Since the Haney test was developed it has been highly scrutinized and questioned for being calibrated to specific regions. Universities, commercial labs, and agronomists have done their fair share of finger waving attempting to discredit and shame the Haney test for not being "calibrated". But is the Haney test calibrated? The answer is yes and no.
Soil health has traditionally been judged in terms of production; however, it recently has gained a wider focus with a global audience, as soil condition is becoming an environmental quality, human health, and political issue. A crucial initial step in evaluating soil health is properly assessing the condition of the soil. Currently most laboratory soil analyses treat soils as non-living, non-integrated systems. Plant available nutrients have traditionally been estimated with methods that utilize harsh chemical extractants in testing soil for inorganic N, P, K, and micronutrients.
The amount of nutrients extracted by any solution in the laboratory is relative to the extractant used. Currently, most commercial soil testing labs use extractants that (1) treat the soil as a non-living non-integrated system, (2) focus on physical and chemical, (3) ignore the biological components of the soil, (3) extract soil with chemistry that soil never sees, and (4) measure the house and not the food source for the biological systems responsible for nutrient cycling. The Bray, Olsen and Mehlich 3 extracts were developed between 1945 and 1984.
Why are we using antiquated extractants and technology to determine nutrient availability in the soils? For perspective, I turn to Jimmy Emmons, who commented, "very few people still farm with a 40-50's tractor, why would they want to use technology that old?"
Regardless, these common soil extractants are used in varying regions depending on the general pH of the soils in that region. Bray is normally used in soils with pH values below 6.8, Olsen in soil with pH values greater than 7, and Mehlich was originally developed for soils with a low pH. Mehlich 3, however, is now used as a "universal" soil extractant despite the fact that the extractant itself has a pH of 2.5. All three extracts are highly buffered so when the soil exposed to these extractants, it assumes the pH of the extractant. Why is this important? The amount of nutrients solubilized is directly related to the pH of the extractant. Lance Gunderson of Regen Ag Lab often uses the metaphor of trying to dissolve a penny in a jar. If you place the penny in a jar of water, it will take a very long time (years possibly) to dissolve the penny. If you place the penny in a jar of soda, the penny will dissolve in weeks as the soda is acidic. If you place the penny in a jar of nitric acid it will dissolve in days as the nitric is highly acidic.
Because Mehlich 3 is so acidic, it will dissolve many P compounds that may or may not be plant available over the growing season such as calcium phosphates and iron/aluminum phosphates. In a field where we are not exposing the soil to pH values that low under natural conditions, these phosphates would not normally be plant available. Essentially, we can change the ph of a soil extractant to dissolve whatever amount of any given nutrient we desire. This is where soil test calibration comes in. To determine the effectiveness of an extract, because it does not represent natural field conditions, correlation and calibration to relative yields are used to determine threshold nutrient availability.
Hochmuth et. al (2018) state that soil test correlation is defined as the "process of determining the relationship between plant nutrient uptake or yield and the amount of nutrient extracted by a particular soil test method" (Mitchell and Mylavarapu 2014). Beegle (Interpretation of Soil Test Results, Cooperative Bulletin No. 493) indicates that the strength of this correlation is the basis for selecting a particular soil test extractant for a given combination of soil, crop, and growing conditions. Beegle explains calibration as follows:
To interpret a soil test we must know the relationship between the amount of a nutrient extracted by a given soil test and the expected crop response for each crop. The process of determining the degree of limitation to crop growth or the probability of getting a growth response to an applied nutrient at a given soil test level is known as soil test calibration and must be determined experimentally in the field. (Dahnke and Olsen, 1990). A common procedure for calibrating a soil test is to grow the crop on soils representative of those where the test will be used that cover the range of soil test extractable nutrients likely to be encountered. This must be done for each crop with which the soil test will be used........Relative yield is the yield with optimum amounts of all nutrients except the nutrient of interest divided by the maximum yield with optimum amounts of all nutrients.
For example, in the case of N below, the optimum fertilization rate is about 180 lbs of N/acre to achieve a non-limiting relative yield of over 200 bu/acre.
