Outsmart Your "K"ompetition

The Role of Potassium in Crop Growth

Potassium (K) increases crop yield, improves crop quality, and is considered one of the primary plant macronutrients, along with nitrogen (N) and phosphorus (P). Except for N, plants often require more K than any other nutrient, as it plays vital roles in crop growth and development.

Unlike phosphate and micronutrients, potassium availability is not generally related to soil pH. Rather, K supply is regulated by a complex interplay between crop demands, fertilization practices, and five forces that compete with your plant for K uptake.

Understanding your "K"ompetition in the soil will help you overcome and deliver much needed potassium to the crop to ensure yield and quality (Figure 1).

5 Sources of “K”ompetition

Potassium Tie Up

Soils differ in their relative amounts of sand, silt and clay particles (e.g., texture). Clay dominated soils tend to have a high cation exchange capacity (CEC), which is the relative ability of a soil to store positively-charged nutrients.  However, nutrient release rates are inversely related to the CEC value. Thus, clay soils can release stored K back to soil solution at a slower rate than actual plant demand. In this sense, clays can ‘tie up’ potassium and slow its availability.

Potassium Fixation

Certain clay minerals can scavenge plant available K from the soil, including K fertilizers, and render the nutrient unavailable to the plant. This is called potassium fixation and the degree of the fixation strength can vary between regions, and even in different areas of a field. Strong K fixation capacity is typically associated with the presence of vermiculite and mica-based minerals in soil. These minerals have a high affinity (e.g., high negative charge) and internal space to attract and sequester plant available K in your soil. Fixed K, once tied up, is no longer available for plant use during the growing season.

Dry Soils

Potassium must be in solution (e.g., soil water) for the plant to take it up through the roots and put it to work for the crop. Make sure water inputs are moving through the entire rooting volume of the crop to ensure adequate K uptake. Excessively dry soils, at any depth throughout the rooting zone, will prevent K uptake.

Leaching

Lightly textured soils, such as sands, are not very good at storing plant nutrients (e.g., low CEC). Furthermore, they also have a high nutrient release rate back to solution. K, in these types of soil situations, is prone to leaching as water can move the potassium ions out of the rooting zone, similar to the way we think about nitrates.

K Uptake Antagonisms with Sodium

Potassium (K) and sodium (Na) atoms are of similar size and charge and can therefore interfere with each other in soil solution for plant root uptake. Wakeel (2013) recently summarized the antagonistic effect of Na on K uptake.

• Poor Uptake of K: Membrane depolarization caused by Na makes it difficult for K to be taken up by inward-rectifying channels (KIRs). Sodium can compromise potassium uptake into plant roots.
• Increased K Leakage: Na increases K leakage from the cell by activating outward-rectifying channels (KORs). Sodium can cause plants to lose potassium through the roots.

Options

Outsmarting your soil competition for K uptake in your crop requires the recognition that your crop is not on a level playing field for maintaining optimum potassium nutrition. A combination of clay mineral interactions, leaching forces, and antagonisms can prevent your crop from acquiring adequate K from soil solution. With these “K”ompetitive agents in mind, it is worth protecting your potassium investment by utilizing the latest in fertilizer formulation technology from Helena.

- Dr. Karl Wyant, Division Agronomist