Agricultural practices have long been a subject of scrutiny due to their environmental impacts, particularly concerning nutrient and metal leaching into groundwater.
A recent study published in Agricultural Water Management (2025) by Vishawjot Sandhu and colleagues sheds light on how cover crops influence the leaching of phosphorus (P) and trace metals in soils fertilized with broiler litter.
This research is particularly relevant in the Southeastern United States, a major broiler-producing region, where the management of broiler litter—a byproduct of poultry farming—poses significant environmental challenges.
Introduction: The Broiler Litter Dilemma
Broiler litter, a mixture of chicken feces, urine, feed, feathers, and bedding materials, is commonly used as an inexpensive fertilizer in agricultural fields.
While it enriches the soil with essential nutrients like nitrogen, phosphorus, and potassium, repeated application can lead to the accumulation of contaminants, including trace metals like zinc (Zn), copper (Cu), and nickel (Ni).
These metals, often added to poultry feed to improve animal health, are excreted and end up in the litter. Over time, the soil’s capacity to retain these metals diminishes, increasing the risk of groundwater contamination.
The study focuses on the role of cover crops—plants grown primarily to protect and enrich the soil—in mitigating or exacerbating the leaching of phosphorus and trace metals.
Cover crops, such as cereal rye and crimson clover, are known to improve soil structure, reduce erosion, and enhance nutrient retention. However, their impact on the leaching of contaminants through soil macropores—large pores that facilitate rapid water movement—remains poorly understood.
The Study: Methodology and Experimental Design
The research was conducted at the E.V. Smith Research Center in Alabama, where soil cores were collected from fields planted with strip-tillage cotton. The study compared two management practices: fields with cover crops (CC) and fields without cover crops (NC).
The cover crop mixture, consisting of cereal rye and crimson clover, was planted in the late fall, while the main crop, cotton, was planted the following spring. After the cotton harvest, undisturbed soil cores were collected from both CC and NC fields.
Half of the cores were treated with broiler litter at a rate of 10 Mg/ha, while the other half served as controls with no litter application.
Rainfall simulations were conducted in the laboratory, and leachate samples were collected and analyzed for total phosphorus (TP), colloidal phosphorus (CP), dissolved reactive phosphorus (DRP), dissolved phosphorus (DP), and trace metals (Zn, Cu, and Ni).
Key Findings: Phosphorus Leaching
The study revealed that cover crops significantly influenced phosphorus leaching, particularly in soils treated with broiler litter. The mean TP concentration in leachate from CC cores fertilized with broiler litter was 4.07 mg/L, compared to 1.54 mg/L in NC cores.
This suggests that cover crops, while beneficial for soil health, may enhance the movement of phosphorus through soil macropores, leading to higher leaching rates. Colloidal phosphorus, a form of phosphorus bound to small soil particles, was also detected in the leachate.
While CP accounted for less than 10% of TP, its presence indicates that colloidal-facilitated transport could be an important mechanism for phosphorus movement in soils with well-connected macropores.
The study found that broiler litter application increased CP loss, particularly in CC soils, where macropore connectivity was higher.
Dissolved phosphorus, primarily in the form of DRP, was the dominant form of phosphorus in the leachate, contributing 83.5–95.6% of TP. The rapid movement of DRP through the soil profile, especially in CC soils, highlights the role of preferential flow pathways in nutrient transport.
Trace Metal Leaching: Zinc, Copper, and Nickel
The study also examined the leaching of trace metals, focusing on Zn, Cu, and Ni. In control columns (no broiler litter), Zn was detectable in the leachate, while Cu and Ni were not.
However, broiler litter application significantly increased the leaching of all three metals, with Zn showing the highest concentrations. The leaching of Zn and Cu was attributed to their complexation with organic matter from broiler litter, which increased their mobility.
In CC soils, the higher macroporosity and connectivity of soil pores facilitated the rapid transport of these metals through the soil profile. Ni leaching, though lower than Zn and Cu, was also observed in broiler litter-amended soils, particularly in acidic conditions where Ni solubility increases.
Implications for Agricultural Management
The findings of this study have important implications for the management of agricultural soils, particularly in regions where broiler litter is extensively used as fertilizer.
While cover crops offer numerous benefits, such as improved soil structure and reduced erosion, they may also enhance the leaching of phosphorus and trace metals through soil macropores.
This is especially concerning in fields with a long history of broiler litter application, where the soil’s capacity to retain these contaminants is already diminished. The study underscores the need for careful management of broiler litter application rates and timing to minimize nutrient and metal leaching.
Additionally, the role of colloidal phosphorus in contaminant transport warrants further investigation, as it could be a significant pathway for phosphorus movement in certain soil types.
Conclusion: Balancing Soil Health and Environmental Protection
This research highlights the complex interplay between agricultural practices and environmental outcomes. While cover crops are a valuable tool for improving soil health, their impact on nutrient and metal leaching must be carefully considered, particularly in soils with high macroporosity and a history of broiler litter application.
Future studies should explore the long-term effects of cover crops on contaminant transport and investigate strategies to mitigate leaching while maintaining the benefits of cover cropping.
Reference: Sandhu, V., Lamba, J., Kaur, P., Malhotra, K., Way, T. R., Balkcom, K. S., & Prasad, R. (2025). Effect of cover crops on phosphorus and trace metal leaching in agricultural soils. Agricultural Water Management, 309, 109343.
