By William McKibben
There is a table (Figure 4) below that I find helpful in deciding how and what level of nutrients to apply to adjust soil nutrients. This table shows the means (mass flow, diffusion or interceptive root growth) by which nutrients are taken up through the roots. This table also shows, with the exception of copper and iron, that most nutrients are picked up out of the liquid solution by either mass flow or diffusion. The mass flow nutrients that are anions (negatively charged ion) are nitrogen (N), sulfur (S), boron (B) and molybdenum (Mo). The cations (positively charged ions) calcium (Ca) and magnesium (Mg) are more free to move in solution and travel longer distances whereas other nutrients such as phosphorus (P), potassium (K) and manganese (Mn) move very little in solution. The later nutrients typically move from areas of high concentration to low concentration, but can bond quite readily in clays and with other nutrients which reduces their mobility.
The other very critical factor to consider is how do most of the nutrients move within the plant once the roots have taken them up. Nutrients like calcium, boron and manganese travel primarily in the xylem or the water transport system of the plant. This is primarily in one direction and very dependent upon the evapotranspiration (ET) happening in the plant. Calcium, boron and manganese are necessary nutrients for root development—especially calcium as it needs to be in contact with the growing root. Nutrients such as potassium and phosphorus on the other hand can travel freely in the phloem. If these nutrients are only surface applied due to no-till practices, then they will become stratified and subject to uptake only as long as the roots are active in the surface.
Looking again at Figure 4, this table can also help with interpretation of plant tissue analysis. For example, plants that are short or deficient in copper may indicate a lack of this nutrient in the soil or a root development problem. This may be a physical structure, compaction, or another nutrient problem (such as calcium level affecting root growth). I have seen an almost fourfold increase in copper deficiencies found in soybeans over corn crops. This might be just the difference between the soybean taproots and the fibrous root system of corn plants. Personally I think a lot of the problem is related to compaction issues since many of our beans are no-till planted into corn stalks. Plants tend to grow at the rate provided by the least available nutrient. Compaction issues will almost always result in many other nutrient deficiencies (especially potassium and phosphorus). There will be more on identifying nutrient deficiencies using tissue analysis later in this book.
Source: The Art of Balancing Soil Nutrients