There is a lot of biological and chemical activity occurring in your average soil. Most happens unseen and unheard, and that’s probably a good thing. Just because these organisms and processes don’t scream out, doesn’t mean growers shouldn’t pay attention to what goes on. In fact, paying attention can make the difference between a good and a great grower. In an earlier Michigan State University Extension article, “Knowing nutrient mobility is helpful in diagnosing plant nutrient deficiencies,” I discussed nutrient mobility within the plant and how understanding mobility helps identify nutrient deficiencies, but it is also important for growers to pay attention to nutrient mobility in the soil.
Plant nutrients exist in the soil as either anions or cations. What are they? Most molecules in natural systems have a positive or negative charge and it is this charge difference that helps drive chemical reactions to keep us all alive – that’s important. Anions are those elements or molecules that in their natural state have a negative (-) charge. Cations are those that in their natural state have a positive (+) charge. Negative charge, positive charge – who cares? Keep reading.
Most soil particles have a negative charge. The amount of negative charge depends on soil texture, such as sand, silt and clay content, which is directly related to soil particle surface area. The cation exchange capacity (CEC) determined by a soil test is a direct indication of the amount of negative charges on your soils. A soil with low CEC has fewer negative charges than a soil with a higher number. High sand soils generally have a low CEC, clay or silt soils are higher and organic soils are highest – all related to particle surface area.
Now the important part! Since soils are negatively charged and plant nutrients are positive and negative, some nutrients are attracted to soil while others are not – the “opposites attract” principle. Those nutrients that exist as anions (-) are moved through soil, meaning growers need to be careful how they are applied regardless of soil type. These nutrients readily travel wherever water carries them, leading to nutrient runoff and leaching and economic loss and environmental concern.
Cations (+) are more readily bound to soil, resulting in these nutrients moving through the soil more slowly. However, since low CEC soils have fewer negative charges, cations will move more quickly through low CEC (sandy-based) soils than they will through high CEC (loamy and silt/clay-based) soils.
All this positive-negative, cation-anion, high CEC-low CEC stuff comes into play when applying nutrients and water. Table 1 gives the soil-borne elements necessary for plant growth, the form taken up by the plant and the element’s mobility in the soil. Note that most mobile elements have a negative charge and the somewhat mobile and immobile elements have a positive charge. Over application of a (-) charged element followed by excessive water will quickly move that element through the system. Likewise, over application of most (+) charged elements on a low CEC soil can move that element through the system since there are not enough (-) charges on the soil particle surface to bind to the cation.
The odd anion is phosphorous. Even though it has a (-) charge, it is not mobile in soil because phosphorous forms are not very soluble. It can, however, still move – not as the anion, but bound to soil particles as the particles move. Therefore, minimizing runoff is helpful in reducing phosphorus pollution.
Understanding nutrient movement in soils helps producers apply nutrients and water to maximize economic effectiveness while minimizing environmental impact.