Should We Test Soils For Molybdenum, Cobalt, and Selenium?

From time to time I get questions from farmers asking how they can have their soil samples tested for molybdenum, cobalt, or selenium. Each of these elements, along with many others that aren’t tested routinely in soils, is important. In some cases, these nutrients are required by the plants, while in others they may be required by us for food, by our livestock for feed, by other organisms that live in the soil, or a combination of these.  So should we ask for soil tests to determine whether we need to do that? Maybe yes, maybe no.

Molybdenum is a key element in enzymes that plants need to be able to metabolize nitrogen. It’s a component of the nitrogenase enzyme that Rhizobial bacteria in legume roots use to fix atmospheric nitrogen and it’s essential for every plant to be able to grow. Molybdenum is required by plants in concentrations that are among the lowest of any of the plant-essential nutrients. Most micronutrients are more available to plants at acidic (low) pH levels, but molybdenum actually becomes more available as soil pH comes up. Except for extremely sandy soils and some acidic peat soils, most soils will supply enough molybdenum for good plant growth if the pH is above 6. Soils that are derived from granite, shale, and young volcanic materials often have higher levels of molybdenum, and so do mineral soils with high organic matter levels. Because the level of molybdenum that plants need in the soil is so low, it makes it harder to test for, so plant analysis is a more reliable way to know if you need to apply this micronutrient.

Cobalt is another element that’s needed by plants at very low concentrations. It’s also a component of the nitrogenase enzyme that Rhizobial bacteria in legume roots use to fix atmospheric nitrogen and has recently been recognized as being essential for all plants to live. It’s also a component of Vitamin B₁₂, so it’s essential for plants to be able to produce the quality of food and feed we all need for good nutrition. So far, we don’t know how to interpret soil tests for cobalt, so it probably doesn’t pay to have soils tested for this nutrient.

Selenium is different from molybdenum and cobalt in that it’s not universally required for plant growth, but it is an extremely important nutrient for livestock and human health, and probably used by many other soil organisms as well. It can be a tricky element to balance in rations because the concentration where it’s “just right” is narrow, and too much can be as bad as, or even worse than, too little. Various regions in the United States have low, variable, or high levels of selenium that are reflected in the selenium content of the forages and grain crops from those areas.

Because selenium isn’t needed by the plants themselves, it’s better to rely on feed testing to know if your soil is supplying enough selenium. If it doesn’t, we’ve traditionally relied on supplementing selenium to livestock instead of fertilizing the soil with it, but recently we’re becoming aware that adding selenium to soils where it’s deficient may indeed be practical. Even so, feed testing is still more reliable than soil testing to see if the crops we raise are getting enough of this vital nutrient.

Although all of these elements are very important, so far it doesn’t make sense to use soil testing to determine if we should apply them to the soil. Even though it’s technically possible to test soil samples for each of these, we can’t tell if they are deficient in the soils based on those tests. Testing feed or plant tissue samples to see if they have adequate levels is a better approach.

If you’d like more information on these or other pasture management topics, please contact me at mark.kopecky@organicvalley.coop or 608-632-9933.