Eighty percent of Illinois agricultural production is under a two-year corn-soybean rotation, with 10.8 million soybean acres planted in 2022 (USDA-NASS, 2022). While 98% of corn acres receive N fertilization and 87% of acreage phosphorus, only 22 and 48% of soybean acres were applied with these same nutrients, respectively, in 2018 (USDA-ERS, 2019). However, the use of in-season foliar applications of nutrients, in particular micronutrients (B, Cu, Mn, Mo, and Zn), has started to increase. Decisions on what nutrients to spray and at what growth stages are often determined through use of a tissue test. The uppermost-matured trifoliate is usually selected for this test as nutrient deficiencies, sufficiencies, or excess accumulation of an actively-growing plant are likely to be identified in the new growth. However, due to soybean’s indeterminate growth, the lower nodes contain fully mature trifoliates that supply photosynthate and remobilized nutrients to developing pods at the same time new nodes and leaves are still emerging at the top of the plant. There is a knowledge gap in the understanding of when certain nodes initiate nutrient remobilization to the grain, and if the standard tissue test accurately assesses nutrient deficiencies at the whole-plant level. This research proposes to identify when during a soybean plant’s life cycle the leaves reach maximum nutrient accumulation and if different nutrients remobilize at the same rates. Building upon the first year of this work, the second year will use stable isotopes supplied to the field to assess the difference in uptake of fertilized nutrients vs. nutrients scavenged from the soil. This will enhance understanding if soybean uptake of fertilizer occurs in the early season, late season, or full-season and therefore identify when optimal timings may be to supply fertilizer so as to maximize its uptake efficiency and minimize off-target movement to the environment.

A replicated small-plot field study with six replications will be conducted with two levels of fertility, none (deficient) or a full rate of nitrogen, phosphorus, potassium, sulfur, zinc, and boron (per acre:19 lbs N, 60 lbs P2O5, 60 lbs K2O, 20 lbs S, 1.5 lbs Zn, and 0.5 lbs B as monoammonium phosphate (MAP; 11-52-0), muriate of potash (MOP/KCl; 0-0-60), ammonium sulfate (AMS; 21-0-0-24S), Ultra-Che Zinc 9% EDTA (7-0-0-9Zn), and Liquid Boron 10% (0-0-0-10B). To distinguish soil vs fertilizer derived nutrients, we will tri-label three nutrients with stable isotopes to allow us to distinguish the source of the nutrients at specified sampling stages of R1 and R8. This will provide novel insight on the source of nutrients (fertilizer or soil) in soybean during vegetative vs. reproductive developmental stages. The KCl and AMS sources will be labeled with stable isotopes of 15N, 41K, and 34S at sufficient levels to track fertilizer uptake into the plant for the nutrients of N, K, and S. Standard plant tissue sampling will begin at the V6 growth stage (5th node) and continue through the R7 growth stage, with tissue samplings being taken by removal of leaf tissue only (leaflets plus petiolules), leaving the main petiole attached to the plant. Whole leaf samples at paired nodes will be collected, dried, weighed, ground, and assessed for total nutrient concentration and accumulation, as well as isotopic nutrient concentrations and accumulations, at different growth stages throughout the season. At physiological maturity (R8), yield will be assessed at the nodal and whole plant levels. Grain will be removed from pods, weighed, and ground for final nutrient analysis to estimate percentage of nutrients in the grain that were remobilized from respective trifoliates, as well as determine percent of nutrients in the grain that were fertilizer vs. soil derived. Final grain yield and grain quality (protein and oil) at each node pair will also be determined.

Results of this research can be used to better identify when leaf tissue testing should be performed to identify the need for managing foliar nutrient applications to soybean production fields for greatest application efficiency, grain yield, and grain quality. Pairing unfertilized to fertilized plots can best assess when nutrient deficiencies occur. The addition of stable isotopes adds a new component to the work to better understand soybean uptake of fertilizer vs soil-derived nutrients on a mid- and full-season basis. Full analysis of the data across growth stages and by node pairs can help identify the optimum plant growth stage and location on the plant to collect a trifoliate sample that is most reflective of final grain yields, and how these values differ over time and location on the plant. Furthermore, the use of stable isotopes will confirm how quickly soybean utilizes fertilizer supplied nutrition and identify at which time in the growing season fertilizer uptake efficiency is at its greatest level. While it is important to know how much fertilizer vs soil nutrient uptake occurs for soybean over the entire season, this work will identify if soybean preferentially uptakes fertilizer during the vegetative or reproductive stages, or in equal proportions to soil nutrients throughout the entire growing season.

Knowing when soybean preferentially takes up fertilizer nutrients can elucidate the optimal timing with which to apply fertilizer so as to maximize its uptake by the plant and mitigate possible nutrient loss from the field.