It’s not really about extractable or available
For a while, this topic was coming up every few months. Here’s an example from an e-mail inquiry. I paraphrase the words here, but keep the punctuation intact, and I omit course, company, and superintendent names:
I may have asked this before, but again it is soil test time and I got my results back. This year I used two different companies. Both samples were analysed by xyz labs but one also includes an abc report.
The numbers are quite similar in both but the abc report shows total ppm of an element plus available ppm of that element!
This is confusing me a little because you said if it is extractable it is also available?!?
I wrote back with this:
If you get your testing done by fertiliser companies you can expect this type of report. I think we have had this conversation before and I am not sure what else I can tell you.
I had a look at the reports that had been sent with the e-mail, and actually there is a little bit of what else. I can try to explain this in a different way. I won’t share the reports, but you may have seen something like them.
In the regular report from xyz labs, the test results are provided in ppm. It is pretty straightforward. One page. Organic matter in %, pH, macronutrients in ppm, micronutrients in ppm. Basically numbers. Also cation exchange capacity (CEC), and calculated percentages of different ions making up the CEC.
The abc report is 3 pages. One page has numbers. It also includes organic matter in %, and pH, and macronutrient and micronutrients in ppm. Also CEC and percentage of ions. This page is mostly numbers. In fact, it is very similar to the regular report. The difference between the abc report and the regular report is that for most elements, there are now two values in ppm: total, and available.
On the other two pages of the abc report, there are lots of charts, some showing a comparison between total and available amounts and others plotting cation ratios. In the cation ratios section, it is shown that total is exchangeable, and available is extractable.
Charts of the total and available Mg and K, and of available ammonium, show them below the targets. The ratio charts come with a table, giving the interpretation that total Mg and K are optimal, but available Mg and K are high. Available ammonium is optimal.
My correspondent wrote, “this is confusing me a little.”
I’d be confused too with the total and exchangeable and available and extractable terminology, and by the contradictory interpretations of high and/or optimal and then, for the same elements, the adjacent page shows them below the target.
The abc report recommends a grand total of twelve different specialty products based on these results, including products to address the low ammonium availability and the low total and available K that in other parts of the report are shown as optimal or high. There are also recommendations for inputs of three generic products. In total, fifteen products recommended, with twelve of them from the company providing the abc report.
How can I respond to the correspondent who said “this is confusing me a little because you said if it is extractable it is also available?!?”
After all that, I don’t think it is really about extractable or available. I’d suggest asking yourself, “what is the purpose of this confusing report?”
Soil nutrient analysis should not be about total or extractable or exchangeable or available. A better way to think about this, and the words that I suggest are more appropriate are enough and not enough. For more, see: