The Seasonal Nature of Soil Test Potassium
Summary
We took soil samples of our 40 plots on 9/3/2016 after corn harvest, but came back and took repeats on 12 plots (1-4, 9-12 and 17-20) on 9/23/2016 (20 days later). For these 12 plots, average pH increased from 5.6 to 5.8 and average potassium (K) increased from 88 lb/ac to 118 lb/ac. The difference in lime and potassium (based on Clemson recommendations) would have saved the farmer $15 an acre if you wanted to follow the corn with small grain and double cropped soybeans behind them. Suppose the farmer had 500 acre of land, the difference in the fertilizer bill would have been $7,500. This is just one sample set and we don’t claim to say that we’ve proved anything except to say timing can make a difference.
The Story
In my lab note on Sept 9, I noted that, based on our soil samples taken around September 3, phosphorus and manganese were actually higher after harvest than before planting (no P or Mn added). By this time I had already realized that phosphorus is seasonal and seems to be related to soil biological activity – our observations being increases in temperature and plant (cash or cover crop) activity – and as we understand it, a much of P and Mn availability is mycorrhizal.
What I didn’t say was that we noticed a steep drop in pH and soil test K. Given that we saw no drop in soil test K values before and after wheat and given a comment by Farmer Carl that pH and potassium seemed to drop most after corn I realized that soil potassium may be seasonal as well. A quick look at the literature confirmed this (see Managing Seasonal Fluctuations of Soil Tests from the University of Kentucky Cooperative Extension Service bulletin AGR-189 for example). We felt like this would be something worth looking at and went out 20 days later (Sept 23) and sampled again on 12 of our 40 plots just to check how seasonal our pH and potassium would be.
It turns out that in the 20 days sample spacing, the average pH for those 12 sites rose from 5.6 to 5.8 and average potassium (K) rose from 88 to 118 lb/ac – in both cases the average increase was statistically significant. Individual plots are shown for pH (Figure 1) and potassium (Figure 2).
What are the implications for the farmer? If we take our state land grant (Clemson University) recommendations for these 12 plots as the guide we reduce lime application from an average of 1100 lb/ac to 650 lb/ac and we reduce our K2O application rates from 103 lb/ac to 86 lb/ac (Muriate of 0-0-60, potash from 172 to 143 lb/ac spread. If we assume a unit price spread of 50/ton for lime and 27c/lb of Muriate of Potash (granted these numbers may be outdated) this amounts to a difference of $6.75 in lime and $8.16 in Muriate of Potash or about $15 difference based on a difference of 20 days when we timed our sample. I suspect we’ll find higher pH’s and K’s when we sample in November and I can’t say which is the “right” value.
What , this raises for me is the question of sample timing – if the farmer is encouraged by a consultant to take the soil samples after the corn land is open, this is to the financial advantage of the fertilizer dealer.
As I dug deeper into this, I came across a paper called “The Potassium Paradox Implications for soil fertility, crop production and human health (2013) by Khan, Mulvaney and Ellsworth that calls into question the scientific basis for managing potassium based on a 6” soil sample amongst other things. The paper is published in Renewable Agriculture and Food Systems: 29(1) (2013) and I believe there are a number of rebuttals. Bottom line however is that we are finding our results so far line up with what these guys have uncovered namely (1) we have not seen a response of wheat, soybeans or corn to potassium (as Muriate of Potash) and (2) over the last 3 years where we have not added potassium, our soil test values, when sampled at the same time of the year tend to remain close to constant.
What to Do?
Well, we know that timing looks like its important in soil sampling - in the cooler wetter part of the year soil test P (and manganese) seem to dip while soil test K seems to increase. University of Kentucky I think gives good advice and that is where possible to take soils samples at the same time each year. This begins to make sense because we have another project, a 3 year conservation innovation grant (CIG) where we have sampled in April and then again in November and found our numbers have been astonishingly constant for P and K despite zero commercial P2O5 and K2O applications.
I think the interpretation of the soil test into a commercial fertilizer recommendation is the trick as I am not sure how and whether seasonality was accounted for in any of the research that underlies our state recommendations. (by the way, try to get your lab to send you a yield response curve for some of the common crops upon which they have based their data, you may be surprised. I am still trying to process this information as well but intend to be very cognizant of soil test timing in the future.
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