Soil fertility is widely recognized to be an important aspect of maintaining and improving farmland.  So, it makes sense to ask the question of what exactly is meant by soil fertility.  The first answer from many working in production agriculture would be that soil fertility is how much Phosphorus (P) and Potassium (K) show up in soil test results.  However, most would likely not argue with the more correct answer, which is that what fertility really means is the capability of a soil to promote robust plant growth and ultimately high yields when the other environmental factors in play allow.  The assumption that soil test results for P and K are a key metric in evaluating that capability, and that the level of each of these in your soil tests will correlate with yield results, has traditionally been widely accepted.

Before we accept or reject the notion that P and K soil test results define soil fertility, let’s take a look at the basics of plant nutrition.  There are 17 nutrients essential for plant growth.  Carbon, Hydrogen, and Oxygen are three of the 17 essential nutrients which are acquired by the plants from air and water in the environment and are considered non-fertilizer nutrients.  That leaves 14 that come from the soil. Of those 14, three are traditionally labelled as macronutrients.  They are placed in this category based on the relative quantity of the nutrient that a crop requires, not because they are necessarily more important than the others.  After all, essential means essential.  Something can be essential or non-essential, but it’s difficult to be more essential or less essential.  There are significant differences in the amount of each nutrient required by a given crop, and that is reflected by the categories used to segment them.  The macronutrients are Nitrogen, Phosphorus, and Potassium.  The secondary nutrients are Sulfur, Calcium, and Magnesium. The rest are micronutrients generally required in trace quantities – zinc, boron, copper, iron, manganese, molybdenum, chlorine, nickel, and cobalt.

If we consider the micronutrients, why is it that there are many acres we could identify that have been planted to crops for many years…150 – 200 in the Midwest, and longer in the east.. that have never had cobalt or nickel or chlorine or molybdenum added?  Why haven’t those soils run out of these or other micronutrients?  If the crop uses up the soil nutrients it needs, it would seem reasonable that we would eventually have to replace what is used if we want to keep increasing yields.

Too often we think of the soil primarily as a place to put our fertilizer, herbicide, and seed for the purpose of growing crops.  The fertility needs of a crop can be oversimplified by thinking in terms similar to a gas tank with a gauge that goes down as crops are produced and consequently needs to have the nutrient tank refilled on a regular basis.  In fact, our soils are a very complex ecosystem with many interacting processes, physical, chemical, and microbial.  These processes are sometimes complimentary to each other, sometimes in opposition, and sometimes neutral.  Soil tests and nutrient levels are important to monitor, but the gas tank analogy is not a good reflection of nutrient management.  For example, while micronutrients are being used by plants, organic matter is being mineralized by soil microorganisms into inorganic plant-available forms of multiple nutrients, including the micronutrients.  The processes that drive mineralization are dependent on many factors… pH, moisture, temperature, soil texture, the health of the microbial population.  Some of these can be easily managed and some cannot be.  We’ll explore more of the basics of soil fertility in future posts.  For example, a logical next question might be, “why don’t we use up the organic matter in the soil if we are constantly mineralizing it to supply many nutrients the crop uses?”  We’ll talk about organic matter in the next blog post on fertility.