Ferrous sulphate reduces phosphate leaching in peat-based substrates

Authors: Altland, James; Shreckhise, Jacob; PANCERZ, MAGDALENA - The Ohio State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2026
Publication Date: 4/28/2026
Citation: Altland, J.E., Shreckhise, J.H., Pancerz, M. 2026. Ferrous sulphate reduces phosphate leaching in peat-based substrates. HortScience. 61(4):675–684. https://doi.org/10.21273/HORTSCI19312-26.
DOI: https://doi.org/10.21273/HORTSCI19312-26

Interpretive Summary: A major challenge in greenhouse and nursery production was identified as the excessive loss of phosphorus (P) from plant containers into the environment. This nutrient, essential for plant growth, often leaches from soilless substrates used in horticulture, contributing to water pollution and wasting valuable resources. Traditional methods, such as using controlled-release fertilizers or reducing irrigation, were found to be only partially effective and difficult to implement on a large scale. To address this problem, a study was conducted to test whether adding ferrous sulfate (FeSO4), a common iron compound, to potting mixes could reduce phosphorus loss without harming plant growth. Marigolds were grown in peat-based substrates treated with different amounts of phosphorus and FeSO4 under greenhouse conditions. Plant size, leaf color, flowering, and nutrient levels were monitored, while water draining from pots was analyzed for phosphorus content. FeSO4 dramatically reduced phosphorus leaching—by more than 90% compared to untreated mixes—while maintaining healthy plant growth when combined with adequate phosphorus levels. This offers a practical and cost-effective solution to a long-standing environmental and production problem. By using FeSO4, growers can significantly cut phosphorus runoff, helping protect water quality while sustaining crop performance.

Technical Abstract: Phosphate leaching from container-grown crops can have negative environmental impacts, and has since been one of the greatest production challenges since the advent of soilless substrates. The objective of this research was to evaluate the effect of ferrous sulfate (FeSO4) on phosphorus (P) leaching and marigold (Tagetes patula L.) growth in peat-based substrates to determine its suitability for short-cycle floriculture crops. Two greenhouse experiments were conducted in Wooster, Ohio, using sphagnum peat:perlite substrates amended with 0 or 3 kg·m'³ FeSO4 and varying rates of triple superphosphate (TSP: 0 to 1 kg·m'³). Marigold cultivars ‘Durango Orange’ and ‘Durango Yellow’ were grown under controlled conditions, fertigated with P-free fertilizer (except controls), and irrigated to a 20% leaching fraction. Leachates and pour-through samples were collected at 2, 4, and 6 weeks to measure pH, electrical conductivity, and nutrient concentrations. Plant growth was assessed via height, width, leaf greenness, and flower counts throughout the experiments, along with root and shoot dry weights at the conclusion of the experiments. Results showed FeSO4 significantly reduced P leaching—by more than 99% initially and 88% to 97% at 4 to 6 weeks—compared to non-amended substrates, even at high TSP rates. Plant growth and development were unaffected by FeSO4 when combined with adequate P (=0.5 kg·m'³ TSP), producing similar size, flowering, and foliar nutrient concentrations as controls; however, low P rates (=0.25 kg·m'³) reduced growth regardless of FeSO4. Foliar P was slightly lower in FeSO4 treatments, suggesting some sorbed P was less available, but FeSO4 did not induce toxicity or major nutrient imbalances. Overall, FeSO4 is a cost-effective, easily incorporated amendment that substantially reduces P leaching without compromising crop performance when paired with sufficient P fertilization. Future research should examine its efficacy under variable leaching fractions and in iron-efficient species prone to Fe toxicity.