Humic and Fulvic acid fertilizer testing and labeling, AAPFCO approved method.

Humic and Fulvic acid plant and soil amendments.

What are humic and fulvic acid?

The Association of American Plant Food Control Officials (AAPFCO) define them as follows:

Humic Substances – the major organic constituents of soil organic matter and the aquatic environment, consisting of complex heterogeneous mixtures of carbon-based substances formed by biochemical reactions during the decay and transformation of plant and microbial remains. They are primarily composed of three main fractions, called humic acids, fulvic acids, and humin, which are operationally defined by their solubility in dilute alkali and acid solutions. High concentrations of humic substances are commercially harvested from terrestrial deposits of Leonardite, oxidized lignite, oxidized sub-bituminous coals, humalite, carbonaceous shales, peat, and sapropel.

Humic Acids – the portions of the alkali extracted
humic substances that are insoluble in strongly acidic
solution. They will precipitate from the alkali extract
in acid solutions of pH 2 or less. They can be used as
either soil amendments, foliar applications, or blended
with liquid fertilizers.

Fulvic Acid Fraction – the portion of humic substances that are soluble in bothalkali and acid aqueous solutions.
Hydrophobic Fulvic Acids – are the portions of humic substances that are soluble in both alkali and acidic aqueous solutions that are separated from non-humic aqueous substances in the fulvic acid fraction by selective adsorption onto a nonionic macroporous acrylic ester resin of moderate polarity i.e. DAX-8 resin, at low pH.
Recently the American Association Of Plant Food Chemist Officials (AAPFCO) has approved a testing method to determine the fulvic acid fraction of plant and soil amendments for the purposes of labeling.
“The analytical procedure developed jointly by the Humic Products Trade Association (HPTA) and the International Humic Substances Society (IHSS) has been approved by AAPFCO as the only officially recognized procedure for the analysis of humic acids and a specific category of fulvic acids. The procedure has been published in theJournal of AOAC International, a peer reviewed scientific journal, and is currently being reviewed by the International Organization for Standardization (ISO) as a possible international analytical standard.” Lawrence Mayhew

The market is saturated with products claiming to have high concentration of either humic or fulvic acid. But I have not seen many products with label claims that have used this new method for the quantification of the humic or fulvic content. The individual state regulatory agency’s are slowly adopting the new method as the required method for labeling claims. I recommend you contact the manufacturer of your humic and fulvic acid products and urge them to lead the way.

Ferti-Organic Soluble Fulvic Acid analysis IHSS-HPTA Lamar method


Comparison of Plant Available Silicon (PAS) in several different fertilizer sources.

Comparison of Plant Available Silicon in fertilizer sources.

Comparison of Plant Available Silicon in fertilizer sources.
Manufacturer Fertilizer name

(common name)

Total Silicon % Soluble Silicon (Si) %

Plant Available Silicon

JAOAC, Vol. #96, #2, 2013

PQ Corp. AgSil 16H

(potassium silicate powder)

24.6% 8.54%
PQ Corp. AgSil 25 – Sil-MATRIX

(potassium silicate liquid)

9.7% 7.6%
SioTex Corp. Grow-Sil

(99.9% SiO2 pure biogenic silica)

46.7% 6.28%
Vanderbilt Minerals VANSIL W10

(natural Wollastonite)

24.2% 3.6%
PQ Corp AgSil 16H stock solution

(560 grams AgSil 16H in one gallon RO water)

~3.34% 3.25%
Harsco Metals llc AgrowSil

(calcium/magnesium silicate blend, stainless steel slag by-product)

12% 2.2%
Harsco Metals llc Crossover

(calcium/magnesium silicate blend, stainless steel by-product)

11.3% 1.9%
Plant Science Concepts

OSA 28

(PEG 400 stabilized silicic acid)

2.09% 1.06%
Mills Nutrients


(stabilized silicic acid)

NA 0.94%

Power Si

(PEG 400 stabilized silicic acid)

1.71% 0.88%


(PEG 400 stabilized silicic acid)

1.61% 0.85%
Loveland Products

SST 28%

(stabilized silica technology)

13% 0.44%

Fossil Shell Flour

(freshwater diatomaceous earth powder)

~42% 0.23%


Total silicon taken directly from the individual fertilizer’s label, or calculated from the labeled SiO2 value. Plant available silicon determined by the American Association of Plant Food Control Officials (AAPFCO) approved method.

Silicon Labeling – The format below should be used for guaranteeing Soluble Silicon (Si) in a dry or liquid fertilizer. The method for Soluble Silicon (Si) is in the Journal of AOAC International, Volume 96, Number 2, 2013, pp 251-259. At this time the method is valid for dry fertilizer and will give a slightly lower number for liquids, but liquids should be analyzed by this method until a method is validated for liquids.   link to source

This method is detailed by Sebastian et al. in the Journal of AOAC International Vol. 96, No. 2, 2013.

A 5-Day Method for Determination of Soluble Silicon Concentrations in Nonliquid Fertilizer Materials Using a Sodium Carbonate-Ammonium Nitrate Extractant Followed by Visible Spectroscopy with Heteropoly Blue Analysis: Single-Laboratory Validation link to source

The values for the Soluble Silicon in the Harsco Metals, and Vanderbilt Minerals products as well as the PQ Corp AgSil 25 are taken directly from the article in the Journal of AOAC International Vol. 96, No. 2, 2013. All other products Soluble Silicon values have been determined by Thornton Labs at the request of a third party. I will display those analysis results below.

AgSil 16H

AgSil 16H soluble silicon, plant available silicon

AgSil 16H stock solution (560 grams AgSil 16H in one gallon RO water)

SioTex corp. Grow-Sil

Grow-sil soluble silicon analysis

Plant Science Concepts, OSA 28

 OSA 28 soluble silicon, plant available silicon

Mills Nutrients, Vitalize

Mills Vitalize soluble silicon, plant available silicon

Fertimaster, Power Si

Power Si soluble silicon, plant available silicon

Aptus, Fasilitor (testing paid for by Plant Science Concepts)

Aptus Fasilitor soluble silicon, plant available silicon

Aptus, Fasilitor (testing paid for by third party product customer)

Aptus Fasilitor (2) soluble silicon, plant available silicon

Loveland Products, SST 28

Loveland SSY 28% stabilized silica technology soluble silicon, plant available silicon

PermaGuard, Fossil Shell Flour, freshwater diatomaceous earth

PermaGuard Fossil Shell Flour freshwater diatomaceous earth soluble silicon, plant available silicon