Where's Mo?

Sweet Tatorman's picture

Not some dude you know by that name but the element Molybdenum (Mo). Of the minor trace elements required by plants, Molybdenum (Mo) and Nickel (Ni) are those needed in the least quantity and were not even recognized as required trace elements until the mid (for Mo) to late (for Ni) 20th century. There is still even some question about whether Ni is needed by all plants. In my gardening I have generally operated on the faith based assumption that since I maintain soil pH in the appropriate range and use wood ash as a soil supplement (good source of trace elements) when available subject to the limitation of not pushing pH too high, trace element deficiencies were unlikely. By nature I am leery of faith based assumptions so recently when the State Ag lab lowered their prices I did a higher level soil test which includes some of the trace elements. Reviewing the results, the values for Mo and Ni were notable. Mo was below the lower limit of detection of 0.04 ppm and Ni was shown as 0.4 ppm. Something is fishy here as elsewhere I have seen typical soil concentrations of 0.5-40 ppm given for Mo and 5-500 ppm for Ni. While the desirable level for Ni is murky, I have seen 1-2 ppm given as optimum for Mo for most crops. I am suspecting there may be an error or deficiency in the lab protocol for these two elements. I am not inclined to attempt any corrective measures without more info. I called the county extension agent to discuss the test results but he was out of his element, so to speak, on my particular questions. He did commit to forward my questions to the soil scientists at the Ag school. I suspect that it will be a few weeks before I hear back. If the test results are legitimate it shouldn't be hard to supplement the soil up to desired levels since 1 ppm is likely all that is needed.

ClareBroommaker's picture

Interesting! So I'm going to have to read up on organic sources for Mo supplement. Maybe the grain straw I sometimes have is a good source as grains seem (from a very quick internet read!) to be big users of Mo.

Do you happen to live in an agricultural area where it would not be difficult to get some sort of Mo fertilizer?

I'm off to go look for some geological mapping of molybdenum bearing rock & soils!

Let us know what you find out.

Sweet Tatorman's picture

If it turns out that the value reported in my soil test it legit I would likely apply Sodium Molybdate which is ~39% actual Molybdenum. Just a pound would get me to about 1.6 lbs/acre or 0.8 ppm in a 6" soil column on my quarter acre. Cost is under $20.

Sweet Tatorman's picture

Bottom line is that I will not be doing any supplementation of the trace elements Ni and Mo this year. I haven't entirely ruled out possibly adding Mo in a future year but will not this year as I don't want it to be a confounding variable in other supplementations I will be doing this year. While I was awaiting input from the soil scientists at our State's Ag school I did quite a bit of web based research into the topic. One thing I learned was that my understanding of the soil testing and the interpretation of results had been fairly shallow. In my original post I had written >Mo was below the lower limit of detection of 0.04 ppm and Ni was shown as 0.4 ppm. Something is fishy here as elsewhere I have seen typical soil concentrations of 0.5-40 ppm given for Mo and 5-500 ppm for Ni<. I was making a bit of an Apples to Oranges comparison error. Those typical concentrations I was referencing above likely are total concentrations, not plant available concentrations. The extractants used in soil testing are engineered to provide a proxy value that is more representative of the plant available. For some soils and some extractants there can be up to several orders of magnitude (100X factor) between element that is plant available and total in the soil. Taking the example of the trace element Ni, nickel has 5 possible valence states but it is only in the form of the Ni+2 ion that it is utilized by the plant. Those values you see on your soil test report, frequently expressed in lbs/acre of a given element, do not necessarily measure the actual plant available lbs/acre of the element in question but are just a proxy value that may be higher or lower than actual depending upon the extractant used, soil type, pH, and other variables. If the test reports 50 lbs/acre phosphorus (P), whether that is considered to be low, medium, or high would depend upon the above variables. My State is divided into 4 soil type regions and these are taken into account in the test report recommendations based upon test result.
My original inquiry to the local county extension agent was resolved through several email exchanges and later phone conversion with the program coordinator of the State soil testing laboratory. Bottom line was that for the soil types in this State there just were not any reported problems involving deficiencies of the two elements of interest here, Ni and Mo, except for Ni deficiency in very sandy, low clay, low organic matter soils in the Southern part of the State and only then with pecans. I am in the extreme North of the State with different soil type. He was of the opinion that if I was properly maintaining soil pH, there was no reason to worry about these elements being deficient. I am not inclined to question the judgement of a Dr of soil science.

lathechuck's picture

I took two semesters of high school chemistry, then a full year of college chemistry, so I thought I'd be pretty well prepared to understand soil chemistry. Not! As you say, the elemental composition of the soil is one thing, but the availability of nutrients is another. Even something as simple as pH throws me, because I can't measure the pH of dry soil, and the more water I add to the soil, the more neutral the pH has to become. (However, since pH is a log scale, I guess I have some wiggle room, as long as I don't use 10x as much water as necessary.) And the ions present in damp but lifeless soil might be very different from those activated by the acids released by plant roots. There's plenty to think about here.

There's also your organic chemistry and what's living in your soil.
I've often thought that we know less about what's living a foot down in our own backyards than we do living a 1/2 mile down in the ocean. You can't see through soil. Move over fifteen feet and the soil might be radically different.

To top it off, depending on the season of the year, an awful lot of the microscopic life is dormant!

If any of you homeschool, you could do a decades long science project by examining selected parts of your yard weekly with a microscope and keep careful records of what you find. It could even be a new species of tiny bug never before identified.