Research Highlights

Delaware Nutrient Survey Uncovers Fertility Trends

By Laura Temple

Do available soil nutrients and applied fertilizer provide everything soybean plants need to maximize yields? 

Soybean varieties, fertilizer delivery and micronutrient products continue to develop and improve. However, soybean nutrient recommendations rely on data that is decades old, according to Jarrod Miller, Assistant Professor and Extension Specialist in Agronomy at the University of Delaware.

“As soybean production inputs and systems improve, do nutrient recommendations need to be updated?” Miller asks. “We have started doing soybean tissue surveys for nutrient concentration to identify what research is needed to answer that question.”

Miller says soybean fertility research is difficult because of the wide number of variables that impact nutrient update, including weather, soil type, soybean variety and production practices. “Any study may uncover the relationship with one nutrient under that year’s environmental conditions,” he says. “A survey of soybeans across different soils and seed varieties can reveal a fuller picture.”

The Delaware Soybean Board funded a survey of nutrient concentration in 2019 and 2020, managed by Miller and Amy Shober, Professor and Extension Specialist in Nutrient Management at the University of Delaware. Their work will uncover opportunities to improve nutrient guidelines for farmers, allowing them to fine-tune soybean nutrient delivery to maximize yield and return on investment.

“While farmers usually fertilize adequately, we expect these surveys to point to patterns for future research,” Miller explains. “For example, we’ve learned that farmers don’t need to split-apply potassium. We plan to investigate potassium uptake in irrigated compared to non-irrigated soybean fields in 2021.”

In the short-term, these survey results let farmers know what nutrients to be paying attention to in their own tissue sampling.

Tissue sampling for nutrient concentration

At the R1 or R2 growth stage, Miller’s team collected upper canopy leaf tissue samples, whole plant samples and soil samples. The University of Delaware Soil Testing Lab analyzed samples for a variety of nutrients, including nitrogen, phosphorus, potassium, calcium, magnesium, boron, manganese, sulfur and zinc. At the end of the season, they also collected yield data.

In 2019, the team sampled 30 fields of full-season soybeans throughout the state in July. However, the wide range of production practices represented in this survey, which included irrigated and dryland fields, made it challenging to correlate nutrient concentrations and yield.

In 2020, the survey relied on University of Delaware soybean variety trials for tissue collection. These trials ensured greater consistency in production practices and allowed double-crop soybeans to be included.

Fertility trends

Though the full analysis of 2020 leaf tissues and plants continues, Miller notes that values for most nutrients fall within the range of current recommendations.

“In this region, the nutrients most likely to be lacking include zinc, manganese and boron,” he says. “The 2019 results showed that calcium was most often below the suggested sufficiency range.”

Over time, he expects to identify nutrient availability and uptake patterns related to weather and soil factors, which could inform additional research and nutrient recommendations. For example, in Delaware’s sandy soils with low cation exchange capacity (CEC) that don’t hold nutrients well, pH can change quickly. 

“We know over-liming can impact soil pH and some nutrient uptake,” Miller says. “Better recommendations can save farmers unnecessary lime application costs and improve other nutrient uptake.” 

He wants to equip farmers to take advantage of opportunities that will help them maintain yields. At the same time, updated guidelines can minimize application of unnecessary nutrients.

“Fine-tuning of nutrient application is now possible with foliar applications,” he adds. “With tight margins, we want farmers to know what they need and what they don’t need to achieve soybean yield goals.”

Published: Jan 18, 2021