Continued from 2 July 2015
In part one I concluded by saying that the mined Phosphate found its way onto our farms by two distinctly different paths. The first is by direct application and the second by making the Phosphorous, (P), soluble by chemically processing the rock with acids prior to application.
In the early years of direct Phosphate rock application to farms, it became apparent that some sources gave much better results than others. In response the NZ fertiliser industry settled on the “Citric Solubility” test as a “Quality Control” measure of Phosphate rock, (PR). A minimum level of 30% of the “P” in the Phosphate rock being dissolved in a 2% Citric acid solution, (Citric solubility), was adopted as being the level required for a Phosphate rock to be classified as “Re-active Phosphate Rock” or RPR. The 2% Citric acid was adopted because it was believed that this was the acid level likely to be found in mildly acidic farm soils and would therefore indicate how available the P would be after direct application.
Unfortunately the laboratory test used to carry out the Citric Solubility test, as well as the reporting methods, can vary considerably from country to country. For example some laboratories will report the dissolved P as a percentage of the total rock sample, while others will report the dissolved P as a percentage of only the total P in the sample.
It is now recognised that the majority of Phosphorous that finds its way into pasture plants, does so via the mutually beneficial relationship that exists between plants and Phosphate solubilising soil bacteria and fungi. In particular, Mycorrhizae fungi are known to excrete powerful enzymes that make insoluble Phosphorous available to plants regardless of soil pH.
In past years, RPR earned a reputation of being a “Rich man’s fert” because it took so long to get results. In my opinion that was simply because the RPR being applied was far to coarse. It is a proven fact, that when RPR or Guano particle sizes are reduced, the surface area increases exponentially and that this results in a significant increase in citric solubility.
I believe that from a farmer’s point of view, when it comes to RPR and Guano, the deciding factors should include:- P content, Citric solubility, heavy metal and fluoride contamination levels, particle size and cost.
In part three I will cover the chemically processed Phosphate rock options.
To be continued…
Andrew de Lautour. PFP Fertilisers Ltd. ©
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