Nitrogen and Iron Fertilizer Turfgrass Field Demonstration

In a SoiLab episode, Matt provides a comprehensive season-long summary of a turfgrass demonstration trial at Walla Walla Community College, focusing on the effects of different nitrogen and iron fertilizer products on lawn color, growth, and soil test values. The demonstration is designed for homeowners and DIYers, aiming to provide practical, visual comparisons of commonly available nitrogen and iron sources.

Site and Study Overview

·       Location: Southeast Washington State, USDA Zone 7a.  This location experiences hot summers and cold winters.

·       Soil: Yakama gravelly silt loam, a medium-textured soil with some gravel, making soil sampling slightly challenging.

·       Plot Design:

o   Nitrogen Treatments (North-South):

§  50% urea + 50% ammonium sulfate

§  100% ammonium sulfate

§  Untreated (no nitrogen)

§  100% urea

§  Each nitrogen source applied at 4 lbs N/1000 sq ft for the year.  The nitrogen was applied across five applications during the season.

o   Iron Treatments (East-West, checkerboarded):

§  Foliar applied iron lignosulfonate

§  Granular ferrous sulfate

§  Foliar applied iron EDDHA

§  Untreated (no iron)

§  Granular iron oxide (Ironite with 1% Nitrogen)

§  Foliar applied iron EDTA

§  Each iron source applied at 1.5 lbs iron/1000 sq ft for the year.  The iron was applied across five applications (0.3 lbs Fe/1000 sq ft/application).

 

Field Observations

Nitrogen Sources

·       Ammonium sulfate (alone or in combination with urea) produced the quickest and darkest green color, especially early and mid-season.

·       By season’s end, visual differences between nitrogen treatments diminished, but ammonium sulfate consistently gave a slightly deeper green.

Iron Sources

·       Iron EDDHA delivered the most noticeable and lasting green-up, especially when watered in after application.

·       Iron lignosulfonate was a close second, providing a rapid greening response.

·       Iron oxide (Ironite) and ferrous sulfate performed similarly to the untreated control, with Ironite offering a slight color improvement, likely due to its small nitrogen content.

·       Iron EDTA showed less visual response, possibly influenced by the presence of bermudagrass in some plots.

Best Nitrogen + Iron Combinations

·       Ammonium sulfate + iron EDDHA and 50/50 ammonium sulfate/urea + iron EDDHA were top performers for season-long turf color.

·       Ammonium sulfate + iron lignosulfonate also maintained strong color throughout the season.

Soil Test Data Insights

·       pH: All plots remained within the optimal range (6.1–6.8), with little variation between treatments.

·       Nitrogen: No significant differences in available soil nitrogen among fertilized plots; only the untreated and Ironite (with its small nitrogen content) plots showed slightly lower N.

·       Iron:

o   EDDHA consistently resulted in the highest available iron levels in the soil.

o   Iron lignosulfonate showed lower soil iron levels, despite strong visual greening-suggesting rapid plant uptake.

o   Ferrous sulfate was inconsistent in raising soil iron levels and had variable results.

o   Ironite and the untreated control tracked closely, with minimal impact on soil iron.

Key Takeaways

·       Nitrogen remains the primary driver of turfgrass color, with ammonium sulfate-based products providing a faster, deeper green than urea alone.

·       Foliar-applied, chelated iron sources (especially EDDHA and lignosulfonate) can enhance turf color, especially when paired with ammonium sulfate.

·       Iron oxide and granular ferrous sulfate are less effective for rapid or dramatic greening, despite their popularity.

·       Soil tests showed that iron EDDHA most reliably increased available soil iron, while lignosulfonate iron was likely absorbed quickly by the grass, explaining its strong visual effect but lower soil iron readings.

·       For homeowners seeking a deep green lawn, combining ammonium sulfate with a quality chelated iron (EDDHA or lignosulfonate) is most effective.

In summary:
For season-long turf color and health, ammonium sulfate-based nitrogen sources paired with foliar-applied chelated iron (EDDHA or lignosulfonate) deliver the best results. Soil test data confirms these products’ effectiveness, and the study offers practical, visually based guidance for DIY lawn enthusiasts seeking the deepest green lawn.