7/2/2013 - By Dr. Travis Shaddox

Part 2 of this article covers liquid fertilizer stains - click here for part 2.

As many of you are aware, granular fertilizer applications can create unsightly stains on sidewalks, driveways, patios, or even in pools (Fig 1).  Fertilizer stains are primarily the product of the oxidation of either iron, manganese, or magnesium.  However, not all sources of these metals create stains.  The objective of the article is to inform you which commonly used fertilizers create stains and which do not.

Figure 1. Sidewalk stained by iron fertilizer.

 In an effort to provide some clarity, an observational study was conducted using 20 commonly used iron, manganese, and magnesium granular fertilizers.  Approximately 10 grams of each fertilizer (Table 1) were equally spaced on a white paver.  Twice a day for 5 days, 10 mLs of water were applied to each fertilizer.  After 5 days, the paver was washed to remove any physical residue and the stains were photographed (Fig. 2).

The staining results are seen in Table 1 while Figure 2 shows the paver and the stains produced.  If you would like to view the labels for each stain, please participate in the online quiz found below.

Iron Stains

Iron sulfate produced a stain quicker than any other iron product with staining observed in less than 10 minutes.  Within 24 hours, staining appeared from iron chelate, polymer-coated iron chelate, and 5n1.  Staining appeared 48, 72, and 120 hours after initiation from Polyon iron sulfate, Vigiron, and iron sucrate, respectively.  No staining was observed from activated sludge, iron oxide, or frit.  In terms of magnitude, staining from iron sources followed iron sulfate > iron chelate polymer coated > iron chelate > 5n1 > Vigiron = iron sucrate > Polyon iron sulfate > iron oxide = activated sludge = frit.

Figure 2. Paver showing stains from iron, manganese, and magnesium fertilizers.

Manganese Stains

Staining was observed from manganese sulfate in less than 10 minutes.  Staining was similar in time and quantity to that of iron sulfate.  Manganese oxysulfate required 24 hours to produce a stain while manganese chelate and manganese sucrate require 48 and 120 hours, respectively.  No stain was observed from nutra plus.  Magnitude of staining followed manganese sulfate > manganese oxysulfate > manganese chelate > managanese sucrate > nutra plus.

Magnesium Stains

The only magnesium source that produced a stain was fairway grade dolomite.  However, 120 hours were required before a stain appeared and the stain was barely noticeable.

Granular fertilizer stains on concrete

Summary

Micronutrient sources containing 100% oxide or organic metals produced no staining.  If staining is the only criteria, then oxide forms of micronutrients are safer to use and will likely produce little to no staining.  Both iron and manganese sucrate required 5 days to produce a visible stain.  Moreover, the stains were almost insignificant, thus, sucrate forms of both iron and manganese are a reasonably safe option when staining is a concern.  Chelated forms of iron and manganese created stains whether they were polymer coated or not.  Caution should be taken when using granular chelated micronutrients, if staining is an issue.  Micronutrients in 100% sulfate forms produced the most pronounced stains and produced them in the least amount of time.  If staining is an issue, sulfate forms of micronutrients should not be used.

Note: This study only tested the staining of granular micronutrient sources and in no way considers their agronomic value.  Finally, the study was only an observational study, thus, no replications were used and no statistical model was used to analyze data.

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