaluate best management practices for P of timing, placement, source and rate to ensure efficient and thus economic P usage for soybean growers. Updating the 4Rs of P management – specifically for soybean, and specific to the distinct soil-climate conditions of central-north vs southern Illinois– is expected to identify options to increase the profitable use of P inputs while reducing off-field nitrogen (N) and P losses. This work will therefore provide updated and evidence-based recommendations on the 4Rs P management options for soybean growers in distinct regions of Illinois. The source, rate, timing and placement of P will be evaluated. Field-based evaluation of timing and placement of P to increase soybean use efficiency of these inputs, and adding P source options to growers’ management toolbox, stands to improve agronomic, economic and environmental outcomes of Illinois soybean production. By quantifying these benefits to water quality, this project will position our state’s soybean growers at the forefront of environmental stewardship as active contributors to water quality improvement. This proposal therefore addresses “Farmer Profitability – Optimize farmer profitability through strategic farm management, yield and sustainable production practices.” Specifically, this proposal will “Provide actionable information through research, events and online resources to support soybean yield and profitability in Illinois” and “Encourage adoption of sustainable and profitable soybean production and nutrient management practices by Illinois soybean producers.”

(i) Problem

Increasing P efficiency for soybean

Managing P nutrition for soybean is critical since this leguminous species is self-sufficient in meeting the majority of its N needs. Additionally, the high nutrient density of soybean grain means that soybean translocates a high proportion of its total P taken into the grain (over 80%). Soybean has a higher P harvest index than corn1, 2, which is why replenishing P removed by harvest of soybean is an important practice for sustaining soybean yields. As soybean yields have increased, total P uptake has concurrently increased. Thus, in contrast to some other grains in which yield gains dilute the same amount of P taken up, higher yielding soybeans necessitates a corresponding increase in how much P is added and how efficiently it is used to support yields.

Currently, the 4Rs of P for are well recognized but relatively under-evaluated for soybean-specific management. For example, a common practice in Illinois is the application of P at the beginning of the corn phase of a corn-soybean rotation, but this practice may limit soybean yields. Targeting P inputs to the soybean phase of the rotation can increase soybean yields3, and specifically in-season P application4. Even in conditions where in-season P fertilization may not necessarily be cost-effective for soybean, there is still reason to consider a P-only source out-of-season: avoiding loss of N added via ammonium phosphate (MAP or DAP) is limited. Precision management of P is enabled by P-only sources that do not require inputs of N to entail P inputs, or vice versa.

Co-benefits for water quality

The common practice in Illinois of applying P at the beginning of the corn phase of a corn-soybean rotation may not only limit soybean yields, it may also encumber economic costs to growers in the form of nutrients exiting the field. These off-farm losses of N and P also have implications for water quality. Illinois is the largest state contributor of P to the Mississippi River watershed (13% of total), and a close second for N losses. Following a federal EPA mandate on N and P loss mitigation for states in the Mississippi River watershed, in 2015 the Nutrient Loss Reduction Strategy (NLRS) was developed by Illinois stakeholders to decrease by 45% the total N from Illinois to the Mississippi River.

A persistent challenge to reducing N losses is hidden in its co-addition in the dominant P sources in Midwestern agriculture: ammonium phosphates. Small changes in P management mean that Illinois soy is uniquely positioned to contribute to N loss reductions without compromising profitability. By decoupling inputs of N and P, mononutrient P sources such as triple superphosphate (TSP; 0-46-0) stand to deliver on soybean P needs without the need to supply additional N, which does not generally increase soybean yields but does pose a water quality risk. In years with wet and warm winters and/or springs, fall-applied MAP (11-52-0) or DAP (18-46-0) undergo substantial nitrification ammonium and subsequent losses by nitrate leaching and/or denitrification. On average, two-thirds of the N from fall-applied MAP or DAP may be unrecovered by crops and lost off-field5. Though agronomic this may correspond to a relatively low amount of N and thus expense, these risked N losses can be substantial for water quality. Averting this N loss by changing timing and/or source of P in soybean production could be a significant contribution to the Illinois NLRS.