We must, however, consider what the data really looks like before the response cure is determined. The response to N fertilization when P and K are not limiting in the research plots above actually looks like this:
So inherently, we are not really measuring "plant available" nutrients under varying field conditions and varying crops but rather crop response to fertilization relative to the particular nutrient extracted in the laboratory and then fitted to a nice curve. This is problematic for many reasons. First and foremost, when we are determining crop response in a plot trial, we are not considering the complex biochemical pathways that are occurring in the plant/soil synergistic relationship under varying climatic and management conditions (the spread in the data above under each fertilizer treatment). From a reductionist viewpoint, if we are treating the soil as though it is not nutrient limited because we have provided the amount of N, P, or K for optimal crop production, but not considered micronutrients and bioavailability of all nutrients under varying conditions in all fields with all crops, what are we really measuring?
Again, is the Haney test, and in particular, the H3A extractant, calibrated? In the standard sense, no, the H3A extractant has not been compared to relative yield under all crop conditions when all nutrients are independently treated as non-limiting. The standard commercial extracts have also not been tested under all of these varying conditions either.
Traditional soil tests typically utilize extractants including Mehlich 3 (Mehlich, 1984) and Olsen (Olsen et al., 1954), which were designed for certain soil pH ranges; however, these extractants are often applied outside their intended pH range because of the benefits of uniform procedures and rapid analysis. This produces inaccurate predictions of plant available P because of the influence of soil pH on soil-solution chemistry (Nelson et al., 1953, Menon et al., 1988) and P solubility (Golterman, 1998, Sharpley, 1993). Thus, the Soil Health Tool uses the H3A extractant (Haney et al., 2006, Haney et al., 2010a), which is composed of weak organic acids that mimic plant root exudates. H3A has been shown to closely match results from the “gold standard” plant available P test that uses FeAlO strip results (Haney et al., 2016).
The Haney test was developed to extract the soil under conditions that best mimic natural conditions in the field. The H3A extract consists of 3 of the most common plant root exudates in row crop conditions, malic acid, citric acid and oxalic acid. The extract is slightly acidic and non-buffered. This means that the pH of the soil remains within approximately 1 unit of the soils natural pH, similar to what happens in the root zone when plant root exudates are pumped into the soil to solubilize nutrients. The H3A extract is used to determine P, K, and micronutrients in the soil that would be solubilized around the root zone and are directly plant available. These extractable nutrients are then considered in combination with microbial activity, carbon to nitrogen ratios, and the carbon compounds that are the food source for the microbes. In combination, we use the metrics to determine if the soil is in balance and if extractable nutrients will actually be available to the plants during the growing season.
Soil Regen is focused on improving producer ROI through thoughtful agronomic practices, including reducing nutrient inputs. While soil testing is not the cause of all problems in agriculture, current soil testing methods and conventional agriculture practices have led to the over-application of nutrients that result in surface and ground water pollution. We believe that soils are the foundation of improving ROI, crop health and the environment. Therefore, a focus on soil testing methods that improve ROI by reducing inputs while improving crop yields is very important. There are many testing methods that can be used in combination to gain a greater understanding of the impact of management practices under varying context and provide guidance to improve crop vigor and therefore producer profitability. In turn, management practices that improve production also improve soil health and mitigate negative impacts to the environment such as sediment and nutrient runoff to streams, lakes and rivers.
From an agricultural standpoint we have long focused solely on the soil physical and chemical properties that relate to plant production, neglecting the inherent biological components of soil that contribute to its overall health. In 1 m3 of agricultural soil there is between 1200 and 1700 kg of soil containing approximately 2.3%–2.6% of the soil’s carbon in the microbial biomass (Anderson and Domsch, 1989). Throughout their life cycle, the microbial biomass (bacteria and fungi) immobilize N during growth and release plant-available N and P upon their death. Microbial nutrient cycling can provide enough N and P to produce a crop without the addition of fertilizers. When the agricultural community accepts the fact that the soil is a biological system and manages it accordingly, it will be able to restore and build soil health while concurrently reducing input costs and maintaining or improving crop yields (Stika, 2013). Additionally, producers have the potential to significantly reduce the negative environmental effects of modern farming practices by managing the soil as a living ecosystem and enhancing its inherent nutrient cycling ability.
So who are you to wave your finger unless you are looking at the soil as the complex ecological system that it is by using soil analyses that focus on biological function and balance as it relates to nutrient release.
