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Title: Continuing assessment of the 5 day sodium carbonate-ammonium nitrate extraction assay as an indicator test for silicon fertilizers

item Zellner, Wendy
item Friedrich, Russell
item Kim, Sujin
item Sturtz, Douglas - Doug
item FRANTZ, JONATHAN - Dupont Pioneer Hi-Bred
item Altland, James
item Krause, Charles

Submitted to: Journal of AOAC International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2014
Publication Date: 8/16/2015
Citation: Zellner, W.L., Friedrich, R.L., Kim, S., Sturtz, D.S., Frantz, J., Altland, J.E., Krause, C.R. 2015. Continuing assessment of the 5 day sodium carbonate-ammonium nitrate extraction assay as an indicator test for silicon fertilizers. Journal of AOAC International. 98(4):890-895.

Interpretive Summary: The 5 day extraction method is a valid indicator test. Our data support the use of either ICP-OES or colorimetric analyses to determine the amount of Si in the extraction solutions. We also want to emphasize that this test is not quantifiable, as the values obtained are bias to the material being tested and do not accurately rank fertilizers based on plant-available Si. While this method has the ability to identify fertilizers that release silicic acid, there still may be other forms of plant-available Si that have not been studied, thus accounting for the variation between the chemical extraction and plant grow-out studies.

Technical Abstract: The five day sodium carbonate-ammonium nitrate extraction assay has been proposed by the AAFPCO as a standard test to identify fertilizers that provide plant-available Si. A single-lab validation test was previously performed; however, the analysis lacked any correlation to a grow-out study. To do this, we performed the 5 day extraction technique on the following Si-containing compounds; Wollastonite, sand, biochar and a BOF slag. Grow-out studies in zinnia were also performed using varying rates of the Wollastonite, biochar and BOF slag. Our results show that using the 5 day extraction technique Wollastonite had the highest detected amounts of silicic acid at 4% followed by biochar (2%), BOF slag (1%) and sand (0%). Use of both the molybdenum blue colorimetric assay and ICP-OES for detection of the silicic acid yielded similar values, supporting the application of either detection method for this type of analysis. However, when extracted values were compared to amounts of silicon taken up by the plants, the 5 day method overestimated both Wollastonite and biochar, suggesting that this method is only valid as an indicator test and should not be used to rate fertilizers based on their plant-available silicon.