(ii) Evidence for project need

This work will provide evidence-based recommendations on P management options for soybean growers to increase efficiency of P inputs. The Illinois Agronomy Handbook employs potentially outdated (pre-2000) soil test P recommendations that have not been specifically calibrated for soybean. Evaluating the timing, placement, source, and rate of P will provide needed updates to the Handbook, for the major soil types of the unglaciated south and glaciated north-central regions of Illinois. These P recommendations can further vary by timing and placement, including the potential of banding to increase soybean P use efficiency. Field-based evaluation of timing and placement of P to increase soybean P use efficiency of these inputs, and adding P source options to growers’ management toolbox, stands to improve agronomic, economic and environmental outcomes of Illinois soy. While these practices are broadly recognized, they are under-evaluated for their potential to increase P use efficiency for soybean specifically.

Non-ammoniacal P sources – superphosphates – were once the dominant P source in Illinois9-12 but now constitute 11% of P fertilizer sales in the state8. For much of the 20th century, single superphosphate (SSP; 0-20-0) or TSP (0-46-0) were the dominant P source in Illinois. University of Illinois field trials through the 1990s evaluated superphosphate as the P source14, and the Illinois Agronomy Handbook listed SSP as the P source of choice through the 1970s15 and TSP as the P source of choice in the 1980 edition16. With recent re-introduction of TSP into the North American market at competitive pricing with ammonium phosphates (e.g., The OCP Group) and increased interest by growers, there is growing interest in and sources of N-free P in Illinois.

In addition to increasing agronomic use efficiency of P inputs, updating the R4s of P management specifically for Illinois soybean –including the right source – stands to deliver water quality benefits. The change in P source, but not necessarily P rate, and the similar pricing of MAP and DAP as TSP per unit P means that adding N-free P options to producers’ nutrient management toolbox stands to maintain producer profitability while improving environmental outcomes of agriculture in Illinois.

Field-based data on how crop production practices can mitigate nutrient losses is useful to demonstrate how the agricultural sector and growers are key leaders in environmentally responsible nutrient management. In particular, this can be achieved by yield-scaled assessments: evaluating nutrient losses with respect to crop productivity6. The yield-scaled metric agronomically contextualizes the impacts of nutrient management practices on nutrient loss mitigation, and can form the basis for the N or P footprint of a given crop – a key metric of sustainability. However, only one study in Illinois has evaluated yield-scaled N leaching for corn at one location7, and no studies to have evaluated yield-scaled N loss mitigation for soybean, despite the strong potential to achieve appreciable N loss reductions in by managing P sources in soybean production.

The overall objective of this work is to enhance efficient usage of P for more profitable soybean production while increasing the water quality benefits, and thus sustainability, of Illinois soy. Specific objectives are:

(1) Quantify yield and profitability of managing P for soybean by placement, timing, and P source (4Rs), including non-N containing sources of P.

(2) Evaluate region-specific considerations for soybean P 4Rs: southern and north-central Illinois.

(3) Determine the extent to which soybean-specific P 4Rs such as in-season application and/or use of P-only source can yield benefits for Illinois water quality.

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.

Land grant institutions, growers, and commodity boards can and should work together to get ahead of potential regulations by providing evidence-based evaluation of environmental impacts and best practices in the agricultural sector. By identifying practices that co-deliver yield, profitability and water quality benefits, this work will position ISA and our state’s soybean growers at the front of environmental stewardship. This project will provide specific recommendations on which P management practices enable synergistic yield, profitability and environmental outcomes. Using information on yield and economics of P sources, timing and placement, soybean growers can make informed decision making on cost-effective P management that includes options for reducing N losses. Finally, increased efficiency of P usage by evaluating timing, placement, source and rate for soybean stands to improve profitability of growers, with the added benefit of keeping more P for the crop and out of our state’s water bodies. These outcomes mean that the investment of ISA dollars will support grower profitability while simultaneously optimizing strategies and providing evidence for the sustainability of Illinois soy. In particular, the hypothesized benefit of using N-free P sources mean that Illinois soy can be a leader in reducing N losses in the state and the Mississippi River Basin.