A couple more thoughts around soil test Phosphorus. 1) To figure out where we are going, it’s often times a good idea to look at history. I’ve attached an interesting paper here that describes what comprises a soil test and also what plant scientists knew from the beginning when they started putting soil tests together. I’ve highlighted that here. I think it is interesting that the first part of a soil test according to the paper is “a reagent to extract the “plant available” nutrient from the soil.” The aim or goal of a soil test is to extract what is plant available. Also, scientists dating back to the 1930s knew that roots excreted organic acids that were assumed to release nutrients from the soil. Knowing what we know now, I think the Haney H3A is the logical conclusion. Especially if we follow what is outlined as criteria of a soil test in this paper. 2) Interpreting soil samples is not always as straight forward as “you have X level Phosphorus, apply Y rate of MAP/DAP to achieve Z yield.” To start, what drives Phosphorus cycling and availability? Biological activity. But what drives biological activity? WEOC (Water Extractable Organic Carbon). I’ve attached the different pools of Phosphorus here. This isn’t a static number, but changes with biological activity. To drive Phosphorus to that plant available form, we need biology to cycle that nutrition. This biology is heavily dependent on WEOC as a food source. This is why I say interpreting soil samples is not always straightforward. Because if we dive further into WEOC, we have to think about what influences that metric. Nitrogen, tillage, plant health, etc. all affect WEOC. All of that eventually plays in to managing Phosphorus. All of this to say, it’s crucial to be able to tie all of these metrics together and have a means to get accurate plant available measurements if we are going to make fertility recommendations based on our soil samples.