Milorganite’s Iron Guarantee

By Jaime Staufenbeil - Milorganite Agronomist

Nutrient amounts in Milorganite, including iron, are guaranteed

Milorganite’s iron guarantee recently changed from 4% to 2.5%. Our faithful customers have expressed concern that less iron in Milorganite may not produce the same green lawns they’ve grown to expect. We’d like to address your concerns and explain the change in analysis.

The analysis on every bag of fertilizer, including Milorganite, guarantees the minimum amount of specific nutrients. The analysis is highly regulated, so rest assured, it’s accurate. The analysis for Milorganite, 6­–4–0, guarantees there’s a minimum of 6% nitrogen, 4% phosphorus, and less than one percent potassium in each bag. There’s one additional nutrient in Milorganite that has made it so popular among homeowners for decades: iron. Iron helps promote the green lawn homeowners desire.

Why is there less iron in Milorganite?

Nutrient levels in Milorganite have fluctuated throughout its 90-year history as a result of the wastewater reclamation and Milorganite production processes. Iron, which doesn’t naturally occur in high quantities in wastewater, has traditionally been used as an effective additive to bind and recover phosphorus and continues to be the primary source of iron in Milorganite. Improvements to our water reclamation operations have reduced the amount of iron needed to maximize phosphorus recovery, improving overall water quality and increasing the phosphorus (P) guarantee of Milorganite from 2% to 4%. As a result, Milorganite’s guaranteed iron has changed from 4% to 2.5%. The overall benefits of Milorganite to lawns, landscapes, and gardens remain the same.

Research demonstrates Milorganite’s ability to green-up lawns doesn’t change with a revised iron guarantee

We conducted independent research a few years ago to ensure that, despite any changes in the iron guarantee, Milorganite continues to provide homeowners the outcomes they expect— lawns that are healthy, with a long-lasting, deep-green color.

We studied both northern and southern grasses to determine how various amounts of iron and nitrogen in Milorganite affected the greenness of lawns. The study of northern grasses was conducted through the University of Wisconsin – Madison, Department of Soil Science.

Two measures were used to evaluate how green the lawn was for each fertilizer mix: one electronic (color index) and the other visual. Under the conditions of the study, both measures were found to be statistically similar to the Milorganite mix with 5.1% iron, as well as the mix with 2.3% iron (see table below). The different levels of iron "...did not significantly affect the color response of the grass."

Average Color Index

Treatment Color Index Visual Quality
Milorganite 2.3% Iron 394 6.7
Milorganite 5.1% Iron 406 6.8

Milorganite Iron Research Test Plots

A 2015 study conducted in Florida by George H. Snyder, Ph.D. compared Bahiagrass grown with various fertilizer treatments, including three of Milorganite with varying amounts of iron—2%, 2.7%, and 4%. The year-long study concluded that “Throughout the study, no differences in leaf appearance or growth were observed among the three Milorganite sources.”

Iron in Milorganite

There are benefits for your lawn when using chelated iron—iron treated to make it water-soluble, which can be expensive. Iron salts are less expensive but have drawbacks, including the potential for burning plants and staining concrete surfaces.

The iron in Milorganite works like chelated iron, but without the expense. It also won’t burn your lawn and releases into the soil where plants need it, compared to foliar applications (applying liquid directly to leaf) of iron that is gradually removed when the lawn is mowed. Milorganite, including its iron, breaks down slowly and is available to plants as microorganisms in the soil use it as a food source. It’s non-staining, so there’ll be no rust stains on your driveway or walkways. It’s also long-lasting, which means fewer applications compared to other types of iron. Best of all, with Milorganite’s iron, you’ll have a deep green lawn!

We don’t recommend using a variety of iron supplements on your lawn. Lawns can use only a limited amount of any nutrient, including iron. Over application can result in run-off into waterways and it’s a waste of money.

Lawn visual quality impacted by more than iron

There are a number of factors that can affect the visual quality of your lawn aside from iron, including temperature, moisture, weeds, diseases, insects, and even cloud cover.

Continue to use Milorganite with confidence

None of us like change, even changes in a product like Milorganite, which you’ve trusted for years. There have always been fluctuations in Milorganite’s nutrient guarantee and we may see changes in the future as processes continue to improve. We will always be honest with you—our faithful customers—and provide accurate information.

Along with Milorganite’s nutrient guarantee, we guarantee we will continue to provide a high-quality product that produces the healthy, green lawns you’ve all grown to expect.