Precision vs. Accurate Agriculture
Intro - This mini exercise was inspired by conversations I had with Will Brinton from Woods End labs and a story he told of Dr. William Liebhardt which can be found here along with the original story in the New farm in 1981! This was spurred by some of my observations in the field where I saw huge differences between commercial lab recommendations and what the land grant university was saying. Finally when I saw this video by Purdue’s Dr. Brad Joern (on the huge differences in land grant fertilizer reccs in states that actually border one another), I decided to do a little test myself.
Method - We went out to our fertilizer plots and took 3 composite samples (about half a bucket full) from 3 known plots (Plots 1, 10 and 20 – a sandy loam, a loamy sand and a sand). Samples were taken after corn harvest on September 23 and were taken down to a depth of 6". We took each sample and mixed it thoroughly in the bucket and split it nine (9) ways and (of course we did this for all three composite samples). We then sent three sets of 3 samples (3 from Plot 1, 3 from Plot 10, 3 from Plot 20) to Clemson (our land grant labs), to a well-used commercial lab (Comm Lab) and a neighboring land grant lab (NLGL). We asked for recommendations for wheat/small (60 bu expected yield). The soil test values for pH, P and K and the fertilizer recommendations for lime, P2O5 and K2O are shown in Figure 1.
Different Extractants - Apples and Oranges - Note that the commercial lab used the Mehlich 1 Extractant and but the neighboring land grant lab (NBLGL) use Mehlich 3 extractant, so comparing the numerical soil test values for P and K is not fair when looking at Clemson or the Comm Lab vs. the NLGL (its like apples and oranges). However both Clemson and the Comm lab used Mehlich 1 so they are directly comparable. What is comparable is the interpretation of these results in terms of P2O5, K2O and lime recommendations.
P2O5 recommendations – even through the soil test P was rated as adequate by the comm lab and in some cases excessive by Clemson, the Comm lab still recommended 40 lb, presumably for maintenance or possibly as a build-up recommendation. Neither Clemson not the NLGL made a P2O5 recommendation.
K2O recommendations - the commercial lab recommended a lot more K2O, especially for the loamy sand where the soil test K was considered adequate (we presume and buildup or maintenance component of the recommendation), the Clemson and NLGL were close.
In terms of the lime recommendation, the commercial lab gave a recommendation that aimed at a target of 6.5 (no other choice of target pH was provided), the subsequent lime recommendation was still higher than the Clemson recommendation (based on Adams-Evans Method) for a target pH of 6.5. Clemson lime recommendation for a target of 6.0 was zero, while the NLGL recommendation for lime (no target pH was explicitly expressed in the NLGL report).
“Maintenance” or “Build-up” application of P2O5 and K2O fertilizer – one of the elements in the above is this idea of adding maintenance or buildup fertilizer applications, assuming that soils are really bad at recycling nutrients. This may have been the case in clean-tilled/degraded soils when fertilizer recommendation research was done, but it does not seem to be the case now. Our experience on the fertilizer plots and out 2013 CIG plots (5 fields, 13 soil polygons 3 different counties in SC), is that for all practical purposes, our soil test P and K has remained static (%OM, pH and soil test calcium have increased). Note that these are long-term no-till fields the soils are primarily kandiudults, paeludults and paleaqualts with kaolinitic clays kicking in between 8 and 15 inches below surface.
Costs – (Figure 2) we added up the costs for fertilizer and lime for 4 options namely (1) comm lab, (2) NLGL, (3) Clemson with target pH of 6 and (4) Clemson with target pH of 6.5 (only affects lime costs). This cost comparison means that between the highest and lowest recommendation you have a $62 an acre difference on average. Supposing the farm was 1,000 acres, and depending who the farmer trusted, the difference between two formal, documented ” fertilizer recommendations (let’s assume the farmer had an even mix of loamy sands, sandy loams and sands) would be 62 $/ac x 1,000 ac = $62,000. Question is: who is right? I don’t know. I did reach out the Clemson and ask them for their yield P and K response curves for corn, wheat, soybeans and cotton and they said they did not have these but that they based their 2007 recommendations on Georgia, Penn State and Indiana’s numbers (and presumably the 2001 recommendations). I reached out to the commercial lab’s agronomist for a rationale for how they made their recommendations and did not receive a reply.
Question is: who is right? I don’t know but maybe the right question to ask is :"is any one of these labs right?". I did reach out the Clemson and ask them for their yield P and K response curves for corn, wheat, soybeans and cotton and they said they did not have these but that they based their 2007 recommendations on Georgia, Penn State and Indiana’s numbers (and presumably the 2001 recommendations). I reached out to the commercial lab’s agronomist for a rationale for how they made their recommendations and did not receive a reply.
caveat emptor
One last word, there is a difference between precision and accuracy. Precision is if I can shoot a 5-round pattern as small as a silver dollar into a target 200 yards away – however, if I am shooting at my neighbor’s target, I may be precise but I will be inaccurate.
0 comments