Ammonium Sulfate: The Preferred Nutrient Source
Author: Dr. S. H. (Norman) Chien, Principal Scientist – Soil Chemistry (Retired), International Fertilizer Development Center (IFDC).
While fertilizers have long been used to provide nitrogen (N) for optimum plant growth, its use to provide sulfur (S) as a nutrient source has increased in recent years. Data have shown that sulfur deficiency in agricultural soils has become a factor in limiting crop yield in many countries because of the extensive and popular use of high-analysis fertilizers that contain little or no sulfur. In addition, stricter EPA emissions regulations in recent years have minimized atmospheric sulfur depositions. In fact, most soils across the U.S. are currently receiving less than 5 lbs. S per acre from the atmosphere. Because of these changing environmental and cropping factors, as well as higher yield potential seed genetics, S nutrition has become more important now than ever before.
Although for over a century ammonium sulfate (AMS) has been a trusted and reliable fertilizer to provide pound for pound the most readily available N and S nutrients to the crop, a new organic-containing fertilizer (SymTRX™ 20S) has been claimed to be more effective than traditional AMS in providing N and S nutrients to the crop. The theory behind that claim is that the SymTRX fertilizer mixes AMS with 16% organic matter from biosolids, forming what the producer calls an Organic MaTRX™ which retains NH4-N and SO4-S in water-insoluble form, providing potential benefits by slowing down the release rate of N and S nutrients into the soil.
AMS vs. SymTRX
Below is an overview of the results obtained from just a few of the studies conducted between traditional AMS and SymTRX fertilizer under my supervision.
- Chemical Composition – The chemical composition for the commercial grades of SymTRX and AMS fertilizers are similar. The ratio of water-soluble NH4-N/SO4-S of the SymTRX fertilizer was 0.86 and the AMS fertilizer was 0.87. This indicates that the water-soluble N and S compounds in the SymTRX fertilizer were derived from the same N and S source as the AMS fertilizer. In other words, the SymTRX fertilizer is merely a physical mixture of AMS and biosolids.
- Soil Leaching – Granular SymTRX fertilizer has been touted as a slow-release N fertilizer. However, soil-releasing data did not show the SymTRX fertilizer to exhibit characteristics of a slow-releasing N fertilizer. In comparing the SymTRX fertilizer with a typical slow-release fertilizer by the AOAC adopted technique, only 15% of total N was released from the typical slow-release N fertilizer as compared to 60% from the SymTRX fertilizer in the first 7 days. Only 50% of total N was released from the typical slow-release N fertilizer as compared to 78% from the SymTRX fertilizer in 28 days. Furthermore, N continued to be slowly released from the typical slow-release N fertilizer from 7 to 56 days without reaching the plateau while the SymTRX attained maximum release between 26 to 56 days. Therefore, the SymTRX fertilizer, like the AMS fertilizer, does not fit the typical release curve expected from a slow-release N fertilizer.
- Soil Incubation – There were no significant differences in KCl-extractable ammonium (NH4-N) from the SymTRX and AMS fertilizers, and there was only a small amount of nitrate (NO3-N) derived from the total 250 mg N kg-1 applied with either the SymTRX or AMS fertilizer based on soil incubation studies. These results suggest that the SymTRX and AMS fertilizers are similar in the rate of N release and transformation in the soil. Like the AMS fertilizer, the SymTRX fertilizer does not behave as a slow N-release fertilizer in the soil.
- Greenhouse Evaluation –In the canola study, there was a significant biomass yield response to S but no significant difference between the two sources in either the Brownfield or Lakeland soils. These results indicate that the SymTRX fertilizer acted like the AMS fertilizer in providing S to the canola crop. Separately, in the wheat study, there were no significant differences in N uptake between the SymTRX and AMS fertilizers as described by the same curvilinear response for Ashland soil and the same linear response for Manhattan soil. These results indicate that both the SymTRX and AMS fertilizers performed equally in providing available N to the wheat crop.
The study results above show that the SymTRX fertilizer has no significant agronomic benefits over AMS fertilizers, as the SymTRX fertilizer is largely comprised of ammonium sulfate as evidenced from canola and wheat crops in greenhouse studies. The SymTRX fertilizer also does not provide as much nutrition to crops as AMS fertilizers. With a nutrient density of 45%, the AMS fertilizer delivers approximately 20% more nutrition to the crops for each pound of fertilizer applied as compared to the SymTRX fertilizer with a 37% nutrient density, making the AMS fertilizer a much more efficient source of nutrition to the crop.
Learn more about the benefits of AMS and how it compares to organic-containing fertilizer at asbenefits.advansix.com.