The Right Place for Potassium

For the past decade, agriculture personnel have been noticing the decline of soil potassium levels.  It is no surprise that as the yield potential for crops increases, so does the demand for more nutrients.  In many circumstances, these required removal rates are simply not achieved with current recommendations.  There may come a point where the correct crop removal rate is not affordable, creates too high salt concentration, soils are unable to support such rates, and/or equipment cannot apply these rates efficiently.  Choosing the right place to apply potassium can lend aid is such times. 

Most potassium (K), or potash fertilizers are spread on top of the surface of the soil.  They are generally blended in with products such as DAP or MAP for a fall or early spring application.  Most potassium fertilizers are soluble but do not move in the soil like other applied fertilizers.  This is because K is a cation and much of the soil, primarily clay and organic matter, is negatively charged.  The process of binding soil particles and K is known as adsorption.  This is why it takes the correct placement of the fertilizer.  It would be counterproductive to place a K application where the roots of the desired crop will not be. 

Crop types and rotations should affect applications.  Some root systems can reach deep into the soil profile, while others are shallow and grow more horizontally.  Already stated above, K is not very mobile in the soil profile.  There are many different cropping techniques and systems that all require potassium for a successful crop.  Let’s use corn as an example.  A majority of the K is surface applied as a dry product.  Once the fertilizer has been spread, or blown on, it must be incorporated. 

How fast the applied product gets to the root zone of the intended crop relies heavily on the product used, soil type, precipitation and current nutrient values.  When the product is mechanically incorporated, it is fairly simple to achieve even distribution.  Tillage makes it easier to apply higher rates without experiencing stratification of K fertilizer.  Although it can happen at shallow working depths.  Fertilizer stratification is when nutrients are unevenly distributed.   Often a surface application in no-till conditions will concentrate the nutrients on the surface, rather than in the root zone. 

In a no-till system, surface applied nutrients must be incorporated with water.  This makes it especially difficult when trying to raise pH and applying K fertilizer.  Both are being concentrated near the surface, binding to negative soil particles.  Soils with a lower CEC, cation exchange capacity, will be able to achieve nutrient accommodations lower in the soil profile much sooner than high CEC soils.  Once the exchange sites are full, nutrients like potassium will infiltrate deeper.  If large amounts of K applications are made near newly planted seeds, salt damage can occur. 

A management strategy to combat nutrient stratification, potential erosion issues with conventional tillage and dispersing inadequate rates via broadcast is strip-tillage.  This is essentially banding the product in, or just below, the root zone.  Some describe it as the best of both worlds, but it too has its challenges.  It can require expensive equipment.  Not only the strip-till bar, but the distribution system, meters, rate controllers, plumbing and enough tractor to have it effectively work.  It can also be slow depending on rate, product, soil type and terrain. 

With any potassium application, it is important to implement the 4Rs for the best environmental practices and return on investment.